€•Š      Œsphinx.addnodes”Œdocument”“”)”}”(Œ	rawsource”Œ ”Œchildren”]”(Œtranslations”ŒLanguagesNode”“”)”}”(hhh]”(h Œpending_xref”“”)”}”(hhh]”Œdocutils.nodes”ŒText”“”ŒChinese (Simplified)”…””}”Œparent”hsbaŒ
attributes”}”(Œids”]”Œclasses”]”Œnames”]”Œdupnames”]”Œbackrefs”]”Œ	refdomain”Œstd”Œreftype”Œdoc”Œ	reftarget”Œ!/translations/zh_CN/RCU/whatisRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuŒtagname”hhhubh)”}”(hhh]”hŒChinese (Traditional)”…””}”hh2sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ!/translations/zh_TW/RCU/whatisRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒItalian”…””}”hhFsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ!/translations/it_IT/RCU/whatisRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒJapanese”…””}”hhZsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ!/translations/ja_JP/RCU/whatisRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒKorean”…””}”hhnsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ!/translations/ko_KR/RCU/whatisRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒPortuguese (Brazilian)”…””}”hh‚sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ!/translations/pt_BR/RCU/whatisRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒSpanish”…””}”hh–sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ!/translations/sp_SP/RCU/whatisRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubeh}”(h]”h ]”h"]”h$]”h&]”Œcurrent_language”ŒEnglish”uh1h
hhŒ	_document”hŒsource”NŒline”NubhŒtarget”“”)”}”(hŒ.. _whatisrcu_doc:”h]”h}”(h]”h ]”h"]”h$]”h&]”Œrefid”Œwhatisrcu-doc”uh1hµh´Khhh²hh³Œ;/var/lib/git/docbuild/linux/Documentation/RCU/whatisRCU.rst”ubhŒsection”“”)”}”(hhh]”(hŒtitle”“”)”}”(hŒ&What is RCU?  --  "Read, Copy, Update"”h]”hŒ*What is RCU?  --  â€œRead, Copy, Updateâ€”…””}”(hhËh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhhÆh²hh³hÃh´KubhŒ	paragraph”“”)”}”(hŒaPlease note that the "What is RCU?" LWN series is an excellent place
to start learning about RCU:”h]”hŒePlease note that the â€œWhat is RCU?â€ LWN series is an excellent place
to start learning about RCU:”…””}”(hhÛh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KhhÆh²hubhŒ
line_block”“”)”}”(hhh]”(hh´“”)”}”(hŒC1.    What is RCU, Fundamentally?  https://lwn.net/Articles/262464/”h]”(hŒ#1.    What is RCU, Fundamentally?  ”…””}”(hhïh²hh³Nh´NubhŒ	reference”“”)”}”(hŒ https://lwn.net/Articles/262464/”h]”hŒ https://lwn.net/Articles/262464/”…””}”(hhùh²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”hûuh1h÷hhïubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´Œindent”K hhëh²hh³hÃh´K	ubhî)”}”(hŒC2.    What is RCU? Part 2: Usage   https://lwn.net/Articles/263130/”h]”(hŒ#2.    What is RCU? Part 2: Usage   ”…””}”(hj  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/263130/”h]”hŒ https://lwn.net/Articles/263130/”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j  uh1h÷hj  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  K hhëh²hh³hÃh´K
ubhî)”}”(hŒC3.    RCU part 3: the RCU API      https://lwn.net/Articles/264090/”h]”(hŒ#3.    RCU part 3: the RCU API      ”…””}”(hj,  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/264090/”h]”hŒ https://lwn.net/Articles/264090/”…””}”(hj4  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j6  uh1h÷hj,  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  K hhëh²hh³hÃh´Kubhî)”}”(hŒC4.    The RCU API, 2010 Edition    https://lwn.net/Articles/418853/”h]”(hŒ#4.    The RCU API, 2010 Edition    ”…””}”(hjI  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/418853/”h]”hŒ https://lwn.net/Articles/418853/”…””}”(hjQ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jS  uh1h÷hjI  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  K hhëh²hh³hÃh´Kubhê)”}”(hhh]”hî)”}”(hŒ=2010 Big API Table           https://lwn.net/Articles/419086/”h]”(hŒ2010 Big API Table           ”…””}”(hji  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/419086/”h]”hŒ https://lwn.net/Articles/419086/”…””}”(hjq  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”js  uh1h÷hji  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  Khjf  h²hh³hÃh´Kubah}”(h]”h ]”h"]”h$]”h&]”uh1héhhëh²hh³hÃh´Kubhî)”}”(hŒC5.    The RCU API, 2014 Edition    https://lwn.net/Articles/609904/”h]”(hŒ#5.    The RCU API, 2014 Edition    ”…””}”(hjŒ  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/609904/”h]”hŒ https://lwn.net/Articles/609904/”…””}”(hj”  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j–  uh1h÷hjŒ  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  K hhëh²hh³hÃh´Kubhê)”}”(hhh]”hî)”}”(hŒ=2014 Big API Table           https://lwn.net/Articles/609973/”h]”(hŒ2014 Big API Table           ”…””}”(hj¬  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/609973/”h]”hŒ https://lwn.net/Articles/609973/”…””}”(hj´  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j¶  uh1h÷hj¬  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  Khj©  h²hh³hÃh´Kubah}”(h]”h ]”h"]”h$]”h&]”uh1héhhëh²hh³hÃh´Kubhî)”}”(hŒC6.    The RCU API, 2019 Edition    https://lwn.net/Articles/777036/”h]”(hŒ#6.    The RCU API, 2019 Edition    ”…””}”(hjÏ  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/777036/”h]”hŒ https://lwn.net/Articles/777036/”…””}”(hj×  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jÙ  uh1h÷hjÏ  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  K hhëh²hh³hÃh´Kubhê)”}”(hhh]”hî)”}”(hŒ=2019 Big API Table           https://lwn.net/Articles/777165/”h]”(hŒ2019 Big API Table           ”…””}”(hjï  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/777165/”h]”hŒ https://lwn.net/Articles/777165/”…””}”(hj÷  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jù  uh1h÷hjï  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  Khjì  h²hh³hÃh´Kubah}”(h]”h ]”h"]”h$]”h&]”uh1héhhëh²hh³hÃh´Kubhî)”}”(hŒC7.    The RCU API, 2024 Edition    https://lwn.net/Articles/988638/”h]”(hŒ#7.    The RCU API, 2024 Edition    ”…””}”(hj  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/988638/”h]”hŒ https://lwn.net/Articles/988638/”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j  uh1h÷hj  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  K hhëh²hh³hÃh´Kubhê)”}”(hhh]”(hî)”}”(hŒ=2024 Background Information  https://lwn.net/Articles/988641/”h]”(hŒ2024 Background Information  ”…””}”(hj2  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/988641/”h]”hŒ https://lwn.net/Articles/988641/”…””}”(hj:  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j<  uh1h÷hj2  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  Khj/  h²hh³hÃh´Kubhî)”}”(hŒ=2024 Big API Table           https://lwn.net/Articles/988666/”h]”(hŒ2024 Big API Table           ”…””}”(hjO  h²hh³Nh´Nubhø)”}”(hŒ https://lwn.net/Articles/988666/”h]”hŒ https://lwn.net/Articles/988666/”…””}”(hjW  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jY  uh1h÷hjO  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  Khj/  h²hh³hÃh´Kubeh}”(h]”h ]”h"]”h$]”h&]”uh1héhhëh²hh³hÃh´Kubeh}”(h]”h ]”h"]”h$]”h&]”uh1héhhÆh²hh³hÃh´K	ubhÚ)”}”(hŒFor those preferring video:”h]”hŒFor those preferring video:”…””}”(hjx  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KhhÆh²hubhê)”}”(hhh]”(hî)”}”(hŒ}1.    Unraveling RCU Mysteries: Fundamentals          https://www.linuxfoundation.org/webinars/unraveling-rcu-usage-mysteries”h]”(hŒ61.    Unraveling RCU Mysteries: Fundamentals          ”…””}”(hj‰  h²hh³Nh´Nubhø)”}”(hŒGhttps://www.linuxfoundation.org/webinars/unraveling-rcu-usage-mysteries”h]”hŒGhttps://www.linuxfoundation.org/webinars/unraveling-rcu-usage-mysteries”…””}”(hj‘  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j“  uh1h÷hj‰  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  K hj†  h²hh³hÃh´Kubhî)”}”(hŒ’2.    Unraveling RCU Mysteries: Additional Use Cases  https://www.linuxfoundation.org/webinars/unraveling-rcu-usage-mysteries-additional-use-cases”h]”(hŒ62.    Unraveling RCU Mysteries: Additional Use Cases  ”…””}”(hj¦  h²hh³Nh´Nubhø)”}”(hŒ\https://www.linuxfoundation.org/webinars/unraveling-rcu-usage-mysteries-additional-use-cases”h]”hŒ\https://www.linuxfoundation.org/webinars/unraveling-rcu-usage-mysteries-additional-use-cases”…””}”(hj®  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j°  uh1h÷hj¦  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1h´j  K hj†  h²hh³hÃh´Kubeh}”(h]”h ]”h"]”h$]”h&]”uh1héhhÆh²hh³hÃh´KubhÚ)”}”(hŒWhat is RCU?”h]”hŒWhat is RCU?”…””}”(hjÉ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KhhÆh²hubhÚ)”}”(hX_  RCU is a synchronization mechanism that was added to the Linux kernel
during the 2.5 development effort that is optimized for read-mostly
situations.  Although RCU is actually quite simple, making effective use
of it requires you to think differently about your code.  Another part
of the problem is the mistaken assumption that there is "one true way" to
describe and to use RCU.  Instead, the experience has been that different
people must take different paths to arrive at an understanding of RCU,
depending on their experiences and use cases.  This document provides
several different paths, as follows:”h]”hXc  RCU is a synchronization mechanism that was added to the Linux kernel
during the 2.5 development effort that is optimized for read-mostly
situations.  Although RCU is actually quite simple, making effective use
of it requires you to think differently about your code.  Another part
of the problem is the mistaken assumption that there is â€œone true wayâ€ to
describe and to use RCU.  Instead, the experience has been that different
people must take different paths to arrive at an understanding of RCU,
depending on their experiences and use cases.  This document provides
several different paths, as follows:”…””}”(hj×  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KhhÆh²hubhÚ)”}”(hŒ+:ref:`1.        RCU OVERVIEW <1_whatisRCU>`”h]”h)”}”(hjç  h]”hŒinline”“”)”}”(hjç  h]”hŒ1.        RCU OVERVIEW”…””}”(hjî  h²hh³Nh´Nubah}”(h]”h ]”(Œxref”Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hjé  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”ŒRCU/whatisRCU”Œ	refdomain”jù  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆŒ	reftarget”Œ1_whatisrcu”uh1hh³hÃh´K(hjå  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K(hhÆh²hubhÚ)”}”(hŒ6:ref:`2.        WHAT IS RCU'S CORE API? <2_whatisRCU>`”h]”h)”}”(hj  h]”jí  )”}”(hj  h]”hŒ#2.        WHAT IS RCUâ€™S CORE API?”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hj  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”j$  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ2_whatisrcu”uh1hh³hÃh´K*hj  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K*hhÆh²hubhÚ)”}”(hŒJ:ref:`3.        WHAT ARE SOME EXAMPLE USES OF CORE RCU API? <3_whatisRCU>`”h]”h)”}”(hj>  h]”jí  )”}”(hj>  h]”hŒ53.        WHAT ARE SOME EXAMPLE USES OF CORE RCU API?”…””}”(hjC  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hj@  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”jM  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ3_whatisrcu”uh1hh³hÃh´K,hj<  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K,hhÆh²hubhÚ)”}”(hŒG:ref:`4.        WHAT IF MY UPDATING THREAD CANNOT BLOCK? <4_whatisRCU>`”h]”h)”}”(hjg  h]”jí  )”}”(hjg  h]”hŒ24.        WHAT IF MY UPDATING THREAD CANNOT BLOCK?”…””}”(hjl  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hji  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”jv  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ4_whatisrcu”uh1hh³hÃh´K.hje  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K.hhÆh²hubhÚ)”}”(hŒK:ref:`5.        WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU? <5_whatisRCU>`”h]”h)”}”(hj  h]”jí  )”}”(hj  h]”hŒ65.        WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?”…””}”(hj•  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hj’  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”jŸ  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ5_whatisrcu”uh1hh³hÃh´K0hjŽ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K0hhÆh²hubhÚ)”}”(hŒA:ref:`6.        ANALOGY WITH READER-WRITER LOCKING <6_whatisRCU>`”h]”h)”}”(hj¹  h]”jí  )”}”(hj¹  h]”hŒ,6.        ANALOGY WITH READER-WRITER LOCKING”…””}”(hj¾  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hj»  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”jÈ  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ6_whatisrcu”uh1hh³hÃh´K2hj·  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K2hhÆh²hubhÚ)”}”(hŒ>:ref:`7.        ANALOGY WITH REFERENCE COUNTING <7_whatisRCU>`”h]”h)”}”(hjâ  h]”jí  )”}”(hjâ  h]”hŒ)7.        ANALOGY WITH REFERENCE COUNTING”…””}”(hjç  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hjä  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”jñ  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ7_whatisrcu”uh1hh³hÃh´K4hjà  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K4hhÆh²hubhÚ)”}”(hŒ4:ref:`8.        FULL LIST OF RCU APIs <8_whatisRCU>`”h]”h)”}”(hj  h]”jí  )”}”(hj  h]”hŒ8.        FULL LIST OF RCU APIs”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hj  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”j  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ8_whatisrcu”uh1hh³hÃh´K6hj	  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K6hhÆh²hubhÚ)”}”(hŒ7:ref:`9.        ANSWERS TO QUICK QUIZZES <9_whatisRCU>`”h]”h)”}”(hj4  h]”jí  )”}”(hj4  h]”hŒ"9.        ANSWERS TO QUICK QUIZZES”…””}”(hj9  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hj6  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”jC  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ9_whatisrcu”uh1hh³hÃh´K8hj2  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K8hhÆh²hubhÚ)”}”(hXv  People who prefer starting with a conceptual overview should focus on
Section 1, though most readers will profit by reading this section at
some point.  People who prefer to start with an API that they can then
experiment with should focus on Section 2.  People who prefer to start
with example uses should focus on Sections 3 and 4.  People who need to
understand the RCU implementation should focus on Section 5, then dive
into the kernel source code.  People who reason best by analogy should
focus on Section 6 and 7.  Section 8 serves as an index to the docbook
API documentation, and Section 9 is the traditional answer key.”h]”hXv  People who prefer starting with a conceptual overview should focus on
Section 1, though most readers will profit by reading this section at
some point.  People who prefer to start with an API that they can then
experiment with should focus on Section 2.  People who prefer to start
with example uses should focus on Sections 3 and 4.  People who need to
understand the RCU implementation should focus on Section 5, then dive
into the kernel source code.  People who reason best by analogy should
focus on Section 6 and 7.  Section 8 serves as an index to the docbook
API documentation, and Section 9 is the traditional answer key.”…””}”(hj[  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K:hhÆh²hubhÚ)”}”(hX>  So, start with the section that makes the most sense to you and your
preferred method of learning.  If you need to know everything about
everything, feel free to read the whole thing -- but if you are really
that type of person, you have perused the source code and will therefore
never need this document anyway.  ;-)”h]”hX>  So, start with the section that makes the most sense to you and your
preferred method of learning.  If you need to know everything about
everything, feel free to read the whole thing -- but if you are really
that type of person, you have perused the source code and will therefore
never need this document anyway.  ;-)”…””}”(hji  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KDhhÆh²hubh¶)”}”(hŒ.. _1_whatisRCU:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒ	whatisrcu”uh1hµh´KJhhÆh²hh³hÃubhÅ)”}”(hhh]”(hÊ)”}”(hŒ1.  RCU OVERVIEW”h]”hŒ1.  RCU OVERVIEW”…””}”(hj…  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhj‚  h²hh³hÃh´KMubhÚ)”}”(hX\  The basic idea behind RCU is to split updates into "removal" and
"reclamation" phases.  The removal phase removes references to data items
within a data structure (possibly by replacing them with references to
new versions of these data items), and can run concurrently with readers.
The reason that it is safe to run the removal phase concurrently with
readers is the semantics of modern CPUs guarantee that readers will see
either the old or the new version of the data structure rather than a
partially updated reference.  The reclamation phase does the work of reclaiming
(e.g., freeing) the data items removed from the data structure during the
removal phase.  Because reclaiming data items can disrupt any readers
concurrently referencing those data items, the reclamation phase must
not start until readers no longer hold references to those data items.”h]”hXd  The basic idea behind RCU is to split updates into â€œremovalâ€ and
â€œreclamationâ€ phases.  The removal phase removes references to data items
within a data structure (possibly by replacing them with references to
new versions of these data items), and can run concurrently with readers.
The reason that it is safe to run the removal phase concurrently with
readers is the semantics of modern CPUs guarantee that readers will see
either the old or the new version of the data structure rather than a
partially updated reference.  The reclamation phase does the work of reclaiming
(e.g., freeing) the data items removed from the data structure during the
removal phase.  Because reclaiming data items can disrupt any readers
concurrently referencing those data items, the reclamation phase must
not start until readers no longer hold references to those data items.”…””}”(hj“  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KOhj‚  h²hubhÚ)”}”(hX9  Splitting the update into removal and reclamation phases permits the
updater to perform the removal phase immediately, and to defer the
reclamation phase until all readers active during the removal phase have
completed, either by blocking until they finish or by registering a
callback that is invoked after they finish.  Only readers that are active
during the removal phase need be considered, because any reader starting
after the removal phase will be unable to gain a reference to the removed
data items, and therefore cannot be disrupted by the reclamation phase.”h]”hX9  Splitting the update into removal and reclamation phases permits the
updater to perform the removal phase immediately, and to defer the
reclamation phase until all readers active during the removal phase have
completed, either by blocking until they finish or by registering a
callback that is invoked after they finish.  Only readers that are active
during the removal phase need be considered, because any reader starting
after the removal phase will be unable to gain a reference to the removed
data items, and therefore cannot be disrupted by the reclamation phase.”…””}”(hj¡  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K\hj‚  h²hubhÚ)”}”(hŒESo the typical RCU update sequence goes something like the following:”h]”hŒESo the typical RCU update sequence goes something like the following:”…””}”(hj¯  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Kehj‚  h²hubhŒenumerated_list”“”)”}”(hhh]”(hŒ	list_item”“”)”}”(hŒ_Remove pointers to a data structure, so that subsequent
readers cannot gain a reference to it.
”h]”hÚ)”}”(hŒ^Remove pointers to a data structure, so that subsequent
readers cannot gain a reference to it.”h]”hŒ^Remove pointers to a data structure, so that subsequent
readers cannot gain a reference to it.”…””}”(hjÈ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KghjÄ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj¿  h²hh³hÃh´NubjÃ  )”}”(hŒQWait for all previous readers to complete their RCU read-side
critical sections.
”h]”hÚ)”}”(hŒPWait for all previous readers to complete their RCU read-side
critical sections.”h]”hŒPWait for all previous readers to complete their RCU read-side
critical sections.”…””}”(hjà  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KjhjÜ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj¿  h²hh³hÃh´NubjÃ  )”}”(hŒŠAt this point, there cannot be any readers who hold references
to the data structure, so it now may safely be reclaimed
(e.g., kfree()d).
”h]”hÚ)”}”(hŒ‰At this point, there cannot be any readers who hold references
to the data structure, so it now may safely be reclaimed
(e.g., kfree()d).”h]”hŒ‰At this point, there cannot be any readers who hold references
to the data structure, so it now may safely be reclaimed
(e.g., kfree()d).”…””}”(hjø  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Kmhjô  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj¿  h²hh³hÃh´Nubeh}”(h]”h ]”h"]”h$]”h&]”Œenumtype”Œ
loweralpha”Œprefix”hŒsuffix”Œ.”uh1j½  hj‚  h²hh³hÃh´KgubhÚ)”}”(hX  Step (b) above is the key idea underlying RCU's deferred destruction.
The ability to wait until all readers are done allows RCU readers to
use much lighter-weight synchronization, in some cases, absolutely no
synchronization at all.  In contrast, in more conventional lock-based
schemes, readers must use heavy-weight synchronization in order to
prevent an updater from deleting the data structure out from under them.
This is because lock-based updaters typically update data items in place,
and must therefore exclude readers.  In contrast, RCU-based updaters
typically take advantage of the fact that writes to single aligned
pointers are atomic on modern CPUs, allowing atomic insertion, removal,
and replacement of data items in a linked structure without disrupting
readers.  Concurrent RCU readers can then continue accessing the old
versions, and can dispense with the atomic operations, memory barriers,
and communications cache misses that are so expensive on present-day
SMP computer systems, even in absence of lock contention.”h]”hX  Step (b) above is the key idea underlying RCUâ€™s deferred destruction.
The ability to wait until all readers are done allows RCU readers to
use much lighter-weight synchronization, in some cases, absolutely no
synchronization at all.  In contrast, in more conventional lock-based
schemes, readers must use heavy-weight synchronization in order to
prevent an updater from deleting the data structure out from under them.
This is because lock-based updaters typically update data items in place,
and must therefore exclude readers.  In contrast, RCU-based updaters
typically take advantage of the fact that writes to single aligned
pointers are atomic on modern CPUs, allowing atomic insertion, removal,
and replacement of data items in a linked structure without disrupting
readers.  Concurrent RCU readers can then continue accessing the old
versions, and can dispense with the atomic operations, memory barriers,
and communications cache misses that are so expensive on present-day
SMP computer systems, even in absence of lock contention.”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Kqhj‚  h²hubhÚ)”}”(hXa  In the three-step procedure shown above, the updater is performing both
the removal and the reclamation step, but it is often helpful for an
entirely different thread to do the reclamation, as is in fact the case
in the Linux kernel's directory-entry cache (dcache).  Even if the same
thread performs both the update step (step (a) above) and the reclamation
step (step (c) above), it is often helpful to think of them separately.
For example, RCU readers and updaters need not communicate at all,
but RCU provides implicit low-overhead communication between readers
and reclaimers, namely, in step (b) above.”h]”hXc  In the three-step procedure shown above, the updater is performing both
the removal and the reclamation step, but it is often helpful for an
entirely different thread to do the reclamation, as is in fact the case
in the Linux kernelâ€™s directory-entry cache (dcache).  Even if the same
thread performs both the update step (step (a) above) and the reclamation
step (step (c) above), it is often helpful to think of them separately.
For example, RCU readers and updaters need not communicate at all,
but RCU provides implicit low-overhead communication between readers
and reclaimers, namely, in step (b) above.”…””}”(hj%  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Khj‚  h²hubhÚ)”}”(hŒ¼So how the heck can a reclaimer tell when a reader is done, given
that readers are not doing any sort of synchronization operations???
Read on to learn about how RCU's API makes this easy.”h]”hŒ¾So how the heck can a reclaimer tell when a reader is done, given
that readers are not doing any sort of synchronization operations???
Read on to learn about how RCUâ€™s API makes this easy.”…””}”(hj3  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K‹hj‚  h²hubh¶)”}”(hŒ.. _2_whatisRCU:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒid1”uh1hµh´Khj‚  h²hh³hÃubeh}”(h]”(Œrcu-overview”j  eh ]”h"]”(Œ1. rcu overview”Œ1_whatisrcu”eh$]”h&]”uh1hÄhhÆh²hh³hÃh´KMŒexpect_referenced_by_name”}”jR  jw  sŒexpect_referenced_by_id”}”j  jw  subhÅ)”}”(hhh]”(hÊ)”}”(hŒ2.  WHAT IS RCU'S CORE API?”h]”hŒ2.  WHAT IS RCUâ€™S CORE API?”…””}”(hj\  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhjY  h²hh³hÃh´K’ubhÚ)”}”(hŒ The core RCU API is quite small:”h]”hŒ The core RCU API is quite small:”…””}”(hjj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K”hjY  h²hubj¾  )”}”(hhh]”(jÃ  )”}”(hŒrcu_read_lock()”h]”hÚ)”}”(hj}  h]”hŒrcu_read_lock()”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K–hj{  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjx  h²hh³hÃh´NubjÃ  )”}”(hŒrcu_read_unlock()”h]”hÚ)”}”(hj”  h]”hŒrcu_read_unlock()”…””}”(hj–  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K—hj’  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjx  h²hh³hÃh´NubjÃ  )”}”(hŒsynchronize_rcu() / call_rcu()”h]”hÚ)”}”(hj«  h]”hŒsynchronize_rcu() / call_rcu()”…””}”(hj­  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K˜hj©  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjx  h²hh³hÃh´NubjÃ  )”}”(hŒrcu_assign_pointer()”h]”hÚ)”}”(hjÂ  h]”hŒrcu_assign_pointer()”…””}”(hjÄ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K™hjÀ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjx  h²hh³hÃh´NubjÃ  )”}”(hŒrcu_dereference()
”h]”hÚ)”}”(hŒrcu_dereference()”h]”hŒrcu_dereference()”…””}”(hjÛ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Kšhj×  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjx  h²hh³hÃh´Nubeh}”(h]”h ]”h"]”h$]”h&]”j  j  j  hj  j  uh1j½  hjY  h²hh³hÃh´K–ubhÚ)”}”(hŒÉThere are many other members of the RCU API, but the rest can be
expressed in terms of these five, though most implementations instead
express synchronize_rcu() in terms of the call_rcu() callback API.”h]”hŒÉThere are many other members of the RCU API, but the rest can be
expressed in terms of these five, though most implementations instead
express synchronize_rcu() in terms of the call_rcu() callback API.”…””}”(hjõ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KœhjY  h²hubhÚ)”}”(hŒ¹The five core RCU APIs are described below, the other 18 will be enumerated
later.  See the kernel docbook documentation for more info, or look directly
at the function header comments.”h]”hŒ¹The five core RCU APIs are described below, the other 18 will be enumerated
later.  See the kernel docbook documentation for more info, or look directly
at the function header comments.”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K hjY  h²hubhÅ)”}”(hhh]”(hÊ)”}”(hŒrcu_read_lock()”h]”hŒrcu_read_lock()”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhj  h²hh³hÃh´K¥ubhŒblock_quote”“”)”}”(hX¸  void rcu_read_lock(void);

This temporal primitive is used by a reader to inform the
reclaimer that the reader is entering an RCU read-side critical
section.  It is illegal to block while in an RCU read-side
critical section, though kernels built with CONFIG_PREEMPT_RCU
can preempt RCU read-side critical sections.  Any RCU-protected
data structure accessed during an RCU read-side critical section
is guaranteed to remain unreclaimed for the full duration of that
critical section.  Reference counts may be used in conjunction
with RCU to maintain longer-term references to data structures.

Note that anything that disables bottom halves, preemption,
or interrupts also enters an RCU read-side critical section.
Acquiring a spinlock also enters an RCU read-side critical
sections, even for spinlocks that do not disable preemption,
as is the case in kernels built with CONFIG_PREEMPT_RT=y.
Sleeplocks do *not* enter RCU read-side critical sections.
”h]”(hÚ)”}”(hŒvoid rcu_read_lock(void);”h]”hŒvoid rcu_read_lock(void);”…””}”(hj(  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K¦hj$  ubhÚ)”}”(hX5  This temporal primitive is used by a reader to inform the
reclaimer that the reader is entering an RCU read-side critical
section.  It is illegal to block while in an RCU read-side
critical section, though kernels built with CONFIG_PREEMPT_RCU
can preempt RCU read-side critical sections.  Any RCU-protected
data structure accessed during an RCU read-side critical section
is guaranteed to remain unreclaimed for the full duration of that
critical section.  Reference counts may be used in conjunction
with RCU to maintain longer-term references to data structures.”h]”hX5  This temporal primitive is used by a reader to inform the
reclaimer that the reader is entering an RCU read-side critical
section.  It is illegal to block while in an RCU read-side
critical section, though kernels built with CONFIG_PREEMPT_RCU
can preempt RCU read-side critical sections.  Any RCU-protected
data structure accessed during an RCU read-side critical section
is guaranteed to remain unreclaimed for the full duration of that
critical section.  Reference counts may be used in conjunction
with RCU to maintain longer-term references to data structures.”…””}”(hj6  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K¨hj$  ubhÚ)”}”(hXe  Note that anything that disables bottom halves, preemption,
or interrupts also enters an RCU read-side critical section.
Acquiring a spinlock also enters an RCU read-side critical
sections, even for spinlocks that do not disable preemption,
as is the case in kernels built with CONFIG_PREEMPT_RT=y.
Sleeplocks do *not* enter RCU read-side critical sections.”h]”(hX9  Note that anything that disables bottom halves, preemption,
or interrupts also enters an RCU read-side critical section.
Acquiring a spinlock also enters an RCU read-side critical
sections, even for spinlocks that do not disable preemption,
as is the case in kernels built with CONFIG_PREEMPT_RT=y.
Sleeplocks do ”…””}”(hjD  h²hh³Nh´NubhŒemphasis”“”)”}”(hŒ*not*”h]”hŒnot”…””}”(hjN  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jL  hjD  ubhŒ' enter RCU read-side critical sections.”…””}”(hjD  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K²hj$  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1j"  h³hÃh´K¦hj  h²hubeh}”(h]”Œrcu-read-lock”ah ]”h"]”Œrcu_read_lock()”ah$]”h&]”uh1hÄhjY  h²hh³hÃh´K¥ubhÅ)”}”(hhh]”(hÊ)”}”(hŒrcu_read_unlock()”h]”hŒrcu_read_unlock()”…””}”(hjw  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhjt  h²hh³hÃh´Kºubj#  )”}”(hXž  void rcu_read_unlock(void);

This temporal primitives is used by a reader to inform the
reclaimer that the reader is exiting an RCU read-side critical
section.  Anything that enables bottom halves, preemption,
or interrupts also exits an RCU read-side critical section.
Releasing a spinlock also exits an RCU read-side critical section.

Note that RCU read-side critical sections may be nested and/or
overlapping.
”h]”(hÚ)”}”(hŒvoid rcu_read_unlock(void);”h]”hŒvoid rcu_read_unlock(void);”…””}”(hj‰  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K»hj…  ubhÚ)”}”(hX3  This temporal primitives is used by a reader to inform the
reclaimer that the reader is exiting an RCU read-side critical
section.  Anything that enables bottom halves, preemption,
or interrupts also exits an RCU read-side critical section.
Releasing a spinlock also exits an RCU read-side critical section.”h]”hX3  This temporal primitives is used by a reader to inform the
reclaimer that the reader is exiting an RCU read-side critical
section.  Anything that enables bottom halves, preemption,
or interrupts also exits an RCU read-side critical section.
Releasing a spinlock also exits an RCU read-side critical section.”…””}”(hj—  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´K½hj…  ubhÚ)”}”(hŒKNote that RCU read-side critical sections may be nested and/or
overlapping.”h]”hŒKNote that RCU read-side critical sections may be nested and/or
overlapping.”…””}”(hj¥  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KÃhj…  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1j"  h³hÃh´K»hjt  h²hubeh}”(h]”Œrcu-read-unlock”ah ]”h"]”Œrcu_read_unlock()”ah$]”h&]”uh1hÄhjY  h²hh³hÃh´KºubhÅ)”}”(hhh]”(hÊ)”}”(hŒsynchronize_rcu()”h]”hŒsynchronize_rcu()”…””}”(hjÄ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhjÁ  h²hh³hÃh´KÇubj#  )”}”(hXº	  void synchronize_rcu(void);

This temporal primitive marks the end of updater code and the
beginning of reclaimer code.  It does this by blocking until
all pre-existing RCU read-side critical sections on all CPUs
have completed.  Note that synchronize_rcu() will **not**
necessarily wait for any subsequent RCU read-side critical
sections to complete.  For example, consider the following
sequence of events::

         CPU 0                  CPU 1                 CPU 2
     ----------------- ------------------------- ---------------
 1.  rcu_read_lock()
 2.                    enters synchronize_rcu()
 3.                                               rcu_read_lock()
 4.  rcu_read_unlock()
 5.                     exits synchronize_rcu()
 6.                                              rcu_read_unlock()

To reiterate, synchronize_rcu() waits only for ongoing RCU
read-side critical sections to complete, not necessarily for
any that begin after synchronize_rcu() is invoked.

Of course, synchronize_rcu() does not necessarily return
**immediately** after the last pre-existing RCU read-side critical
section completes.  For one thing, there might well be scheduling
delays.  For another thing, many RCU implementations process
requests in batches in order to improve efficiencies, which can
further delay synchronize_rcu().

Since synchronize_rcu() is the API that must figure out when
readers are done, its implementation is key to RCU.  For RCU
to be useful in all but the most read-intensive situations,
synchronize_rcu()'s overhead must also be quite small.

The call_rcu() API is an asynchronous callback form of
synchronize_rcu(), and is described in more detail in a later
section.  Instead of blocking, it registers a function and
argument which are invoked after all ongoing RCU read-side
critical sections have completed.  This callback variant is
particularly useful in situations where it is illegal to block
or where update-side performance is critically important.

However, the call_rcu() API should not be used lightly, as use
of the synchronize_rcu() API generally results in simpler code.
In addition, the synchronize_rcu() API has the nice property
of automatically limiting update rate should grace periods
be delayed.  This property results in system resilience in face
of denial-of-service attacks.  Code using call_rcu() should limit
update rate in order to gain this same sort of resilience.  See
checklist.rst for some approaches to limiting the update rate.
”h]”(hÚ)”}”(hŒvoid synchronize_rcu(void);”h]”hŒvoid synchronize_rcu(void);”…””}”(hjÖ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KÈhjÒ  ubhÚ)”}”(hX|  This temporal primitive marks the end of updater code and the
beginning of reclaimer code.  It does this by blocking until
all pre-existing RCU read-side critical sections on all CPUs
have completed.  Note that synchronize_rcu() will **not**
necessarily wait for any subsequent RCU read-side critical
sections to complete.  For example, consider the following
sequence of events::”h]”(hŒêThis temporal primitive marks the end of updater code and the
beginning of reclaimer code.  It does this by blocking until
all pre-existing RCU read-side critical sections on all CPUs
have completed.  Note that synchronize_rcu() will ”…””}”(hjä  h²hh³Nh´NubhŒstrong”“”)”}”(hŒ**not**”h]”hŒnot”…””}”(hjî  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hjä  ubhŒŠ
necessarily wait for any subsequent RCU read-side critical
sections to complete.  For example, consider the following
sequence of events:”…””}”(hjä  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KÊhjÒ  ubhŒliteral_block”“”)”}”(hX…          CPU 0                  CPU 1                 CPU 2
    ----------------- ------------------------- ---------------
1.  rcu_read_lock()
2.                    enters synchronize_rcu()
3.                                               rcu_read_lock()
4.  rcu_read_unlock()
5.                     exits synchronize_rcu()
6.                                              rcu_read_unlock()”h]”hX…          CPU 0                  CPU 1                 CPU 2
    ----------------- ------------------------- ---------------
1.  rcu_read_lock()
2.                    enters synchronize_rcu()
3.                                               rcu_read_lock()
4.  rcu_read_unlock()
5.                     exits synchronize_rcu()
6.                                              rcu_read_unlock()”…””}”hj  sbah}”(h]”h ]”h"]”h$]”h&]”Œ	xml:space”Œpreserve”uh1j  h³hÃh´KÒhjÒ  ubhÚ)”}”(hŒªTo reiterate, synchronize_rcu() waits only for ongoing RCU
read-side critical sections to complete, not necessarily for
any that begin after synchronize_rcu() is invoked.”h]”hŒªTo reiterate, synchronize_rcu() waits only for ongoing RCU
read-side critical sections to complete, not necessarily for
any that begin after synchronize_rcu() is invoked.”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KÛhjÒ  ubhÚ)”}”(hX[  Of course, synchronize_rcu() does not necessarily return
**immediately** after the last pre-existing RCU read-side critical
section completes.  For one thing, there might well be scheduling
delays.  For another thing, many RCU implementations process
requests in batches in order to improve efficiencies, which can
further delay synchronize_rcu().”h]”(hŒ9Of course, synchronize_rcu() does not necessarily return
”…””}”(hj&  h²hh³Nh´Nubjí  )”}”(hŒ**immediately**”h]”hŒimmediately”…””}”(hj.  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hj&  ubhX   after the last pre-existing RCU read-side critical
section completes.  For one thing, there might well be scheduling
delays.  For another thing, many RCU implementations process
requests in batches in order to improve efficiencies, which can
further delay synchronize_rcu().”…””}”(hj&  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KßhjÒ  ubhÚ)”}”(hŒìSince synchronize_rcu() is the API that must figure out when
readers are done, its implementation is key to RCU.  For RCU
to be useful in all but the most read-intensive situations,
synchronize_rcu()'s overhead must also be quite small.”h]”hŒîSince synchronize_rcu() is the API that must figure out when
readers are done, its implementation is key to RCU.  For RCU
to be useful in all but the most read-intensive situations,
synchronize_rcu()â€™s overhead must also be quite small.”…””}”(hjF  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KæhjÒ  ubhÚ)”}”(hXŸ  The call_rcu() API is an asynchronous callback form of
synchronize_rcu(), and is described in more detail in a later
section.  Instead of blocking, it registers a function and
argument which are invoked after all ongoing RCU read-side
critical sections have completed.  This callback variant is
particularly useful in situations where it is illegal to block
or where update-side performance is critically important.”h]”hXŸ  The call_rcu() API is an asynchronous callback form of
synchronize_rcu(), and is described in more detail in a later
section.  Instead of blocking, it registers a function and
argument which are invoked after all ongoing RCU read-side
critical sections have completed.  This callback variant is
particularly useful in situations where it is illegal to block
or where update-side performance is critically important.”…””}”(hjT  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KëhjÒ  ubhÚ)”}”(hX÷  However, the call_rcu() API should not be used lightly, as use
of the synchronize_rcu() API generally results in simpler code.
In addition, the synchronize_rcu() API has the nice property
of automatically limiting update rate should grace periods
be delayed.  This property results in system resilience in face
of denial-of-service attacks.  Code using call_rcu() should limit
update rate in order to gain this same sort of resilience.  See
checklist.rst for some approaches to limiting the update rate.”h]”hX÷  However, the call_rcu() API should not be used lightly, as use
of the synchronize_rcu() API generally results in simpler code.
In addition, the synchronize_rcu() API has the nice property
of automatically limiting update rate should grace periods
be delayed.  This property results in system resilience in face
of denial-of-service attacks.  Code using call_rcu() should limit
update rate in order to gain this same sort of resilience.  See
checklist.rst for some approaches to limiting the update rate.”…””}”(hjb  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´KóhjÒ  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1j"  h³hÃh´KÈhjÁ  h²hubeh}”(h]”Œsynchronize-rcu”ah ]”h"]”Œsynchronize_rcu()”ah$]”h&]”uh1hÄhjY  h²hh³hÃh´KÇubhÅ)”}”(hhh]”(hÊ)”}”(hŒrcu_assign_pointer()”h]”hŒrcu_assign_pointer()”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhj~  h²hh³hÃh´Kýubj#  )”}”(hX¦  void rcu_assign_pointer(p, typeof(p) v);

Yes, rcu_assign_pointer() **is** implemented as a macro, though
it would be cool to be able to declare a function in this manner.
(And there has been some discussion of adding overloaded functions
to the C language, so who knows?)

The updater uses this spatial macro to assign a new value to an
RCU-protected pointer, in order to safely communicate the change
in value from the updater to the reader.  This is a spatial (as
opposed to temporal) macro.  It does not evaluate to an rvalue,
but it does provide any compiler directives and memory-barrier
instructions required for a given compile or CPU architecture.
Its ordering properties are that of a store-release operation,
that is, any prior loads and stores required to initialize the
structure are ordered before the store that publishes the pointer
to that structure.

Perhaps just as important, rcu_assign_pointer() serves to document
(1) which pointers are protected by RCU and (2) the point at which
a given structure becomes accessible to other CPUs.  That said,
rcu_assign_pointer() is most frequently used indirectly, via
the _rcu list-manipulation primitives such as list_add_rcu().
”h]”(hÚ)”}”(hŒ(void rcu_assign_pointer(p, typeof(p) v);”h]”hŒ(void rcu_assign_pointer(p, typeof(p) v);”…””}”(hj“  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Kþhj  ubhÚ)”}”(hŒæYes, rcu_assign_pointer() **is** implemented as a macro, though
it would be cool to be able to declare a function in this manner.
(And there has been some discussion of adding overloaded functions
to the C language, so who knows?)”h]”(hŒYes, rcu_assign_pointer() ”…””}”(hj¡  h²hh³Nh´Nubjí  )”}”(hŒ**is**”h]”hŒis”…””}”(hj©  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hj¡  ubhŒÆ implemented as a macro, though
it would be cool to be able to declare a function in this manner.
(And there has been some discussion of adding overloaded functions
to the C language, so who knows?)”…””}”(hj¡  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M hj  ubhÚ)”}”(hXQ  The updater uses this spatial macro to assign a new value to an
RCU-protected pointer, in order to safely communicate the change
in value from the updater to the reader.  This is a spatial (as
opposed to temporal) macro.  It does not evaluate to an rvalue,
but it does provide any compiler directives and memory-barrier
instructions required for a given compile or CPU architecture.
Its ordering properties are that of a store-release operation,
that is, any prior loads and stores required to initialize the
structure are ordered before the store that publishes the pointer
to that structure.”h]”hXQ  The updater uses this spatial macro to assign a new value to an
RCU-protected pointer, in order to safely communicate the change
in value from the updater to the reader.  This is a spatial (as
opposed to temporal) macro.  It does not evaluate to an rvalue,
but it does provide any compiler directives and memory-barrier
instructions required for a given compile or CPU architecture.
Its ordering properties are that of a store-release operation,
that is, any prior loads and stores required to initialize the
structure are ordered before the store that publishes the pointer
to that structure.”…””}”(hjÁ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj  ubhÚ)”}”(hX@  Perhaps just as important, rcu_assign_pointer() serves to document
(1) which pointers are protected by RCU and (2) the point at which
a given structure becomes accessible to other CPUs.  That said,
rcu_assign_pointer() is most frequently used indirectly, via
the _rcu list-manipulation primitives such as list_add_rcu().”h]”hX@  Perhaps just as important, rcu_assign_pointer() serves to document
(1) which pointers are protected by RCU and (2) the point at which
a given structure becomes accessible to other CPUs.  That said,
rcu_assign_pointer() is most frequently used indirectly, via
the _rcu list-manipulation primitives such as list_add_rcu().”…””}”(hjÏ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1j"  h³hÃh´Kþhj~  h²hubeh}”(h]”Œrcu-assign-pointer”ah ]”h"]”Œrcu_assign_pointer()”ah$]”h&]”uh1hÄhjY  h²hh³hÃh´KýubhÅ)”}”(hhh]”(hÊ)”}”(hŒrcu_dereference()”h]”hŒrcu_dereference()”…””}”(hjî  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhjë  h²hh³hÃh´Mubj#  )”}”(hXX
  typeof(p) rcu_dereference(p);

Like rcu_assign_pointer(), rcu_dereference() must be implemented
as a macro.

The reader uses the spatial rcu_dereference() macro to fetch
an RCU-protected pointer, which returns a value that may
then be safely dereferenced.  Note that rcu_dereference()
does not actually dereference the pointer, instead, it
protects the pointer for later dereferencing.  It also
executes any needed memory-barrier instructions for a given
CPU architecture.  Currently, only Alpha needs memory barriers
within rcu_dereference() -- on other CPUs, it compiles to a
volatile load.  However, no mainstream C compilers respect
address dependencies, so rcu_dereference() uses volatile casts,
which, in combination with the coding guidelines listed in
rcu_dereference.rst, prevent current compilers from breaking
these dependencies.

Common coding practice uses rcu_dereference() to copy an
RCU-protected pointer to a local variable, then dereferences
this local variable, for example as follows::

        p = rcu_dereference(head.next);
        return p->data;

However, in this case, one could just as easily combine these
into one statement::

        return rcu_dereference(head.next)->data;

If you are going to be fetching multiple fields from the
RCU-protected structure, using the local variable is of
course preferred.  Repeated rcu_dereference() calls look
ugly, do not guarantee that the same pointer will be returned
if an update happened while in the critical section, and incur
unnecessary overhead on Alpha CPUs.

Note that the value returned by rcu_dereference() is valid
only within the enclosing RCU read-side critical section [1]_.
For example, the following is **not** legal::

        rcu_read_lock();
        p = rcu_dereference(head.next);
        rcu_read_unlock();
        x = p->address; /* BUG!!! */
        rcu_read_lock();
        y = p->data;    /* BUG!!! */
        rcu_read_unlock();

Holding a reference from one RCU read-side critical section
to another is just as illegal as holding a reference from
one lock-based critical section to another!  Similarly,
using a reference outside of the critical section in which
it was acquired is just as illegal as doing so with normal
locking.

As with rcu_assign_pointer(), an important function of
rcu_dereference() is to document which pointers are protected by
RCU, in particular, flagging a pointer that is subject to changing
at any time, including immediately after the rcu_dereference().
And, again like rcu_assign_pointer(), rcu_dereference() is
typically used indirectly, via the _rcu list-manipulation
primitives, such as list_for_each_entry_rcu() [2]_.
”h]”(hÚ)”}”(hŒtypeof(p) rcu_dereference(p);”h]”hŒtypeof(p) rcu_dereference(p);”…””}”(hj   h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjü  ubhÚ)”}”(hŒLLike rcu_assign_pointer(), rcu_dereference() must be implemented
as a macro.”h]”hŒLLike rcu_assign_pointer(), rcu_dereference() must be implemented
as a macro.”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjü  ubhÚ)”}”(hXÛ  The reader uses the spatial rcu_dereference() macro to fetch
an RCU-protected pointer, which returns a value that may
then be safely dereferenced.  Note that rcu_dereference()
does not actually dereference the pointer, instead, it
protects the pointer for later dereferencing.  It also
executes any needed memory-barrier instructions for a given
CPU architecture.  Currently, only Alpha needs memory barriers
within rcu_dereference() -- on other CPUs, it compiles to a
volatile load.  However, no mainstream C compilers respect
address dependencies, so rcu_dereference() uses volatile casts,
which, in combination with the coding guidelines listed in
rcu_dereference.rst, prevent current compilers from breaking
these dependencies.”h]”hXÛ  The reader uses the spatial rcu_dereference() macro to fetch
an RCU-protected pointer, which returns a value that may
then be safely dereferenced.  Note that rcu_dereference()
does not actually dereference the pointer, instead, it
protects the pointer for later dereferencing.  It also
executes any needed memory-barrier instructions for a given
CPU architecture.  Currently, only Alpha needs memory barriers
within rcu_dereference() -- on other CPUs, it compiles to a
volatile load.  However, no mainstream C compilers respect
address dependencies, so rcu_dereference() uses volatile casts,
which, in combination with the coding guidelines listed in
rcu_dereference.rst, prevent current compilers from breaking
these dependencies.”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjü  ubhÚ)”}”(hŒ£Common coding practice uses rcu_dereference() to copy an
RCU-protected pointer to a local variable, then dereferences
this local variable, for example as follows::”h]”hŒ¢Common coding practice uses rcu_dereference() to copy an
RCU-protected pointer to a local variable, then dereferences
this local variable, for example as follows:”…””}”(hj*  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M+hjü  ubj  )”}”(hŒ/p = rcu_dereference(head.next);
return p->data;”h]”hŒ/p = rcu_dereference(head.next);
return p->data;”…””}”hj8  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M/hjü  ubhÚ)”}”(hŒRHowever, in this case, one could just as easily combine these
into one statement::”h]”hŒQHowever, in this case, one could just as easily combine these
into one statement:”…””}”(hjF  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M2hjü  ubj  )”}”(hŒ(return rcu_dereference(head.next)->data;”h]”hŒ(return rcu_dereference(head.next)->data;”…””}”hjT  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M5hjü  ubhÚ)”}”(hXJ  If you are going to be fetching multiple fields from the
RCU-protected structure, using the local variable is of
course preferred.  Repeated rcu_dereference() calls look
ugly, do not guarantee that the same pointer will be returned
if an update happened while in the critical section, and incur
unnecessary overhead on Alpha CPUs.”h]”hXJ  If you are going to be fetching multiple fields from the
RCU-protected structure, using the local variable is of
course preferred.  Repeated rcu_dereference() calls look
ugly, do not guarantee that the same pointer will be returned
if an update happened while in the critical section, and incur
unnecessary overhead on Alpha CPUs.”…””}”(hjb  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M7hjü  ubhÚ)”}”(hŒ§Note that the value returned by rcu_dereference() is valid
only within the enclosing RCU read-side critical section [1]_.
For example, the following is **not** legal::”h]”(hŒtNote that the value returned by rcu_dereference() is valid
only within the enclosing RCU read-side critical section ”…””}”(hjp  h²hh³Nh´NubhŒfootnote_reference”“”)”}”(hŒ[1]_”h]”hŒ1”…””}”(hjz  h²hh³Nh´Nubah}”(h]”Œid2”ah ]”h"]”h$]”h&]”hÁŒid4”Œdocname”j  uh1jx  hjp  Œresolved”KubhŒ .
For example, the following is ”…””}”(hjp  h²hh³Nh´Nubjí  )”}”(hŒ**not**”h]”hŒnot”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hjp  ubhŒ legal:”…””}”(hjp  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M>hjü  ubj  )”}”(hŒ¡rcu_read_lock();
p = rcu_dereference(head.next);
rcu_read_unlock();
x = p->address; /* BUG!!! */
rcu_read_lock();
y = p->data;    /* BUG!!! */
rcu_read_unlock();”h]”hŒ¡rcu_read_lock();
p = rcu_dereference(head.next);
rcu_read_unlock();
x = p->address; /* BUG!!! */
rcu_read_lock();
y = p->data;    /* BUG!!! */
rcu_read_unlock();”…””}”hj¨  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´MBhjü  ubhÚ)”}”(hX,  Holding a reference from one RCU read-side critical section
to another is just as illegal as holding a reference from
one lock-based critical section to another!  Similarly,
using a reference outside of the critical section in which
it was acquired is just as illegal as doing so with normal
locking.”h]”hX,  Holding a reference from one RCU read-side critical section
to another is just as illegal as holding a reference from
one lock-based critical section to another!  Similarly,
using a reference outside of the critical section in which
it was acquired is just as illegal as doing so with normal
locking.”…””}”(hj¶  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MJhjü  ubhÚ)”}”(hX£  As with rcu_assign_pointer(), an important function of
rcu_dereference() is to document which pointers are protected by
RCU, in particular, flagging a pointer that is subject to changing
at any time, including immediately after the rcu_dereference().
And, again like rcu_assign_pointer(), rcu_dereference() is
typically used indirectly, via the _rcu list-manipulation
primitives, such as list_for_each_entry_rcu() [2]_.”h]”(hXž  As with rcu_assign_pointer(), an important function of
rcu_dereference() is to document which pointers are protected by
RCU, in particular, flagging a pointer that is subject to changing
at any time, including immediately after the rcu_dereference().
And, again like rcu_assign_pointer(), rcu_dereference() is
typically used indirectly, via the _rcu list-manipulation
primitives, such as list_for_each_entry_rcu() ”…””}”(hjÄ  h²hh³Nh´Nubjy  )”}”(hŒ[2]_”h]”hŒ2”…””}”(hjÌ  h²hh³Nh´Nubah}”(h]”Œid3”ah ]”h"]”h$]”h&]”hÁŒid5”jŠ  j  uh1jx  hjÄ  j‹  KubhŒ.”…””}”(hjÄ  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MQhjü  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1j"  h³hÃh´Mhjë  h²hubhŒfootnote”“”)”}”(hXë  The variant rcu_dereference_protected() can be used outside
of an RCU read-side critical section as long as the usage is
protected by locks acquired by the update-side code.  This variant
avoids the lockdep warning that would happen when using (for
example) rcu_dereference() without rcu_read_lock() protection.
Using rcu_dereference_protected() also has the advantage
of permitting compiler optimizations that rcu_dereference()
must prohibit.  The rcu_dereference_protected() variant takes
a lockdep expression to indicate which locks must be acquired
by the caller. If the indicated protection is not provided,
a lockdep splat is emitted.  See Design/Requirements/Requirements.rst
and the API's code comments for more details and example usage.
”h]”(hŒlabel”“”)”}”(hŒ1”h]”hŒ1”…””}”(hjô  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jò  hjî  ubhÚ)”}”(hXê  The variant rcu_dereference_protected() can be used outside
of an RCU read-side critical section as long as the usage is
protected by locks acquired by the update-side code.  This variant
avoids the lockdep warning that would happen when using (for
example) rcu_dereference() without rcu_read_lock() protection.
Using rcu_dereference_protected() also has the advantage
of permitting compiler optimizations that rcu_dereference()
must prohibit.  The rcu_dereference_protected() variant takes
a lockdep expression to indicate which locks must be acquired
by the caller. If the indicated protection is not provided,
a lockdep splat is emitted.  See Design/Requirements/Requirements.rst
and the API's code comments for more details and example usage.”h]”hXì  The variant rcu_dereference_protected() can be used outside
of an RCU read-side critical section as long as the usage is
protected by locks acquired by the update-side code.  This variant
avoids the lockdep warning that would happen when using (for
example) rcu_dereference() without rcu_read_lock() protection.
Using rcu_dereference_protected() also has the advantage
of permitting compiler optimizations that rcu_dereference()
must prohibit.  The rcu_dereference_protected() variant takes
a lockdep expression to indicate which locks must be acquired
by the caller. If the indicated protection is not provided,
a lockdep splat is emitted.  See Design/Requirements/Requirements.rst
and the APIâ€™s code comments for more details and example usage.”…””}”(hj	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MYhjî  ubeh}”(h]”j‰  ah ]”h"]”Œ1”ah$]”h&]”j„  ajŠ  j  uh1jì  h³hÃh´MYhjë  h²hj‹  Kubjí  )”}”(hX  If the list_for_each_entry_rcu() instance might be used by
update-side code as well as by RCU readers, then an additional
lockdep expression can be added to its list of arguments.
For example, given an additional "lock_is_held(&mylock)" argument,
the RCU lockdep code would complain only if this instance was
invoked outside of an RCU read-side critical section and without
the protection of mylock.
”h]”(jó  )”}”(hŒ2”h]”hŒ2”…””}”(hj	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jò  hj	  ubhÚ)”}”(hX  If the list_for_each_entry_rcu() instance might be used by
update-side code as well as by RCU readers, then an additional
lockdep expression can be added to its list of arguments.
For example, given an additional "lock_is_held(&mylock)" argument,
the RCU lockdep code would complain only if this instance was
invoked outside of an RCU read-side critical section and without
the protection of mylock.”h]”hX“  If the list_for_each_entry_rcu() instance might be used by
update-side code as well as by RCU readers, then an additional
lockdep expression can be added to its list of arguments.
For example, given an additional â€œlock_is_held(&mylock)â€ argument,
the RCU lockdep code would complain only if this instance was
invoked outside of an RCU read-side critical section and without
the protection of mylock.”…””}”(hj)	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mfhj	  ubeh}”(h]”jÛ  ah ]”h"]”Œ2”ah$]”h&]”jÖ  ajŠ  j  uh1jì  h³hÃh´Mfhjë  h²hj‹  KubhÚ)”}”(hŒbThe following diagram shows how each API communicates among the
reader, updater, and reclaimer.
::”h]”hŒ_The following diagram shows how each API communicates among the
reader, updater, and reclaimer.”…””}”(hj>	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mnhjë  h²hubj  )”}”(hXç  rcu_assign_pointer()
                        +--------+
+---------------------->| reader |---------+
|                       +--------+         |
|                           |              |
|                           |              | Protect:
|                           |              | rcu_read_lock()
|                           |              | rcu_read_unlock()
|        rcu_dereference()  |              |
+---------+                 |              |
| updater |<----------------+              |
+---------+                                V
|                                    +-----------+
+----------------------------------->| reclaimer |
                                     +-----------+
  Defer:
  synchronize_rcu() & call_rcu()”h]”hXç  rcu_assign_pointer()
                        +--------+
+---------------------->| reader |---------+
|                       +--------+         |
|                           |              |
|                           |              | Protect:
|                           |              | rcu_read_lock()
|                           |              | rcu_read_unlock()
|        rcu_dereference()  |              |
+---------+                 |              |
| updater |<----------------+              |
+---------+                                V
|                                    +-----------+
+----------------------------------->| reclaimer |
                                     +-----------+
  Defer:
  synchronize_rcu() & call_rcu()”…””}”hjL	  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mshjë  h²hubhÚ)”}”(hXI  The RCU infrastructure observes the temporal sequence of rcu_read_lock(),
rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
order to determine when (1) synchronize_rcu() invocations may return
to their callers and (2) call_rcu() callbacks may be invoked.  Efficient
implementations of the RCU infrastructure make heavy use of batching in
order to amortize their overhead over many uses of the corresponding APIs.
The rcu_assign_pointer() and rcu_dereference() invocations communicate
spatial changes via stores to and loads from the RCU-protected pointer in
question.”h]”hXI  The RCU infrastructure observes the temporal sequence of rcu_read_lock(),
rcu_read_unlock(), synchronize_rcu(), and call_rcu() invocations in
order to determine when (1) synchronize_rcu() invocations may return
to their callers and (2) call_rcu() callbacks may be invoked.  Efficient
implementations of the RCU infrastructure make heavy use of batching in
order to amortize their overhead over many uses of the corresponding APIs.
The rcu_assign_pointer() and rcu_dereference() invocations communicate
spatial changes via stores to and loads from the RCU-protected pointer in
question.”…””}”(hjZ	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M†hjë  h²hubhÚ)”}”(hXR  There are at least three flavors of RCU usage in the Linux kernel. The diagram
above shows the most common one. On the updater side, the rcu_assign_pointer(),
synchronize_rcu() and call_rcu() primitives used are the same for all three
flavors. However for protection (on the reader side), the primitives used vary
depending on the flavor:”h]”hXR  There are at least three flavors of RCU usage in the Linux kernel. The diagram
above shows the most common one. On the updater side, the rcu_assign_pointer(),
synchronize_rcu() and call_rcu() primitives used are the same for all three
flavors. However for protection (on the reader side), the primitives used vary
depending on the flavor:”…””}”(hjh	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjë  h²hubj¾  )”}”(hhh]”(jÃ  )”}”(hŒ6rcu_read_lock() / rcu_read_unlock()
rcu_dereference()
”h]”hÚ)”}”(hŒ5rcu_read_lock() / rcu_read_unlock()
rcu_dereference()”h]”hŒ5rcu_read_lock() / rcu_read_unlock()
rcu_dereference()”…””}”(hj}	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M–hjy	  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjv	  h²hh³hÃh´NubjÃ  )”}”(hŒfrcu_read_lock_bh() / rcu_read_unlock_bh()
local_bh_disable() / local_bh_enable()
rcu_dereference_bh()
”h]”hÚ)”}”(hŒercu_read_lock_bh() / rcu_read_unlock_bh()
local_bh_disable() / local_bh_enable()
rcu_dereference_bh()”h]”hŒercu_read_lock_bh() / rcu_read_unlock_bh()
local_bh_disable() / local_bh_enable()
rcu_dereference_bh()”…””}”(hj•	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M™hj‘	  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjv	  h²hh³hÃh´NubjÃ  )”}”(hŒÆrcu_read_lock_sched() / rcu_read_unlock_sched()
preempt_disable() / preempt_enable()
local_irq_save() / local_irq_restore()
hardirq enter / hardirq exit
NMI enter / NMI exit
rcu_dereference_sched()
”h]”hÚ)”}”(hŒÅrcu_read_lock_sched() / rcu_read_unlock_sched()
preempt_disable() / preempt_enable()
local_irq_save() / local_irq_restore()
hardirq enter / hardirq exit
NMI enter / NMI exit
rcu_dereference_sched()”•     h]”hŒÅrcu_read_lock_sched() / rcu_read_unlock_sched()
preempt_disable() / preempt_enable()
local_irq_save() / local_irq_restore()
hardirq enter / hardirq exit
NMI enter / NMI exit
rcu_dereference_sched()”…””}”(hj­	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj©	  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjv	  h²hh³hÃh´Nubeh}”(h]”h ]”h"]”h$]”h&]”j  j  j  hj  j  uh1j½  hjë  h²hh³hÃh´M–ubhÚ)”}”(hŒ(These three flavors are used as follows:”h]”hŒ(These three flavors are used as follows:”…””}”(hjÇ	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M¤hjë  h²hubj¾  )”}”(hhh]”(jÃ  )”}”(hŒ'RCU applied to normal data structures.
”h]”hÚ)”}”(hŒ&RCU applied to normal data structures.”h]”hŒ&RCU applied to normal data structures.”…””}”(hjÜ	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M¦hjØ	  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjÕ	  h²hh³hÃh´NubjÃ  )”}”(hŒeRCU applied to networking data structures that may be subjected
to remote denial-of-service attacks.
”h]”hÚ)”}”(hŒdRCU applied to networking data structures that may be subjected
to remote denial-of-service attacks.”h]”hŒdRCU applied to networking data structures that may be subjected
to remote denial-of-service attacks.”…””}”(hjô	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M¨hjð	  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjÕ	  h²hh³hÃh´NubjÃ  )”}”(hŒ:RCU applied to scheduler and interrupt/NMI-handler tasks.
”h]”hÚ)”}”(hŒ9RCU applied to scheduler and interrupt/NMI-handler tasks.”h]”hŒ9RCU applied to scheduler and interrupt/NMI-handler tasks.”…””}”(hj
  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M«hj
  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjÕ	  h²hh³hÃh´Nubeh}”(h]”h ]”h"]”h$]”h&]”j  j  j  hj  j  uh1j½  hjë  h²hh³hÃh´M¦ubhÚ)”}”(hŒîAgain, most uses will be of (a).  The (b) and (c) cases are important
for specialized uses, but are relatively uncommon.  The SRCU, RCU-Tasks,
RCU-Tasks-Rude, and RCU-Tasks-Trace have similar relationships among
their assorted primitives.”h]”hŒîAgain, most uses will be of (a).  The (b) and (c) cases are important
for specialized uses, but are relatively uncommon.  The SRCU, RCU-Tasks,
RCU-Tasks-Rude, and RCU-Tasks-Trace have similar relationships among
their assorted primitives.”…””}”(hj&
  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M­hjë  h²hubh¶)”}”(hŒ.. _3_whatisRCU:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒid6”uh1hµh´M²hjë  h²hh³hÃubeh}”(h]”Œrcu-dereference”ah ]”h"]”Œrcu_dereference()”ah$]”h&]”uh1hÄhjY  h²hh³hÃh´Mubeh}”(h]”(Œwhat-is-rcu-s-core-api”jK  eh ]”h"]”(Œ2. what is rcu's core api?”Œ2_whatisrcu”eh$]”h&]”uh1hÄhhÆh²hh³hÃh´K’jU  }”jM
  jA  sjW  }”jK  jA  subhÅ)”}”(hhh]”(hÊ)”}”(hŒ/3.  WHAT ARE SOME EXAMPLE USES OF CORE RCU API?”h]”hŒ/3.  WHAT ARE SOME EXAMPLE USES OF CORE RCU API?”…””}”(hjU
  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhjR
  h²hh³hÃh´MµubhÚ)”}”(hŒ¿This section shows a simple use of the core RCU API to protect a
global pointer to a dynamically allocated structure.  More-typical
uses of RCU may be found in listRCU.rst and NMI-RCU.rst.
::”h]”hŒ¼This section shows a simple use of the core RCU API to protect a
global pointer to a dynamically allocated structure.  More-typical
uses of RCU may be found in listRCU.rst and NMI-RCU.rst.”…””}”(hjc
  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M·hjR
  h²hubj  )”}”(hX&  struct foo {
        int a;
        char b;
        long c;
};
DEFINE_SPINLOCK(foo_mutex);

struct foo __rcu *gbl_foo;

/*
 * Create a new struct foo that is the same as the one currently
 * pointed to by gbl_foo, except that field "a" is replaced
 * with "new_a".  Points gbl_foo to the new structure, and
 * frees up the old structure after a grace period.
 *
 * Uses rcu_assign_pointer() to ensure that concurrent readers
 * see the initialized version of the new structure.
 *
 * Uses synchronize_rcu() to ensure that any readers that might
 * have references to the old structure complete before freeing
 * the old structure.
 */
void foo_update_a(int new_a)
{
        struct foo *new_fp;
        struct foo *old_fp;

        new_fp = kmalloc_obj(*new_fp);
        spin_lock(&foo_mutex);
        old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
        *new_fp = *old_fp;
        new_fp->a = new_a;
        rcu_assign_pointer(gbl_foo, new_fp);
        spin_unlock(&foo_mutex);
        synchronize_rcu();
        kfree(old_fp);
}

/*
 * Return the value of field "a" of the current gbl_foo
 * structure.  Use rcu_read_lock() and rcu_read_unlock()
 * to ensure that the structure does not get deleted out
 * from under us, and use rcu_dereference() to ensure that
 * we see the initialized version of the structure (important
 * for DEC Alpha and for people reading the code).
 */
int foo_get_a(void)
{
        int retval;

        rcu_read_lock();
        retval = rcu_dereference(gbl_foo)->a;
        rcu_read_unlock();
        return retval;
}”h]”hX&  struct foo {
        int a;
        char b;
        long c;
};
DEFINE_SPINLOCK(foo_mutex);

struct foo __rcu *gbl_foo;

/*
 * Create a new struct foo that is the same as the one currently
 * pointed to by gbl_foo, except that field "a" is replaced
 * with "new_a".  Points gbl_foo to the new structure, and
 * frees up the old structure after a grace period.
 *
 * Uses rcu_assign_pointer() to ensure that concurrent readers
 * see the initialized version of the new structure.
 *
 * Uses synchronize_rcu() to ensure that any readers that might
 * have references to the old structure complete before freeing
 * the old structure.
 */
void foo_update_a(int new_a)
{
        struct foo *new_fp;
        struct foo *old_fp;

        new_fp = kmalloc_obj(*new_fp);
        spin_lock(&foo_mutex);
        old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
        *new_fp = *old_fp;
        new_fp->a = new_a;
        rcu_assign_pointer(gbl_foo, new_fp);
        spin_unlock(&foo_mutex);
        synchronize_rcu();
        kfree(old_fp);
}

/*
 * Return the value of field "a" of the current gbl_foo
 * structure.  Use rcu_read_lock() and rcu_read_unlock()
 * to ensure that the structure does not get deleted out
 * from under us, and use rcu_dereference() to ensure that
 * we see the initialized version of the structure (important
 * for DEC Alpha and for people reading the code).
 */
int foo_get_a(void)
{
        int retval;

        rcu_read_lock();
        retval = rcu_dereference(gbl_foo)->a;
        rcu_read_unlock();
        return retval;
}”…””}”hjq
  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M¼hjR
  h²hubhÚ)”}”(hŒSo, to sum up:”h]”hŒSo, to sum up:”…””}”(hj
  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MôhjR
  h²hubhŒbullet_list”“”)”}”(hhh]”(jÃ  )”}”(hŒTUse rcu_read_lock() and rcu_read_unlock() to guard RCU
read-side critical sections.
”h]”hÚ)”}”(hŒSUse rcu_read_lock() and rcu_read_unlock() to guard RCU
read-side critical sections.”h]”hŒSUse rcu_read_lock() and rcu_read_unlock() to guard RCU
read-side critical sections.”…””}”(hj–
  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Möhj’
  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj
  h²hh³hÃh´NubjÃ  )”}”(hŒgWithin an RCU read-side critical section, use rcu_dereference()
to dereference RCU-protected pointers.
”h]”hÚ)”}”(hŒfWithin an RCU read-side critical section, use rcu_dereference()
to dereference RCU-protected pointers.”h]”hŒfWithin an RCU read-side critical section, use rcu_dereference()
to dereference RCU-protected pointers.”…””}”(hj®
  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mùhjª
  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj
  h²hh³hÃh´NubjÃ  )”}”(hŒqUse some solid design (such as locks or semaphores) to
keep concurrent updates from interfering with each other.
”h]”hÚ)”}”(hŒpUse some solid design (such as locks or semaphores) to
keep concurrent updates from interfering with each other.”h]”hŒpUse some solid design (such as locks or semaphores) to
keep concurrent updates from interfering with each other.”…””}”(hjÆ
  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MühjÂ
  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj
  h²hh³hÃh´NubjÃ  )”}”(hX;  Use rcu_assign_pointer() to update an RCU-protected pointer.
This primitive protects concurrent readers from the updater,
**not** concurrent updates from each other!  You therefore still
need to use locking (or something similar) to keep concurrent
rcu_assign_pointer() primitives from interfering with each other.
”h]”hÚ)”}”(hX:  Use rcu_assign_pointer() to update an RCU-protected pointer.
This primitive protects concurrent readers from the updater,
**not** concurrent updates from each other!  You therefore still
need to use locking (or something similar) to keep concurrent
rcu_assign_pointer() primitives from interfering with each other.”h]”(hŒzUse rcu_assign_pointer() to update an RCU-protected pointer.
This primitive protects concurrent readers from the updater,
”…””}”(hjÞ
  h²hh³Nh´Nubjí  )”}”(hŒ**not**”h]”hŒnot”…””}”(hjæ
  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hjÞ
  ubhŒ¹ concurrent updates from each other!  You therefore still
need to use locking (or something similar) to keep concurrent
rcu_assign_pointer() primitives from interfering with each other.”…””}”(hjÞ
  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÿhjÚ
  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj
  h²hh³hÃh´NubjÃ  )”}”(hX  Use synchronize_rcu() **after** removing a data element from an
RCU-protected data structure, but **before** reclaiming/freeing
the data element, in order to wait for the completion of all
RCU read-side critical sections that might be referencing that
data item.
”h]”hÚ)”}”(hX  Use synchronize_rcu() **after** removing a data element from an
RCU-protected data structure, but **before** reclaiming/freeing
the data element, in order to wait for the completion of all
RCU read-side critical sections that might be referencing that
data item.”h]”(hŒUse synchronize_rcu() ”…””}”(hj  h²hh³Nh´Nubjí  )”}”(hŒ	**after**”h]”hŒafter”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hj  ubhŒC removing a data element from an
RCU-protected data structure, but ”…””}”(hj  h²hh³Nh´Nubjí  )”}”(hŒ
**before**”h]”hŒbefore”…””}”(hj"  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hj  ubhŒš reclaiming/freeing
the data element, in order to wait for the completion of all
RCU read-side critical sections that might be referencing that
data item.”…””}”(hj  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj
  h²hh³hÃh´Nubeh}”(h]”h ]”h"]”h$]”h&]”Œbullet”Œ-”uh1j
  h³hÃh´MöhjR
  h²hubhÚ)”}”(hŒ‘See checklist.rst for additional rules to follow when using RCU.
And again, more-typical uses of RCU may be found in listRCU.rst
and NMI-RCU.rst.”h]”hŒ‘See checklist.rst for additional rules to follow when using RCU.
And again, more-typical uses of RCU may be found in listRCU.rst
and NMI-RCU.rst.”…””}”(hjH  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MhjR
  h²hubh¶)”}”(hŒ.. _4_whatisRCU:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒid7”uh1hµh´MhjR
  h²hh³hÃubeh}”(h]”(Œ*what-are-some-example-uses-of-core-rcu-api”j>
  eh ]”h"]”(Œ.3. what are some example uses of core rcu api?”Œ3_whatisrcu”eh$]”h&]”uh1hÄhhÆh²hh³hÃh´MµjU  }”jg  j4
  sjW  }”j>
  j4
  subhÅ)”}”(hhh]”(hÊ)”}”(hŒ,4.  WHAT IF MY UPDATING THREAD CANNOT BLOCK?”h]”hŒ,4.  WHAT IF MY UPDATING THREAD CANNOT BLOCK?”…””}”(hjo  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhjl  h²hh³hÃh´MubhÚ)”}”(hŒÊIn the example above, foo_update_a() blocks until a grace period elapses.
This is quite simple, but in some cases one cannot afford to wait so
long -- there might be other high-priority work to be done.”h]”hŒÊIn the example above, foo_update_a() blocks until a grace period elapses.
This is quite simple, but in some cases one cannot afford to wait so
long -- there might be other high-priority work to be done.”…””}”(hj}  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjl  h²hubhÚ)”}”(hŒdIn such cases, one uses call_rcu() rather than synchronize_rcu().
The call_rcu() API is as follows::”h]”hŒcIn such cases, one uses call_rcu() rather than synchronize_rcu().
The call_rcu() API is as follows:”…””}”(hj‹  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjl  h²hubj  )”}”(hŒ:void call_rcu(struct rcu_head *head, rcu_callback_t func);”h]”hŒ:void call_rcu(struct rcu_head *head, rcu_callback_t func);”…””}”hj™  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mhjl  h²hubhÚ)”}”(hX  This function invokes func(head) after a grace period has elapsed.
This invocation might happen from either softirq or process context,
so the function is not permitted to block.  The foo struct needs to
have an rcu_head structure added, perhaps as follows::”h]”hX  This function invokes func(head) after a grace period has elapsed.
This invocation might happen from either softirq or process context,
so the function is not permitted to block.  The foo struct needs to
have an rcu_head structure added, perhaps as follows:”…””}”(hj§  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjl  h²hubj  )”}”(hŒ[struct foo {
        int a;
        char b;
        long c;
        struct rcu_head rcu;
};”h]”hŒ[struct foo {
        int a;
        char b;
        long c;
        struct rcu_head rcu;
};”…””}”hjµ  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M"hjl  h²hubhÚ)”}”(hŒ>The foo_update_a() function might then be written as follows::”h]”hŒ=The foo_update_a() function might then be written as follows:”…””}”(hjÃ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M)hjl  h²hubj  )”}”(hXž  /*
 * Create a new struct foo that is the same as the one currently
 * pointed to by gbl_foo, except that field "a" is replaced
 * with "new_a".  Points gbl_foo to the new structure, and
 * frees up the old structure after a grace period.
 *
 * Uses rcu_assign_pointer() to ensure that concurrent readers
 * see the initialized version of the new structure.
 *
 * Uses call_rcu() to ensure that any readers that might have
 * references to the old structure complete before freeing the
 * old structure.
 */
void foo_update_a(int new_a)
{
        struct foo *new_fp;
        struct foo *old_fp;

        new_fp = kmalloc_obj(*new_fp);
        spin_lock(&foo_mutex);
        old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
        *new_fp = *old_fp;
        new_fp->a = new_a;
        rcu_assign_pointer(gbl_foo, new_fp);
        spin_unlock(&foo_mutex);
        call_rcu(&old_fp->rcu, foo_reclaim);
}”h]”hXž  /*
 * Create a new struct foo that is the same as the one currently
 * pointed to by gbl_foo, except that field "a" is replaced
 * with "new_a".  Points gbl_foo to the new structure, and
 * frees up the old structure after a grace period.
 *
 * Uses rcu_assign_pointer() to ensure that concurrent readers
 * see the initialized version of the new structure.
 *
 * Uses call_rcu() to ensure that any readers that might have
 * references to the old structure complete before freeing the
 * old structure.
 */
void foo_update_a(int new_a)
{
        struct foo *new_fp;
        struct foo *old_fp;

        new_fp = kmalloc_obj(*new_fp);
        spin_lock(&foo_mutex);
        old_fp = rcu_dereference_protected(gbl_foo, lockdep_is_held(&foo_mutex));
        *new_fp = *old_fp;
        new_fp->a = new_a;
        rcu_assign_pointer(gbl_foo, new_fp);
        spin_unlock(&foo_mutex);
        call_rcu(&old_fp->rcu, foo_reclaim);
}”…””}”hjÑ  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M+hjl  h²hubhÚ)”}”(hŒ4The foo_reclaim() function might appear as follows::”h]”hŒ3The foo_reclaim() function might appear as follows:”…””}”(hjß  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MGhjl  h²hubj  )”}”(hŒ–void foo_reclaim(struct rcu_head *rp)
{
        struct foo *fp = container_of(rp, struct foo, rcu);

        foo_cleanup(fp->a);

        kfree(fp);
}”h]”hŒ–void foo_reclaim(struct rcu_head *rp)
{
        struct foo *fp = container_of(rp, struct foo, rcu);

        foo_cleanup(fp->a);

        kfree(fp);
}”…””}”hjí  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´MIhjl  h²hubhÚ)”}”(hŒÂThe container_of() primitive is a macro that, given a pointer into a
struct, the type of the struct, and the pointed-to field within the
struct, returns a pointer to the beginning of the struct.”h]”hŒÂThe container_of() primitive is a macro that, given a pointer into a
struct, the type of the struct, and the pointed-to field within the
struct, returns a pointer to the beginning of the struct.”…””}”(hjû  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MRhjl  h²hubhÚ)”}”(hX&  The use of call_rcu() permits the caller of foo_update_a() to
immediately regain control, without needing to worry further about the
old version of the newly updated element.  It also clearly shows the
RCU distinction between updater, namely foo_update_a(), and reclaimer,
namely foo_reclaim().”h]”hX&  The use of call_rcu() permits the caller of foo_update_a() to
immediately regain control, without needing to worry further about the
old version of the newly updated element.  It also clearly shows the
RCU distinction between updater, namely foo_update_a(), and reclaimer,
namely foo_reclaim().”…””}”(hj	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MVhjl  h²hubhÚ)”}”(hŒ…The summary of advice is the same as for the previous section, except
that we are now using call_rcu() rather than synchronize_rcu():”h]”hŒ…The summary of advice is the same as for the previous section, except
that we are now using call_rcu() rather than synchronize_rcu():”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M\hjl  h²hubjŽ
  )”}”(hhh]”jÃ  )”}”(hŒúUse call_rcu() **after** removing a data element from an
RCU-protected data structure in order to register a callback
function that will be invoked after the completion of all RCU
read-side critical sections that might be referencing that
data item.
”h]”hÚ)”}”(hŒùUse call_rcu() **after** removing a data element from an
RCU-protected data structure in order to register a callback
function that will be invoked after the completion of all RCU
read-side critical sections that might be referencing that
data item.”h]”(hŒUse call_rcu() ”…””}”(hj,  h²hh³Nh´Nubjí  )”}”(hŒ	**after**”h]”hŒafter”…””}”(hj4  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hj,  ubhŒá removing a data element from an
RCU-protected data structure in order to register a callback
function that will be invoked after the completion of all RCU
read-side critical sections that might be referencing that
data item.”…””}”(hj,  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M_hj(  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj%  h²hh³hÃh´Nubah}”(h]”h ]”h"]”h$]”h&]”jF  jG  uh1j
  h³hÃh´M_hjl  h²hubhÚ)”}”(hŒ»If the callback for call_rcu() is not doing anything more than calling
kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
to avoid having to write your own callback::”h]”hŒºIf the callback for call_rcu() is not doing anything more than calling
kfree() on the structure, you can use kfree_rcu() instead of call_rcu()
to avoid having to write your own callback:”…””}”(hjX  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mehjl  h²hubj  )”}”(hŒkfree_rcu(old_fp, rcu);”h]”hŒkfree_rcu(old_fp, rcu);”…””}”hjf  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mihjl  h²hubhÚ)”}”(hŒ|If the occasional sleep is permitted, the single-argument form may
be used, omitting the rcu_head structure from struct foo.”h]”hŒ|If the occasional sleep is permitted, the single-argument form may
be used, omitting the rcu_head structure from struct foo.”…””}”(hjt  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mkhjl  h²hubj#  )”}”(hŒkfree_rcu_mightsleep(old_fp);
”h]”hÚ)”}”(hŒkfree_rcu_mightsleep(old_fp);”h]”hŒkfree_rcu_mightsleep(old_fp);”…””}”(hj†  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mnhj‚  ubah}”(h]”h ]”h"]”h$]”h&]”uh1j"  h³hÃh´Mnhjl  h²hubhÚ)”}”(hŒyThis variant almost never blocks, but might do so by invoking
synchronize_rcu() in response to memory-allocation failure.”h]”hŒyThis variant almost never blocks, but might do so by invoking
synchronize_rcu() in response to memory-allocation failure.”…””}”(hjš  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mphjl  h²hubhÚ)”}”(hŒGAgain, see checklist.rst for additional rules governing the use of RCU.”h]”hŒGAgain, see checklist.rst for additional rules governing the use of RCU.”…””}”(hj¨  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mshjl  h²hubh¶)”}”(hŒ.. _5_whatisRCU:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒid8”uh1hµh´Muhjl  h²hh³hÃubeh}”(h]”(Œ'what-if-my-updating-thread-cannot-block”j`  eh ]”h"]”(Œ+4. what if my updating thread cannot block?”Œ4_whatisrcu”eh$]”h&]”uh1hÄhhÆh²hh³hÃh´MjU  }”jÇ  jV  sjW  }”j`  jV  subhÅ)”}”(hhh]”(hÊ)”}”(hŒ05.  WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?”h]”hŒ05.  WHAT ARE SOME SIMPLE IMPLEMENTATIONS OF RCU?”…””}”(hjÏ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhjÌ  h²hh³hÃh´MxubhÚ)”}”(hX\  One of the nice things about RCU is that it has extremely simple "toy"
implementations that are a good first step towards understanding the
production-quality implementations in the Linux kernel.  This section
presents two such "toy" implementations of RCU, one that is implemented
in terms of familiar locking primitives, and another that more closely
resembles "classic" RCU.  Both are way too simple for real-world use,
lacking both functionality and performance.  However, they are useful
in getting a feel for how RCU works.  See kernel/rcu/update.c for a
production-quality implementation, and see:”h]”hXh  One of the nice things about RCU is that it has extremely simple â€œtoyâ€
implementations that are a good first step towards understanding the
production-quality implementations in the Linux kernel.  This section
presents two such â€œtoyâ€ implementations of RCU, one that is implemented
in terms of familiar locking primitives, and another that more closely
resembles â€œclassicâ€ RCU.  Both are way too simple for real-world use,
lacking both functionality and performance.  However, they are useful
in getting a feel for how RCU works.  See kernel/rcu/update.c for a
production-quality implementation, and see:”…””}”(hjÝ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MzhjÌ  h²hubj#  )”}”(hŒUhttps://docs.google.com/document/d/1X0lThx8OK0ZgLMqVoXiR4ZrGURHrXK6NyLRbeXe3Xac/edit
”h]”hÚ)”}”(hŒThttps://docs.google.com/document/d/1X0lThx8OK0ZgLMqVoXiR4ZrGURHrXK6NyLRbeXe3Xac/edit”h]”hø)”}”(hjñ  h]”hŒThttps://docs.google.com/document/d/1X0lThx8OK0ZgLMqVoXiR4ZrGURHrXK6NyLRbeXe3Xac/edit”…””}”(hjó  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jñ  uh1h÷hjï  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M„hjë  ubah}”(h]”h ]”h"]”h$]”h&]”uh1j"  h³hÃh´M„hjÌ  h²hubhÚ)”}”(hŒÌfor papers describing the Linux kernel RCU implementation.  The OLS'01
and OLS'02 papers are a good introduction, and the dissertation provides
more details on the current implementation as of early 2004.”h]”hŒÐfor papers describing the Linux kernel RCU implementation.  The OLSâ€™01
and OLSâ€™02 papers are a good introduction, and the dissertation provides
more details on the current implementation as of early 2004.”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M†hjÌ  h²hubhÅ)”}”(hhh]”(hÊ)”}”(hŒ%5A.  "TOY" IMPLEMENTATION #1: LOCKING”h]”hŒ)5A.  â€œTOYâ€ IMPLEMENTATION #1: LOCKING”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhj  h²hh³hÃh´MŒubhÚ)”}”(hX×  This section presents a "toy" RCU implementation that is based on
familiar locking primitives.  Its overhead makes it a non-starter for
real-life use, as does its lack of scalability.  It is also unsuitable
for realtime use, since it allows scheduling latency to "bleed" from
one read-side critical section to another.  It also assumes recursive
reader-writer locks:  If you try this with non-recursive locks, and
you allow nested rcu_read_lock() calls, you can deadlock.”h]”hXß  This section presents a â€œtoyâ€ RCU implementation that is based on
familiar locking primitives.  Its overhead makes it a non-starter for
real-life use, as does its lack of scalability.  It is also unsuitable
for realtime use, since it allows scheduling latency to â€œbleedâ€ from
one read-side critical section to another.  It also assumes recursive
reader-writer locks:  If you try this with non-recursive locks, and
you allow nested rcu_read_lock() calls, you can deadlock.”…””}”(hj,  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj  h²hubhÚ)”}”(hŒ]However, it is probably the easiest implementation to relate to, so is
a good starting point.”h]”hŒ]However, it is probably the easiest implementation to relate to, so is
a good starting point.”…””}”(hj:  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M•hj  h²hubhÚ)”}”(hŒIt is extremely simple::”h]”hŒIt is extremely simple:”…””}”(hjH  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M˜hj  h²hubj  )”}”(hX1  static DEFINE_RWLOCK(rcu_gp_mutex);

void rcu_read_lock(void)
{
        read_lock(&rcu_gp_mutex);
}

void rcu_read_unlock(void)
{
        read_unlock(&rcu_gp_mutex);
}

void synchronize_rcu(void)
{
        write_lock(&rcu_gp_mutex);
        smp_mb__after_spinlock();
        write_unlock(&rcu_gp_mutex);
}”h]”hX1  static DEFINE_RWLOCK(rcu_gp_mutex);

void rcu_read_lock(void)
{
        read_lock(&rcu_gp_mutex);
}

void rcu_read_unlock(void)
{
        read_unlock(&rcu_gp_mutex);
}

void synchronize_rcu(void)
{
        write_lock(&rcu_gp_mutex);
        smp_mb__after_spinlock();
        write_unlock(&rcu_gp_mutex);
}”…””}”hjV  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mšhj  h²hubhÚ)”}”(hŒÖ[You can ignore rcu_assign_pointer() and rcu_dereference() without missing
much.  But here are simplified versions anyway.  And whatever you do,
don't forget about them when submitting patches making use of RCU!]::”h]”hŒ×[You can ignore rcu_assign_pointer() and rcu_dereference() without missing
much.  But here are simplified versions anyway.  And whatever you do,
donâ€™t forget about them when submitting patches making use of RCU!]:”…””}”(hjd  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M­hj  h²hubj  )”}”(hŒÁ#define rcu_assign_pointer(p, v) \
({ \
        smp_store_release(&(p), (v)); \
})

#define rcu_dereference(p) \
({ \
        typeof(p) _________p1 = READ_ONCE(p); \
        (_________p1); \
})”h]”hŒÁ#define rcu_assign_pointer(p, v) \
({ \
        smp_store_release(&(p), (v)); \
})

#define rcu_dereference(p) \
({ \
        typeof(p) _________p1 = READ_ONCE(p); \
        (_________p1); \
})”…””}”hjr  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M±hj  h²hubhÚ)”}”(hX]  The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
and release a global reader-writer lock.  The synchronize_rcu()
primitive write-acquires this same lock, then releases it.  This means
that once synchronize_rcu() exits, all RCU read-side critical sections
that were in progress before synchronize_rcu() was called are guaranteed
to have completed -- there is no way that synchronize_rcu() would have
been able to write-acquire the lock otherwise.  The smp_mb__after_spinlock()
promotes synchronize_rcu() to a full memory barrier in compliance with
the "Memory-Barrier Guarantees" listed in:”h]”hXa  The rcu_read_lock() and rcu_read_unlock() primitive read-acquire
and release a global reader-writer lock.  The synchronize_rcu()
primitive write-acquires this same lock, then releases it.  This means
that once synchronize_rcu() exits, all RCU read-side critical sections
that were in progress before synchronize_rcu() was called are guaranteed
to have completed -- there is no way that synchronize_rcu() would have
been able to write-acquire the lock otherwise.  The smp_mb__after_spinlock()
promotes synchronize_rcu() to a full memory barrier in compliance with
the â€œMemory-Barrier Guaranteesâ€ listed in:”…””}”(hj€  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M½hj  h²hubj#  )”}”(hŒ%Design/Requirements/Requirements.rst
”h]”hÚ)”}”(hŒ$Design/Requirements/Requirements.rst”h]”hŒ$Design/Requirements/Requirements.rst”…””}”(hj’  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÇhjŽ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1j"  h³hÃh´MÇhj  h²hubhÚ)”}”(hX‰  It is possible to nest rcu_read_lock(), since reader-writer locks may
be recursively acquired.  Note also that rcu_read_lock() is immune
from deadlock (an important property of RCU).  The reason for this is
that the only thing that can block rcu_read_lock() is a synchronize_rcu().
But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
so there can be no deadlock cycle.”h]”hX‰  It is possible to nest rcu_read_lock(), since reader-writer locks may
be recursively acquired.  Note also that rcu_read_lock() is immune
from deadlock (an important property of RCU).  The reason for this is
that the only thing that can block rcu_read_lock() is a synchronize_rcu().
But synchronize_rcu() does not acquire any locks while holding rcu_gp_mutex,
so there can be no deadlock cycle.”…””}”(hj¦  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÉhj  h²hubh¶)”}”(hŒ.. _quiz_1:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒquiz-1”uh1hµh´MÐhj  h²hh³hÃubhŒdefinition_list”“”)”}”(hhh]”hŒdefinition_list_item”“”)”}”(hŒ¤Quick Quiz #1:
Why is this argument naive?  How could a deadlock
occur when using this algorithm in a real-world Linux
kernel?  How could this deadlock be avoided?
”h]”(hŒterm”“”)”}”(hŒQuick Quiz #1:”h]”hŒQuick Quiz #1:”…””}”(hjÌ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÊ  h³hÃh´MÕhjÆ  ubhŒ
definition”“”)”}”(hhh]”hÚ)”}”(hŒ”Why is this argument naive?  How could a deadlock
occur when using this algorithm in a real-world Linux
kernel?  How could this deadlock be avoided?”h]”hŒ”Why is this argument naive?  How could a deadlock
occur when using this algorithm in a real-world Linux
kernel?  How could this deadlock be avoided?”…””}”(hjß  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÓhjÜ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÚ  hjÆ  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  h³hÃh´MÕhjÁ  ubah}”(h]”j¾  ah ]”h"]”Œquiz_1”ah$]”h&]”uh1j¿  hj  h²hh³hÃh´NjU  }”jý  j´  sjW  }”j¾  j´  subhÚ)”}”(hŒ*:ref:`Answers to Quick Quiz <9_whatisRCU>`”h]”h)”}”(hj  h]”jí  )”}”(hj  h]”hŒAnswers to Quick Quiz”…””}”(hj	  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hj  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”j  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ9_whatisrcu”uh1hh³hÃh´M×hj  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M×hj  h²hubeh}”(h]”Œa-toy-implementation-1-locking”ah ]”h"]”Œ$5a. "toy" implementation #1: locking”ah$]”h&]”uh1hÄhjÌ  h²hh³hÃh´MŒubhÅ)”}”(hhh]”(hÊ)”}”(hŒ"5B.  "TOY" EXAMPLE #2: CLASSIC RCU”h]”hŒ&5B.  â€œTOYâ€ EXAMPLE #2: CLASSIC RCU”…””}”(hj6  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhj3  h²hh³hÃh´MÚubhÚ)”}”(hXn  This section presents a "toy" RCU implementation that is based on
"classic RCU".  It is also short on performance (but only for updates) and
on features such as hotplug CPU and the ability to run in CONFIG_PREEMPTION
kernels.  The definitions of rcu_dereference() and rcu_assign_pointer()
are the same as those shown in the preceding section, so they are omitted.
::”h]”hXs  This section presents a â€œtoyâ€ RCU implementation that is based on
â€œclassic RCUâ€.  It is also short on performance (but only for updates) and
on features such as hotplug CPU and the ability to run in CONFIG_PREEMPTION
kernels.  The definitions of rcu_dereference() and rcu_assign_pointer()
are the same as those shown in the preceding section, so they are omitted.”…””}”(hjD  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÛhj3  h²hubj  )”}”(hŒ®void rcu_read_lock(void) { }

void rcu_read_unlock(void) { }

void synchronize_rcu(void)
{
        int cpu;

        for_each_possible_cpu(cpu)
                run_on(cpu);
}”h]”hŒ®void rcu_read_lock(void) { }

void rcu_read_unlock(void) { }

void synchronize_rcu(void)
{
        int cpu;

        for_each_possible_cpu(cpu)
                run_on(cpu);
}”…””}”hjR  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mâhj3  h²hubhÚ)”}”(hX0  Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
This is the great strength of classic RCU in a non-preemptive kernel:
read-side overhead is precisely zero, at least on non-Alpha CPUs.
And there is absolutely no way that rcu_read_lock() can possibly
participate in a deadlock cycle!”h]”hX0  Note that rcu_read_lock() and rcu_read_unlock() do absolutely nothing.
This is the great strength of classic RCU in a non-preemptive kernel:
read-side overhead is precisely zero, at least on non-Alpha CPUs.
And there is absolutely no way that rcu_read_lock() can possibly
participate in a deadlock cycle!”…””}”(hj`  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mîhj3  h²hubhÚ)”}”(hX€  The implementation of synchronize_rcu() simply schedules itself on each
CPU in turn.  The run_on() primitive can be implemented straightforwardly
in terms of the sched_setaffinity() primitive.  Of course, a somewhat less
"toy" implementation would restore the affinity upon completion rather
than just leaving all tasks running on the last CPU, but when I said
"toy", I meant **toy**!”h]”(hX€  The implementation of synchronize_rcu() simply schedules itself on each
CPU in turn.  The run_on() primitive can be implemented straightforwardly
in terms of the sched_setaffinity() primitive.  Of course, a somewhat less
â€œtoyâ€ implementation would restore the affinity upon completion rather
than just leaving all tasks running on the last CPU, but when I said
â€œtoyâ€, I meant ”…””}”(hjn  h²hh³Nh´Nubjí  )”}”(hŒ**toy**”h]”hŒtoy”…””}”(hjv  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hjn  ubhŒ!”…””}”(hjn  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Môhj3  h²hubhÚ)”}”(hŒ+So how the heck is this supposed to work???”h]”hŒ+So how the heck is this supposed to work???”…””}”(hjŽ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mûhj3  h²hubhÚ)”}”(hXW  Remember that it is illegal to block while in an RCU read-side critical
section.  Therefore, if a given CPU executes a context switch, we know
that it must have completed all preceding RCU read-side critical sections.
Once **all** CPUs have executed a context switch, then **all** preceding
RCU read-side critical sections will have completed.”h]”(hŒßRemember that it is illegal to block while in an RCU read-side critical
section.  Therefore, if a given CPU executes a context switch, we know
that it must have completed all preceding RCU read-side critical sections.
Once ”…””}”(hjœ  h²hh³Nh´Nubjí  )”}”(hŒ**all**”h]”hŒall”…””}”(hj¤  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hjœ  ubhŒ+ CPUs have executed a context switch, then ”…””}”(hjœ  h²hh³Nh´Nubjí  )”}”(hŒ**all**”h]”hŒall”…””}”(hj¶  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hjœ  ubhŒ? preceding
RCU read-side critical sections will have completed.”…””}”(hjœ  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mýhj3  h²hubhÚ)”}”(hX  So, suppose that we remove a data item from its structure and then invoke
synchronize_rcu().  Once synchronize_rcu() returns, we are guaranteed
that there are no RCU read-side critical sections holding a reference
to that data item, so we can safely reclaim it.”h]”hX  So, suppose that we remove a data item from its structure and then invoke
synchronize_rcu().  Once synchronize_rcu() returns, we are guaranteed
that there are no RCU read-side critical sections holding a reference
to that data item, so we can safely reclaim it.”…””}”(hjÎ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj3  h²hubh¶)”}”(hŒ.. _quiz_2:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒquiz-2”uh1hµh´Mhj3  h²hh³hÃubjÀ  )”}”(hhh]”jÅ  )”}”(hŒWQuick Quiz #2:
Give an example where Classic RCU's read-side
overhead is **negative**.
”h]”(jË  )”}”(hŒQuick Quiz #2:”h]”hŒQuick Quiz #2:”…””}”(hjî  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÊ  h³hÃh´Mhjê  ubjÛ  )”}”(hhh]”hÚ)”}”(hŒGGive an example where Classic RCU's read-side
overhead is **negative**.”h]”(hŒ<Give an example where Classic RCUâ€™s read-side
overhead is ”…””}”(hjÿ  h²hh³Nh´Nubjí  )”}”(hŒ**negative**”h]”hŒnegative”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hjÿ  ubhŒ.”…””}”(hjÿ  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjü  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÚ  hjê  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  h³hÃh´Mhjç  ubah}”(h]”jæ  ah ]”h"]”Œquiz_2”ah$]”h&]”uh1j¿  hj3  h²hh³hÃh´NjU  }”j/  jÜ  sjW  }”jæ  jÜ  subhÚ)”}”(hŒ*:ref:`Answers to Quick Quiz <9_whatisRCU>`”h]”h)”}”(hj6  h]”jí  )”}”(hj6  h]”hŒAnswers to Quick Quiz”…””}”(hj;  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hj8  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”jE  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ9_whatisrcu”uh1hh³hÃh´Mhj4  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj3  h²hubh¶)”}”(hŒ.. _quiz_3:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒquiz-3”uh1hµh´Mhj3  h²hh³hÃubjÀ  )”}”(hhh]”jÅ  )”}”(hŒ¡Quick Quiz #3:
If it is illegal to block in an RCU read-side
critical section, what the heck do you do in
CONFIG_PREEMPT_RT, where normal spinlocks can block???
”h]”(jË  )”}”(hŒQuick Quiz #3:”h]”hŒQuick Quiz #3:”…””}”(hjo  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÊ  h³hÃh´Mhjk  ubjÛ  )”}”(hhh]”hÚ)”}”(hŒ‘If it is illegal to block in an RCU read-side
critical section, what the heck do you do in
CONFIG_PREEMPT_RT, where normal spinlocks can block???”h]”hŒ‘If it is illegal to block in an RCU read-side
critical section, what the heck do you do in
CONFIG_PREEMPT_RT, where normal spinlocks can block???”…””}”(hj€  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj}  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÚ  hjk  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  h³hÃh´Mhjh  ubah}”(h]”jg  ah ]”h"]”Œquiz_3”ah$]”h&]”uh1j¿  hj3  h²hh³hÃh´NjU  }”jž  j]  sjW  }”jg  j]  subhÚ)”}”(hŒ*:ref:`Answers to Quick Quiz <9_whatisRCU>`”h]”h)”}”(hj¥  h]”jí  )”}”(hj¥  h]”hŒAnswers to Quick Quiz”…””}”(hjª  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hj§  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”j´  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œ9_whatisrcu”uh1hh³hÃh´Mhj£  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj3  h²hubh¶)”}”(hŒ.. _6_whatisRCU:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒid9”uh1hµh´Mhj3  h²hh³hÃubeh}”(h]”Œb-toy-example-2-classic-rcu”ah ]”h"]”Œ!5b. "toy" example #2: classic rcu”ah$]”h&]”uh1hÄhjÌ  h²hh³hÃh´MÚubeh}”(h]”(Œ+what-are-some-simple-implementations-of-rcu”jÀ  eh ]”h"]”(Œ/5. what are some simple implementations of rcu?”Œ5_whatisrcu”eh$]”h&]”uh1hÄhhÆh²hh³hÃh´MxjU  }”jå  j¶  sjW  }”jÀ  j¶  subhÅ)”}”(hhh]”(hÊ)”}”(hŒ&6.  ANALOGY WITH READER-WRITER LOCKING”h]”hŒ&6.  ANALOGY WITH READER-WRITER LOCKING”…””}”(hjí  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhjê  h²hh³hÃh´MubhÚ)”}”(hŒÏAlthough RCU can be used in many different ways, a very common use of
RCU is analogous to reader-writer locking.  The following unified
diff shows how closely related RCU and reader-writer locking can be.
::”h]”hŒÌAlthough RCU can be used in many different ways, a very common use of
RCU is analogous to reader-writer locking.  The following unified
diff shows how closely related RCU and reader-writer locking can be.”…””}”(hjû  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjê  h²hubj  )”}”(hX  @@ -5,5 +5,5 @@ struct el {
        int data;
        /* Other data fields */
 };
-rwlock_t listmutex;
+spinlock_t listmutex;
 struct el head;

@@ -13,15 +14,15 @@
        struct list_head *lp;
        struct el *p;

-       read_lock(&listmutex);
-       list_for_each_entry(p, head, lp) {
+       rcu_read_lock();
+       list_for_each_entry_rcu(p, head, lp) {
                if (p->key == key) {
                        *result = p->data;
-                       read_unlock(&listmutex);
+                       rcu_read_unlock();
                        return 1;
                }
        }
-       read_unlock(&listmutex);
+       rcu_read_unlock();
        return 0;
 }

@@ -29,15 +30,16 @@
 {
        struct el *p;

-       write_lock(&listmutex);
+       spin_lock(&listmutex);
        list_for_each_entry(p, head, lp) {
                if (p->key == key) {
-                       list_del(&p->list);
-                       write_unlock(&listmutex);
+                       list_del_rcu(&p->list);
+                       spin_unlock(&listmutex);
+                       synchronize_rcu();
                        kfree(p);
                        return 1;
                }
        }
-       write_unlock(&listmutex);
+       spin_unlock(&listmutex);
        return 0;
 }”h]”hX  @@ -5,5 +5,5 @@ struct el {
        int data;
        /* Other data fields */
 };
-rwlock_t listmutex;
+spinlock_t listmutex;
 struct el head;

@@ -13,15 +14,15 @@
        struct list_head *lp;
        struct el *p;

-       read_lock(&listmutex);
-       list_for_each_entry(p, head, lp) {
+       rcu_read_lock();
+       list_for_each_entry_rcu(p, head, lp) {
                if (p->key == key) {
                        *result = p->data;
-                       read_unlock(&listmutex);
+                       rcu_read_unlock();
                        return 1;
                }
        }
-       read_unlock(&listmutex);
+       rcu_read_unlock();
        return 0;
 }

@@ -29,15 +30,16 @@
 {
        struct el *p;

-       write_lock(&listmutex);
+       spin_lock(&listmutex);
        list_for_each_entry(p, head, lp) {
                if (p->key == key) {
-                       list_del(&p->list);
-                       write_unlock(&listmutex);
+                       list_del_rcu(&p->list);
+                       spin_unlock(&listmutex);
+                       synchronize_rcu();
                        kfree(p);
                        return 1;
                }
        }
-       write_unlock(&listmutex);
+       spin_unlock(&listmutex);
        return 0;
 }”…””}”hj	  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M#hjê  h²hubhÚ)”}”(hŒ1Or, for those who prefer a side-by-side listing::”h]”hŒ0Or, for those who prefer a side-by-side listing:”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MUhjê  h²hubj  )”}”(hX  1 struct el {                          1 struct el {
2   struct list_head list;             2   struct list_head list;
3   long key;                          3   long key;
4   spinlock_t mutex;                  4   spinlock_t mutex;
5   int data;                          5   int data;
6   /* Other data fields */            6   /* Other data fields */
7 };                                   7 };
8 rwlock_t listmutex;                  8 spinlock_t listmutex;
9 struct el head;                      9 struct el head;”h]”hX  1 struct el {                          1 struct el {
2   struct list_head list;             2   struct list_head list;
3   long key;                          3   long key;
4   spinlock_t mutex;                  4   spinlock_t mutex;
5   int data;                          5   int data;
6   /* Other data fields */            6   /* Other data fields */
7 };                                   7 };
8 rwlock_t listmutex;                  8 spinlock_t listmutex;
9 struct el head;                      9 struct el head;”…””}”hj%  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´MWhjê  h²hubj  )”}”(hX¯   1 int search(long key, int *result)    1 int search(long key, int *result)
 2 {                                    2 {
 3   struct list_head *lp;              3   struct list_head *lp;
 4   struct el *p;                      4   struct el *p;
 5                                      5
 6   read_lock(&listmutex);             6   rcu_read_lock();
 7   list_for_each_entry(p, head, lp) { 7   list_for_each_entry_rcu(p, head, lp) {
 8     if (p->key == key) {             8     if (p->key == key) {
 9       *result = p->data;             9       *result = p->data;
10       read_unlock(&listmutex);      10       rcu_read_unlock();
11       return 1;                     11       return 1;
12     }                               12     }
13   }                                 13   }
14   read_unlock(&listmutex);          14   rcu_read_unlock();
15   return 0;                         15   return 0;
16 }                                   16 }”h]”hX¯   1 int search(long key, int *result)    1 int search(long key, int *result)
 2 {                                    2 {
 3   struct list_head *lp;              3   struct list_head *lp;
 4   struct el *p;                      4   struct el *p;
 5                                      5
 6   read_lock(&listmutex);             6   rcu_read_lock();
 7   list_for_each_entry(p, head, lp) { 7   list_for_each_entry_rcu(p, head, lp) {
 8     if (p->key == key) {             8     if (p->key == key) {
 9       *result = p->data;             9       *result = p->data;
10       read_unlock(&listmutex);      10       rcu_read_unlock();
11       return 1;                     11       return 1;
12     }                               12     }
13   }                                 13   }
14   read_unlock(&listmutex);          14   rcu_read_unlock();
15   return 0;                         15   return 0;
16 }                                   16 }”…””}”hj3  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mchjê  h²hubj  )”}”(hXð   1 int delete(long key)                 1 int delete(long key)
 2 {                                    2 {
 3   struct el *p;                      3   struct el *p;
 4                                      4
 5   write_lock(&listmutex);            5   spin_lock(&listmutex);
 6   list_for_each_entry(p, head, lp) { 6   list_for_each_entry(p, head, lp) {
 7     if (p->key == key) {             7     if (p->key == key) {
 8       list_del(&p->list);            8       list_del_rcu(&p->list);
 9       write_unlock(&listmutex);      9       spin_unlock(&listmutex);
                                       10       synchronize_rcu();
10       kfree(p);                     11       kfree(p);
11       return 1;                     12       return 1;
12     }                               13     }
13   }                                 14   }
14   write_unlock(&listmutex);         15   spin_unlock(&listmutex);
15   return 0;                         16   return 0;
16 }                                   17 }”h]”hXð   1 int delete(long key)                 1 int delete(long key)
 2 {                                    2 {
 3   struct el *p;                      3   struct el *p;
 4                                      4
 5   write_lock(&listmutex);            5   spin_lock(&listmutex);
 6   list_for_each_entry(p, head, lp) { 6   list_for_each_entry(p, head, lp) {
 7     if (p->key == key) {             7     if (p->key == key) {
 8       list_del(&p->list);            8       list_del_rcu(&p->list);
 9       write_unlock(&listmutex);      9       spin_unlock(&listmutex);
                                       10       synchronize_rcu();
10       kfree(p);                     11       kfree(p);
11       return 1;                     12       return 1;
12     }                               13     }
13   }                                 14   }
14   write_unlock(&listmutex);         15   spin_unlock(&listmutex);
15   return 0;                         16   return 0;
16 }                                   17 }”…””}”hjA  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mvhjê  h²hubhÚ)”}”(hŒåEither way, the differences are quite small.  Read-side locking moves
to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
a reader-writer lock to a simple spinlock, and a synchronize_rcu()
precedes the kfree().”h]”hŒåEither way, the differences are quite small.  Read-side locking moves
to rcu_read_lock() and rcu_read_unlock, update-side locking moves from
a reader-writer lock to a simple spinlock, and a synchronize_rcu()
precedes the kfree().”…””}”(hjO  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mˆhjê  h²hubhÚ)”}”(hXV  However, there is one potential catch: the read-side and update-side
critical sections can now run concurrently.  In many cases, this will
not be a problem, but it is necessary to check carefully regardless.
For example, if multiple independent list updates must be seen as
a single atomic update, converting to RCU will require special care.”h]”hXV  However, there is one potential catch: the read-side and update-side
critical sections can now run concurrently.  In many cases, this will
not be a problem, but it is necessary to check carefully regardless.
For example, if multiple independent list updates must be seen as
a single atomic update, converting to RCU will require special care.”…””}”(hj]  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjê  h²hubhÚ)”}”(hŒýAlso, the presence of synchronize_rcu() means that the RCU version of
delete() can now block.  If this is a problem, there is a callback-based
mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
be used in place of synchronize_rcu().”h]”hŒýAlso, the presence of synchronize_rcu() means that the RCU version of
delete() can now block.  If this is a problem, there is a callback-based
mechanism that never blocks, namely call_rcu() or kfree_rcu(), that can
be used in place of synchronize_rcu().”…””}”(hjk  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M“hjê  h²hubh¶)”}”(hŒ.. _7_whatisRCU:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒid10”uh1hµh´M˜hjê  h²hh³hÃubeh}”(h]”(Œ"analogy-with-reader-writer-locking”jÖ  eh ]”h"]”(Œ%6. analogy with reader-writer locking”Œ6_whatisrcu”eh$]”h&]”uh1hÄhhÆh²hh³hÃh´MjU  }”jŠ  jÌ  sjW  }”jÖ  jÌ  subhÅ)”}”(hhh]”(hÊ)”}”(hŒ#7.  ANALOGY WITH REFERENCE COUNTING”h]”hŒ#7.  ANALOGY WITH REFERENCE COUNTING”…””}”(hj’  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhj  h²hh³hÃh´M›ubhÚ)”}”(hŒáThe reader-writer analogy (illustrated by the previous section) is not
always the best way to think about using RCU.  Another helpful analogy
considers RCU an effective reference count on everything which is
protected by RCU.”h]”hŒáThe reader-writer analogy (illustrated by the previous section) is not
always the best way to think about using RCU.  Another helpful analogy
considers RCU an effective reference count on everything which is
protected by RCU.”…””}”(hj   h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj  h²hubhÚ)”}”(hX'  A reference count typically does not prevent the referenced object's
values from changing, but does prevent changes to type -- particularly the
gross change of type that happens when that object's memory is freed and
re-allocated for some other purpose.  Once a type-safe reference to the
object is obtained, some other mechanism is needed to ensure consistent
access to the data in the object.  This could involve taking a spinlock,
but with RCU the typical approach is to perform reads with SMP-aware
operations such as smp_load_acquire(), to perform updates with atomic
read-modify-write operations, and to provide the necessary ordering.
RCU provides a number of support functions that embed the required
operations and ordering, such as the list_for_each_entry_rcu() macro
used in the previous section.”h]”hX+  A reference count typically does not prevent the referenced objectâ€™s
values from changing, but does prevent changes to type -- particularly the
gross change of type that happens when that objectâ€™s memory is freed and
re-allocated for some other purpose.  Once a type-safe reference to the
object is obtained, some other mechanism is needed to ensure consistent
access to the data in the object.  This could involve taking a spinlock,
but with RCU the typical approach is to perform reads with SMP-aware
operations such as smp_load_acquire(), to perform updates with atomic
read-modify-write operations, and to provide the necessary ordering.
RCU provides a number of support functions that embed the required
operations and ordering, such as the list_for_each_entry_rcu() macro
used in the previous section.”…””}”(hj®  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M¢hj  h²hubhÚ)”}”(hXC  A more focused view of the reference counting behavior is that,
between rcu_read_lock() and rcu_read_unlock(), any reference taken with
rcu_dereference() on a pointer marked as ``__rcu`` can be treated as
though a reference-count on that object has been temporarily increased.
This prevents the object from changing type.  Exactly what this means
will depend on normal expectations of objects of that type, but it
typically includes that spinlocks can still be safely locked, normal
reference counters can be safely manipulated, and ``__rcu`` pointers
can be safely dereferenced.”h]”(hŒ±A more focused view of the reference counting behavior is that,
between rcu_read_lock() and rcu_read_unlock(), any reference taken with
rcu_dereference() on a pointer marked as ”…””}”(hj¼  h²hh³Nh´NubhŒliteral”“”)”}”(hŒ	``__rcu``”h]”hŒ__rcu”…””}”(hjÆ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  hj¼  ubhX[   can be treated as
though a reference-count on that object has been temporarily increased.
This prevents the object from changing type.  Exactly what this means
will depend on normal expectations of objects of that type, but it
typically includes that spinlocks can still be safely locked, normal
reference counters can be safely manipulated, and ”…””}”(hj¼  h²hh³Nh´NubjÅ  )”}”(hŒ	``__rcu``”h]”hŒ__rcu”…””}”(hjØ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  hj¼  ubhŒ% pointers
can be safely dereferenced.”…””}”(hj¼  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M¯hj  h²hubhÚ)”}”(hŒeSome operations that one might expect to see on an object for
which an RCU reference is held include:”h]”hŒeSome operations that one might expect to see on an object for
which an RCU reference is held include:”…””}”(hjð  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M¹hj  h²hubj#  )”}”(hX2  - Copying out data that is guaranteed to be stable by the object's type.
- Using kref_get_unless_zero() or similar to get a longer-term
  reference.  This may fail of course.
- Acquiring a spinlock in the object, and checking if the object still
  is the expected object and if so, manipulating it freely.
”h]”jŽ
  )”}”(hhh]”(jÃ  )”}”(hŒFCopying out data that is guaranteed to be stable by the object's type.”h]”hÚ)”}”(hj  h]”hŒHCopying out data that is guaranteed to be stable by the objectâ€™s type.”…””}”(hj	  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M¼hj  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj  ubjÃ  )”}”(hŒaUsing kref_get_unless_zero() or similar to get a longer-term
reference.  This may fail of course.”h]”hÚ)”}”(hŒaUsing kref_get_unless_zero() or similar to get a longer-term
reference.  This may fail of course.”h]”hŒaUsing kref_get_unless_zero() or similar to get a longer-term
reference.  This may fail of course.”…””}”(hj   h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M½hj  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj  ubjÃ  )”}”(hŒAcquiring a spinlock in the object, and checking if the object still
is the expected object and if so, manipulating it freely.
”h]”hÚ)”}”(hŒ~Acquiring a spinlock in the object, and checking if the object still
is the expected object and if so, manipulating it freely.”h]”hŒ~Acquiring a spinlock in the object, and checking if the object still
is the expected object and if so, manipulating it freely.”…””}”(hj8  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M¿hj4  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj  ubeh}”(h]”h ]”h"]”h$]”h&]”jF  jG  uh1j
  h³hÃh´M¼hjþ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1j"  h³hÃh´M¼hj  h²hubhÚ)”}”(hXM  The understanding that RCU provides a reference that only prevents a
change of type is particularly visible with objects allocated from a
slab cache marked ``SLAB_TYPESAFE_BY_RCU``.  RCU operations may yield a
reference to an object from such a cache that has been concurrently freed
and the memory reallocated to a completely different object, though of
the same type.  In this case RCU doesn't even protect the identity of the
object from changing, only its type.  So the object found may not be the
one expected, but it will be one where it is safe to take a reference
(and then potentially acquiring a spinlock), allowing subsequent code
to check whether the identity matches expectations.  It is tempting
to simply acquire the spinlock without first taking the reference, but
unfortunately any spinlock in a ``SLAB_TYPESAFE_BY_RCU`` object must be
initialized after each and every call to kmem_cache_alloc(), which renders
reference-free spinlock acquisition completely unsafe.  Therefore, when
using ``SLAB_TYPESAFE_BY_RCU``, make proper use of a reference counter.
If using refcount_t, the specialized refcount_{add|inc}_not_zero_acquire()
and refcount_set_release() APIs should be used to ensure correct operation
ordering when verifying object identity and when initializing newly
allocated objects. Acquire fence in refcount_{add|inc}_not_zero_acquire()
ensures that identity checks happen *after* reference count is taken.
refcount_set_release() should be called after a newly allocated object is
fully initialized and release fence ensures that new values are visible
*before* refcount can be successfully taken by other users. Once
refcount_set_release() is called, the object should be considered visible
by other tasks.
(Those willing to initialize their locks in a kmem_cache constructor
may also use locking, including cache-friendly sequence locking.)”h]”(hŒœThe understanding that RCU provides a reference that only prevents a
change of type is particularly visible with objects allocated from a
slab cache marked ”…””}”(hjX  h²hh³Nh´NubjÅ  )”}”(hŒ``SLAB_TYPESAFE_BY_RCU``”h]”hŒSLAB_TYPESAFE_BY_RCU”…””}”(hj`  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  hjX  ubhX{  .  RCU operations may yield a
reference to an object from such a cache that has been concurrently freed
and the memory reallocated to a completely different object, though of
the same type.  In this case RCU doesnâ€™t even protect the identity of the
object from changing, only its type.  So the object found may not be the
one expected, but it will be one where it is safe to take a reference
(and then potentially acquiring a spinlock), allowing subsequent code
to check whether the identity matches expectations.  It is tempting
to simply acquire the spinlock without first taking the reference, but
unfortunately any spinlock in a ”…””}”(hjX  h²hh³Nh´NubjÅ  )”}”(hŒ``SLAB_TYPESAFE_BY_RCU``”h]”hŒSLAB_TYPESAFE_BY_RCU”…””}”(hjr  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  hjX  ubhŒ© object must be
initialized after each and every call to kmem_cache_alloc(), which renders
reference-free spinlock acquisition completely unsafe.  Therefore, when
using ”…””}”(hjX  h²hh³Nh´NubjÅ  )”}”(hŒ``SLAB_TYPESAFE_BY_RCU``”h]”hŒSLAB_TYPESAFE_BY_RCU”…””}”(hj„  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  hjX  ubhXr  , make proper use of a reference counter.
If using refcount_t, the specialized refcount_{add|inc}_not_zero_acquire()
and refcount_set_release() APIs should be used to ensure correct operation
ordering when verifying object identity and when initializing newly
allocated objects. Acquire fence in refcount_{add|inc}_not_zero_acquire()
ensures that identity checks happen ”…””}”(hjX  h²hh³Nh´NubjM  )”}”(hŒ*after*”h]”hŒafter”…””}”(hj–  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jL  hjX  ubhŒ­ reference count is taken.
refcount_set_release() should be called after a newly allocated object is
fully initialized and release fence ensures that new values are visible
”…””}”(hjX  h²hh³Nh´NubjM  )”}”(hŒ*before*”h]”hŒbefore”…””}”(hj¨  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jL  hjX  ubhX   refcount can be successfully taken by other users. Once
refcount_set_release() is called, the object should be considered visible
by other tasks.
(Those willing to initialize their locks in a kmem_cache constructor
may also use locking, including cache-friendly sequence locking.)”…””}”(hjX  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÂhj  h²hubhÚ)”}”(hXq  With traditional reference counting -- such as that implemented by the
kref library in Linux -- there is typically code that runs when the last
reference to an object is dropped.  With kref, this is the function
passed to kref_put().  When RCU is being used, such finalization code
must not be run until all ``__rcu`` pointers referencing the object have
been updated, and then a grace period has passed.  Every remaining
globally visible pointer to the object must be considered to be a
potential counted reference, and the finalization code is typically run
using call_rcu() only after all those pointers have been changed.”h]”(hX4  With traditional reference counting -- such as that implemented by the
kref library in Linux -- there is typically code that runs when the last
reference to an object is dropped.  With kref, this is the function
passed to kref_put().  When RCU is being used, such finalization code
must not be run until all ”…””}”(hjÀ  h²hh³Nh´NubjÅ  )”}”(hŒ	``__rcu``”h]”hŒ__rcu”…””}”(hjÈ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  hjÀ  ubhX4   pointers referencing the object have
been updated, and then a grace period has passed.  Every remaining
globally visible pointer to the object must be considered to be a
potential counted reference, and the finalization code is typically run
using call_rcu() only after all those pointers have been changed.”…””}”(hjÀ  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÞhj  h²hubhÚ)”}”(hX  To see how to choose between these two analogies -- of RCU as a
reader-writer lock and RCU as a reference counting system -- it is useful
to reflect on the scale of the thing being protected.  The reader-writer
lock analogy looks at larger multi-part objects such as a linked list
and shows how RCU can facilitate concurrency while elements are added
to, and removed from, the list.  The reference-count analogy looks at
the individual objects and looks at how they can be accessed safely
within whatever whole they are a part of.”h]”hX  To see how to choose between these two analogies -- of RCU as a
reader-writer lock and RCU as a reference counting system -- it is useful
to reflect on the scale of the thing being protected.  The reader-writer
lock analogy looks at larger multi-part objects such as a linked list
and shows how RCU can facilitate concurrency while elements are added
to, and removed from, the list.  The reference-count analogy looks at
the individual objects and looks at how they can be accessed safely
within whatever whole they are a part of.”…””}”(hjà  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mèhj  h²hubh¶)”}”(hŒ.. _8_whatisRCU:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒid11”uh1hµh´Mñhj  h²hh³hÃubeh}”(h]”(Œanalogy-with-reference-counting”jƒ  eh ]”h"]”(Œ"7. analogy with reference counting”Œ7_whatisrcu”eh$]”h&]”uh1hÄhhÆh²hh³hÃh´M›jU  }”jÿ  jy  sjW  }”jƒ  jy  subhÅ)”}”(hhh]”(hÊ)”}”(hŒ8.  FULL LIST OF RCU APIs”h]”hŒ8.  FULL LIST OF RCU APIs”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhj  h²hh³hÃh´MôubhÚ)”}”(hŒóThe RCU APIs are documented in docbook-format header comments in the
Linux-kernel source code, but it helps to have a full list of the
APIs, since there does not appear to be a way to categorize them
in docbook.  Here is the list, by category.”h]”hŒóThe RCU APIs are documented in docbook-format header comments in the
Linux-kernel source code, but it helps to have a full list of the
APIs, since there does not appear to be a way to categorize them
in docbook.  Here is the list, by category.”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Möhj  h²hubhÚ)”}”(hŒRCU list traversal::”h]”hŒRCU list traversal:”…””}”(hj#  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mûhj  h²hubj  )”}”(hXv  list_entry_rcu
list_entry_lockless
list_first_entry_rcu
list_first_or_null_rcu
list_tail_rcu
list_next_rcu
list_next_or_null_rcu
list_for_each_entry_rcu
list_for_each_entry_continue_rcu
list_for_each_entry_from_rcu
list_for_each_entry_lockless
hlist_first_rcu
hlist_next_rcu
hlist_pprev_rcu
hlist_for_each_entry_rcu
hlist_for_each_entry_rcu_notrace
hlist_for_each_entry_rcu_bh
hlist_for_each_entry_from_rcu
hlist_for_each_entry_continue_rcu
hlist_for_each_entry_continue_rcu_bh
hlist_nulls_first_rcu
hlist_nulls_next_rcu
hlist_nulls_for_each_entry_rcu
hlist_nulls_for_each_entry_safe
hlist_bl_first_rcu
hlist_bl_for_each_entry_rcu”•S¶      h]”hXv  list_entry_rcu
list_entry_lockless
list_first_entry_rcu
list_first_or_null_rcu
list_tail_rcu
list_next_rcu
list_next_or_null_rcu
list_for_each_entry_rcu
list_for_each_entry_continue_rcu
list_for_each_entry_from_rcu
list_for_each_entry_lockless
hlist_first_rcu
hlist_next_rcu
hlist_pprev_rcu
hlist_for_each_entry_rcu
hlist_for_each_entry_rcu_notrace
hlist_for_each_entry_rcu_bh
hlist_for_each_entry_from_rcu
hlist_for_each_entry_continue_rcu
hlist_for_each_entry_continue_rcu_bh
hlist_nulls_first_rcu
hlist_nulls_next_rcu
hlist_nulls_for_each_entry_rcu
hlist_nulls_for_each_entry_safe
hlist_bl_first_rcu
hlist_bl_for_each_entry_rcu”…””}”hj1  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mýhj  h²hubhÚ)”}”(hŒRCU pointer/list update::”h]”hŒRCU pointer/list update:”…””}”(hj?  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj  h²hubj  )”}”(hXð  rcu_assign_pointer
rcu_replace_pointer
INIT_LIST_HEAD_RCU
list_add_rcu
list_add_tail_rcu
list_del_rcu
list_replace_rcu
list_splice_init_rcu
list_splice_tail_init_rcu
hlist_add_behind_rcu
hlist_add_before_rcu
hlist_add_head_rcu
hlist_add_tail_rcu
hlist_del_rcu
hlist_del_init_rcu
hlist_replace_rcu
hlist_nulls_del_init_rcu
hlist_nulls_del_rcu
hlist_nulls_add_head_rcu
hlist_nulls_add_tail_rcu
hlist_nulls_add_fake
hlists_swap_heads_rcu
hlist_bl_add_head_rcu
hlist_bl_del_rcu
hlist_bl_set_first_rcu”h]”hXð  rcu_assign_pointer
rcu_replace_pointer
INIT_LIST_HEAD_RCU
list_add_rcu
list_add_tail_rcu
list_del_rcu
list_replace_rcu
list_splice_init_rcu
list_splice_tail_init_rcu
hlist_add_behind_rcu
hlist_add_before_rcu
hlist_add_head_rcu
hlist_add_tail_rcu
hlist_del_rcu
hlist_del_init_rcu
hlist_replace_rcu
hlist_nulls_del_init_rcu
hlist_nulls_del_rcu
hlist_nulls_add_head_rcu
hlist_nulls_add_tail_rcu
hlist_nulls_add_fake
hlists_swap_heads_rcu
hlist_bl_add_head_rcu
hlist_bl_del_rcu
hlist_bl_set_first_rcu”…””}”hjM  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mhj  h²hubhÚ)”}”(hŒRCU::”h]”hŒRCU:”…””}”(hj[  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M4hj  h²hubj  )”}”(hXñ  Critical sections               Grace period            Barrier

rcu_read_lock                   synchronize_net         rcu_barrier
rcu_read_unlock                 synchronize_rcu
guard(rcu)()                    synchronize_rcu_expedited
scoped_guard(rcu)               synchronize_rcu_mult
rcu_dereference                 call_rcu
rcu_dereference_check           call_rcu_hurry
rcu_dereference_protected       kfree_rcu
rcu_read_lock_held              kvfree_rcu
rcu_read_lock_any_held          kfree_rcu_mightsleep
rcu_pointer_handoff             cond_synchronize_rcu
unrcu_pointer                   cond_synchronize_rcu_full
                                cond_synchronize_rcu_expedited
                                cond_synchronize_rcu_expedited_full
                                get_completed_synchronize_rcu
                                get_completed_synchronize_rcu_full
                                get_state_synchronize_rcu
                                get_state_synchronize_rcu_full
                                poll_state_synchronize_rcu
                                poll_state_synchronize_rcu_full
                                same_state_synchronize_rcu
                                same_state_synchronize_rcu_full
                                start_poll_synchronize_rcu
                                start_poll_synchronize_rcu_full
                                start_poll_synchronize_rcu_expedited
                                start_poll_synchronize_rcu_expedited_full”h]”hXñ  Critical sections               Grace period            Barrier

rcu_read_lock                   synchronize_net         rcu_barrier
rcu_read_unlock                 synchronize_rcu
guard(rcu)()                    synchronize_rcu_expedited
scoped_guard(rcu)               synchronize_rcu_mult
rcu_dereference                 call_rcu
rcu_dereference_check           call_rcu_hurry
rcu_dereference_protected       kfree_rcu
rcu_read_lock_held              kvfree_rcu
rcu_read_lock_any_held          kfree_rcu_mightsleep
rcu_pointer_handoff             cond_synchronize_rcu
unrcu_pointer                   cond_synchronize_rcu_full
                                cond_synchronize_rcu_expedited
                                cond_synchronize_rcu_expedited_full
                                get_completed_synchronize_rcu
                                get_completed_synchronize_rcu_full
                                get_state_synchronize_rcu
                                get_state_synchronize_rcu_full
                                poll_state_synchronize_rcu
                                poll_state_synchronize_rcu_full
                                same_state_synchronize_rcu
                                same_state_synchronize_rcu_full
                                start_poll_synchronize_rcu
                                start_poll_synchronize_rcu_full
                                start_poll_synchronize_rcu_expedited
                                start_poll_synchronize_rcu_expedited_full”…””}”hji  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M6hj  h²hubhÚ)”}”(hŒbh::”h]”hŒbh:”…””}”(hjw  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MRhj  h²hubj  )”}”(hX	  Critical sections       Grace period            Barrier

rcu_read_lock_bh        [Same as RCU]           [Same as RCU]
rcu_read_unlock_bh
[local_bh_disable]
[and friends]
rcu_dereference_bh
rcu_dereference_bh_check
rcu_dereference_bh_protected
rcu_read_lock_bh_held”h]”hX	  Critical sections       Grace period            Barrier

rcu_read_lock_bh        [Same as RCU]           [Same as RCU]
rcu_read_unlock_bh
[local_bh_disable]
[and friends]
rcu_dereference_bh
rcu_dereference_bh_check
rcu_dereference_bh_protected
rcu_read_lock_bh_held”…””}”hj…  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´MThj  h²hubhÚ)”}”(hŒsched::”h]”hŒsched:”…””}”(hj“  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M_hj  h²hubj  )”}”(hXQ  Critical sections       Grace period            Barrier

rcu_read_lock_sched     [Same as RCU]           [Same as RCU]
rcu_read_unlock_sched
[preempt_disable]
[and friends]
rcu_read_lock_sched_notrace
rcu_read_unlock_sched_notrace
rcu_dereference_sched
rcu_dereference_sched_check
rcu_dereference_sched_protected
rcu_read_lock_sched_held”h]”hXQ  Critical sections       Grace period            Barrier

rcu_read_lock_sched     [Same as RCU]           [Same as RCU]
rcu_read_unlock_sched
[preempt_disable]
[and friends]
rcu_read_lock_sched_notrace
rcu_read_unlock_sched_notrace
rcu_dereference_sched
rcu_dereference_sched_check
rcu_dereference_sched_protected
rcu_read_lock_sched_held”…””}”hj¡  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mahj  h²hubhÚ)”}”(hŒ&RCU: Initialization/cleanup/ordering::”h]”hŒ%RCU: Initialization/cleanup/ordering:”…””}”(hj¯  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mohj  h²hubj  )”}”(hŒRCU_INIT_POINTER
RCU_INITIALIZER
RCU_POINTER_INITIALIZER
init_rcu_head
destroy_rcu_head
init_rcu_head_on_stack
destroy_rcu_head_on_stack
SLAB_TYPESAFE_BY_RCU”h]”hŒRCU_INIT_POINTER
RCU_INITIALIZER
RCU_POINTER_INITIALIZER
init_rcu_head
destroy_rcu_head
init_rcu_head_on_stack
destroy_rcu_head_on_stack
SLAB_TYPESAFE_BY_RCU”…””}”hj½  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mqhj  h²hubhÚ)”}”(hŒ$RCU: Quiescents states and control::”h]”hŒ#RCU: Quiescents states and control:”…””}”(hjË  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M{hj  h²hubj  )”}”(hŒÄcond_resched_tasks_rcu_qs
rcu_all_qs
rcu_softirq_qs_periodic
rcu_end_inkernel_boot
rcu_expedite_gp
rcu_gp_is_expedited
rcu_unexpedite_gp
rcu_cpu_stall_reset
rcu_head_after_call_rcu
rcu_is_watching”h]”hŒÄcond_resched_tasks_rcu_qs
rcu_all_qs
rcu_softirq_qs_periodic
rcu_end_inkernel_boot
rcu_expedite_gp
rcu_gp_is_expedited
rcu_unexpedite_gp
rcu_cpu_stall_reset
rcu_head_after_call_rcu
rcu_is_watching”…””}”hjÙ  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M}hj  h²hubhÚ)”}”(hŒRCU-sync primitive::”h]”hŒRCU-sync primitive:”…””}”(hjç  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M‰hj  h²hubj  )”}”(hŒIrcu_sync_is_idle
rcu_sync_init
rcu_sync_enter
rcu_sync_exit
rcu_sync_dtor”h]”hŒIrcu_sync_is_idle
rcu_sync_init
rcu_sync_enter
rcu_sync_exit
rcu_sync_dtor”…””}”hjõ  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M‹hj  h²hubhÚ)”}”(hŒRCU-Tasks::”h]”hŒ
RCU-Tasks:”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M’hj  h²hubj  )”}”(hŒ¸Critical sections       Grace period                    Barrier

N/A                     call_rcu_tasks                  rcu_barrier_tasks
                        synchronize_rcu_tasks”h]”hŒ¸Critical sections       Grace period                    Barrier

N/A                     call_rcu_tasks                  rcu_barrier_tasks
                        synchronize_rcu_tasks”…””}”hj  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M”hj  h²hubhÚ)”}”(hŒRCU-Tasks-Rude::”h]”hŒRCU-Tasks-Rude:”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mšhj  h²hubj  )”}”(hŒ»Critical sections       Grace period                    Barrier

N/A                     synchronize_rcu_tasks_rude      rcu_barrier_tasks_rude
                        call_rcu_tasks_rude”h]”hŒ»Critical sections       Grace period                    Barrier

N/A                     synchronize_rcu_tasks_rude      rcu_barrier_tasks_rude
                        call_rcu_tasks_rude”…””}”hj-  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Mœhj  h²hubhÚ)”}”(hŒRCU-Tasks-Trace::”h]”hŒRCU-Tasks-Trace:”…””}”(hj;  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M¢hj  h²hubj  )”}”(hŒûCritical sections       Grace period                    Barrier

rcu_read_lock_trace     call_rcu_tasks_trace            rcu_barrier_tasks_trace
rcu_read_unlock_trace   synchronize_rcu_tasks_trace
guard(rcu_tasks_trace)()
scoped_guard(rcu_tasks_trace)”h]”hŒûCritical sections       Grace period                    Barrier

rcu_read_lock_trace     call_rcu_tasks_trace            rcu_barrier_tasks_trace
rcu_read_unlock_trace   synchronize_rcu_tasks_trace
guard(rcu_tasks_trace)()
scoped_guard(rcu_tasks_trace)”…””}”hjI  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M¤hj  h²hubjÀ  )”}”(hhh]”jÅ  )”}”(hŒJSRCU list traversal::
list_for_each_entry_srcu
hlist_for_each_entry_srcu

”h]”(jË  )”}”(hŒSRCU list traversal::”h]”hŒSRCU list traversal::”…””}”(hj^  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÊ  h³hÃh´M¯hjZ  ubjÛ  )”}”(hhh]”hÚ)”}”(hŒ2list_for_each_entry_srcu
hlist_for_each_entry_srcu”h]”hŒ2list_for_each_entry_srcu
hlist_for_each_entry_srcu”…””}”(hjo  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M­hjl  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÚ  hjZ  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  h³hÃh´M¯hjW  ubah}”(h]”h ]”h"]”h$]”h&]”uh1j¿  hj  h²hh³hÃh´NubhÚ)”}”(hŒSRCU::”h]”hŒSRCU:”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M±hj  h²hubj  )”}”(hXÏ  Critical sections               Grace period            Barrier

srcu_read_lock                  call_srcu               srcu_barrier
srcu_read_unlock                synchronize_srcu
srcu_read_lock_fast             synchronize_srcu_expedited
srcu_read_unlock_fast           get_state_synchronize_srcu
srcu_read_lock_nmisafe          start_poll_synchronize_srcu
srcu_read_unlock_nmisafe        start_poll_synchronize_srcu_expedited
srcu_read_lock_notrace          poll_state_synchronize_srcu
srcu_read_unlock_notrace
srcu_down_read
srcu_up_read
srcu_down_read_fast
srcu_up_read_fast
guard(srcu)()
scoped_guard(srcu)
srcu_read_lock_held
srcu_dereference
srcu_dereference_check
srcu_dereference_notrace
srcu_read_lock_held”h]”hXÏ  Critical sections               Grace period            Barrier

srcu_read_lock                  call_srcu               srcu_barrier
srcu_read_unlock                synchronize_srcu
srcu_read_lock_fast             synchronize_srcu_expedited
srcu_read_unlock_fast           get_state_synchronize_srcu
srcu_read_lock_nmisafe          start_poll_synchronize_srcu
srcu_read_unlock_nmisafe        start_poll_synchronize_srcu_expedited
srcu_read_lock_notrace          poll_state_synchronize_srcu
srcu_read_unlock_notrace
srcu_down_read
srcu_up_read
srcu_down_read_fast
srcu_up_read_fast
guard(srcu)()
scoped_guard(srcu)
srcu_read_lock_held
srcu_dereference
srcu_dereference_check
srcu_dereference_notrace
srcu_read_lock_held”…””}”hj  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M³hj  h²hubhÚ)”}”(hŒ'SRCU: Initialization/cleanup/ordering::”h]”hŒ&SRCU: Initialization/cleanup/ordering:”…””}”(hj«  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÊhj  h²hubj  )”}”(hŒúDEFINE_SRCU
DEFINE_STATIC_SRCU
DEFINE_SRCU_FAST        // for srcu_read_lock_fast() and friends
DEFINE_STATIC_SRCU_FAST // for srcu_read_lock_fast() and friends
init_srcu_struct
init_srcu_struct_fast
cleanup_srcu_struct
smp_mb__after_srcu_read_unlock”h]”hŒúDEFINE_SRCU
DEFINE_STATIC_SRCU
DEFINE_SRCU_FAST        // for srcu_read_lock_fast() and friends
DEFINE_STATIC_SRCU_FAST // for srcu_read_lock_fast() and friends
init_srcu_struct
init_srcu_struct_fast
cleanup_srcu_struct
smp_mb__after_srcu_read_unlock”…””}”hj¹  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´MÌhj  h²hubhÚ)”}”(hŒ'All: lockdep-checked RCU utility APIs::”h]”hŒ&All: lockdep-checked RCU utility APIs:”…””}”(hjÇ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÕhj  h²hubj  )”}”(hŒ RCU_LOCKDEP_WARN
rcu_sleep_check”h]”hŒ RCU_LOCKDEP_WARN
rcu_sleep_check”…””}”hjÕ  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´M×hj  h²hubhÚ)”}”(hŒ-All: Unchecked RCU-protected pointer access::”h]”hŒ,All: Unchecked RCU-protected pointer access:”…””}”(hjã  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÚhj  h²hubj  )”}”(hŒrcu_dereference_raw”h]”hŒrcu_dereference_raw”…””}”hjñ  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´MÜhj  h²hubhÚ)”}”(hŒKAll: Unchecked RCU-protected pointer access with dereferencing prohibited::”h]”hŒJAll: Unchecked RCU-protected pointer access with dereferencing prohibited:”…””}”(hjÿ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MÞhj  h²hubj  )”}”(hŒrcu_access_pointer”h]”hŒrcu_access_pointer”…””}”hj  sbah}”(h]”h ]”h"]”h$]”h&]”j  j  uh1j  h³hÃh´Màhj  h²hubhÚ)”}”(hŒeSee the comment headers in the source code (or the docbook generated
from them) for more information.”h]”hŒeSee the comment headers in the source code (or the docbook generated
from them) for more information.”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mâhj  h²hubhÚ)”}”(hŒ¢However, given that there are no fewer than four families of RCU APIs
in the Linux kernel, how do you choose which one to use?  The following
list can be helpful:”h]”hŒ¢However, given that there are no fewer than four families of RCU APIs
in the Linux kernel, how do you choose which one to use?  The following
list can be helpful:”…””}”(hj)  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Måhj  h²hubj¾  )”}”(hhh]”(jÃ  )”}”(hŒ3Will readers need to block?  If so, you need SRCU.
”h]”hÚ)”}”(hŒ2Will readers need to block?  If so, you need SRCU.”h]”hŒ2Will readers need to block?  If so, you need SRCU.”…””}”(hj>  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Méhj:  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj7  h²hh³hÃh´NubjÃ  )”}”(hŒ–Will readers need to block and are you doing tracing, for
example, ftrace or BPF?  If so, you need RCU-tasks,
RCU-tasks-rude, and/or RCU-tasks-trace.
”h]”hÚ)”}”(hŒ•Will readers need to block and are you doing tracing, for
example, ftrace or BPF?  If so, you need RCU-tasks,
RCU-tasks-rude, and/or RCU-tasks-trace.”h]”hŒ•Will readers need to block and are you doing tracing, for
example, ftrace or BPF?  If so, you need RCU-tasks,
RCU-tasks-rude, and/or RCU-tasks-trace.”…””}”(hjV  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MëhjR  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj7  h²hh³hÃh´NubjÃ  )”}”(hX#  What about the -rt patchset?  If readers would need to block in
an non-rt kernel, you need SRCU.  If readers would block when
acquiring spinlocks in a -rt kernel, but not in a non-rt kernel,
SRCU is not necessary.  (The -rt patchset turns spinlocks into
sleeplocks, hence this distinction.)
”h]”hÚ)”}”(hX"  What about the -rt patchset?  If readers would need to block in
an non-rt kernel, you need SRCU.  If readers would block when
acquiring spinlocks in a -rt kernel, but not in a non-rt kernel,
SRCU is not necessary.  (The -rt patchset turns spinlocks into
sleeplocks, hence this distinction.)”h]”hX"  What about the -rt patchset?  If readers would need to block in
an non-rt kernel, you need SRCU.  If readers would block when
acquiring spinlocks in a -rt kernel, but not in a non-rt kernel,
SRCU is not necessary.  (The -rt patchset turns spinlocks into
sleeplocks, hence this distinction.)”…””}”(hjn  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mïhjj  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj7  h²hh³hÃh´NubjÃ  )”}”(hXs  Do you need to treat NMI handlers, hardirq handlers,
and code segments with preemption disabled (whether
via preempt_disable(), local_irq_save(), local_bh_disable(),
or some other mechanism) as if they were explicit RCU readers?
If so, RCU-sched readers are the only choice that will work
for you, but since about v4.20 you use can use the vanilla RCU
update primitives.
”h]”hÚ)”}”(hXr  Do you need to treat NMI handlers, hardirq handlers,
and code segments with preemption disabled (whether
via preempt_disable(), local_irq_save(), local_bh_disable(),
or some other mechanism) as if they were explicit RCU readers?
If so, RCU-sched readers are the only choice that will work
for you, but since about v4.20 you use can use the vanilla RCU
update primitives.”h]”hXr  Do you need to treat NMI handlers, hardirq handlers,
and code segments with preemption disabled (whether
via preempt_disable(), local_irq_save(), local_bh_disable(),
or some other mechanism) as if they were explicit RCU readers?
If so, RCU-sched readers are the only choice that will work
for you, but since about v4.20 you use can use the vanilla RCU
update primitives.”…””}”(hj†  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mõhj‚  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj7  h²hh³hÃh´NubjÃ  )”}”(hXg  Do you need RCU grace periods to complete even in the face of
softirq monopolization of one or more of the CPUs?  For example,
is your code subject to network-based denial-of-service attacks?
If so, you should disable softirq across your readers, for
example, by using rcu_read_lock_bh().  Since about v4.20 you
use can use the vanilla RCU update primitives.
”h]”hÚ)”}”(hXf  Do you need RCU grace periods to complete even in the face of
softirq monopolization of one or more of the CPUs?  For example,
is your code subject to network-based denial-of-service attacks?
If so, you should disable softirq across your readers, for
example, by using rcu_read_lock_bh().  Since about v4.20 you
use can use the vanilla RCU update primitives.”h]”hXf  Do you need RCU grace periods to complete even in the face of
softirq monopolization of one or more of the CPUs?  For example,
is your code subject to network-based denial-of-service attacks?
If so, you should disable softirq across your readers, for
example, by using rcu_read_lock_bh().  Since about v4.20 you
use can use the vanilla RCU update primitives.”…””}”(hjž  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mýhjš  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj7  h²hh³hÃh´NubjÃ  )”}”(hŒäIs your workload too update-intensive for normal use of
RCU, but inappropriate for other synchronization mechanisms?
If so, consider SLAB_TYPESAFE_BY_RCU (which was originally
named SLAB_DESTROY_BY_RCU).  But please be careful!
”h]”hÚ)”}”(hŒãIs your workload too update-intensive for normal use of
RCU, but inappropriate for other synchronization mechanisms?
If so, consider SLAB_TYPESAFE_BY_RCU (which was originally
named SLAB_DESTROY_BY_RCU).  But please be careful!”h]”hŒãIs your workload too update-intensive for normal use of
RCU, but inappropriate for other synchronization mechanisms?
If so, consider SLAB_TYPESAFE_BY_RCU (which was originally
named SLAB_DESTROY_BY_RCU).  But please be careful!”…””}”(hj¶  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj²  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj7  h²hh³hÃh´NubjÃ  )”}”(hX7  Do you need read-side critical sections that are respected even
on CPUs that are deep in the idle loop, during entry to or exit
from user-mode execution, or on an offlined CPU?  If so, SRCU
and RCU Tasks Trace are the only choices that will work for you,
with SRCU being strongly preferred in almost all cases.
”h]”hÚ)”}”(hX6  Do you need read-side critical sections that are respected even
on CPUs that are deep in the idle loop, during entry to or exit
from user-mode execution, or on an offlined CPU?  If so, SRCU
and RCU Tasks Trace are the only choices that will work for you,
with SRCU being strongly preferred in almost all cases.”h]”hX6  Do you need read-side critical sections that are respected even
on CPUs that are deep in the idle loop, during entry to or exit
from user-mode execution, or on an offlined CPU?  If so, SRCU
and RCU Tasks Trace are the only choices that will work for you,
with SRCU being strongly preferred in almost all cases.”…””}”(hjÎ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M	hjÊ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj7  h²hh³hÃh´NubjÃ  )”}”(hŒOtherwise, use RCU.
”h]”hÚ)”}”(hŒOtherwise, use RCU.”h]”hŒOtherwise, use RCU.”…””}”(hjæ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhjâ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hj7  h²hh³hÃh´Nubeh}”(h]”h ]”h"]”h$]”h&]”j  j  j  hj  j  uh1j½  hj  h²hh³hÃh´MéubhÚ)”}”(hŒeOf course, this all assumes that you have determined that RCU is in fact
the right tool for your job.”h]”hŒeOf course, this all assumes that you have determined that RCU is in fact
the right tool for your job.”…””}”(hj   h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj  h²hubh¶)”}”(hŒ.. _9_whatisRCU:”h]”h}”(h]”h ]”h"]”h$]”h&]”hÁŒid12”uh1hµh´Mhj  h²hh³hÃubeh}”(h]”(Œfull-list-of-rcu-apis”jø  eh ]”h"]”(Œ8. full list of rcu apis”Œ8_whatisrcu”eh$]”h&]”uh1hÄhhÆh²hh³hÃh´MôjU  }”j  jî  sjW  }”jø  jî  subhÅ)”}”(hhh]”(hÊ)”}”(hŒ9.  ANSWERS TO QUICK QUIZZES”h]”hŒ9.  ANSWERS TO QUICK QUIZZES”…””}”(hj'  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÉhj$  h²hh³hÃh´MubjÀ  )”}”(hhh]”(jÅ  )”}”(hŒ³Quick Quiz #1:
Why is this argument naive?  How could a deadlock
occur when using this algorithm in a real-world Linux
kernel?  [Referring to the lock-based "toy" RCU
algorithm.]
”h]”(jË  )”}”(hŒQuick Quiz #1:”h]”hŒQuick Quiz #1:”…””}”(hj<  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÊ  h³hÃh´Mhj8  ubjÛ  )”}”(hhh]”hÚ)”}”(hŒ£Why is this argument naive?  How could a deadlock
occur when using this algorithm in a real-world Linux
kernel?  [Referring to the lock-based "toy" RCU
algorithm.]”h]”hŒ§Why is this argument naive?  How could a deadlock
occur when using this algorithm in a real-world Linux
kernel?  [Referring to the lock-based â€œtoyâ€ RCU
algorithm.]”…””}”(hjM  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MhjJ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÚ  hj8  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  h³hÃh´Mhj5  ubjÅ  )”}”(hX  Answer:
Consider the following sequence of events:

1.      CPU 0 acquires some unrelated lock, call it
        "problematic_lock", disabling irq via
        spin_lock_irqsave().

2.      CPU 1 enters synchronize_rcu(), write-acquiring
        rcu_gp_mutex.

3.      CPU 0 enters rcu_read_lock(), but must wait
        because CPU 1 holds rcu_gp_mutex.

4.      CPU 1 is interrupted, and the irq handler
        attempts to acquire problematic_lock.

The system is now deadlocked.

One way to avoid this deadlock is to use an approach like
that of CONFIG_PREEMPT_RT, where all normal spinlocks
become blocking locks, and all irq handlers execute in
the context of special tasks.  In this case, in step 4
above, the irq handler would block, allowing CPU 1 to
release rcu_gp_mutex, avoiding the deadlock.

Even in the absence of deadlock, this RCU implementation
allows latency to "bleed" from readers to other
readers through synchronize_rcu().  To see this,
consider task A in an RCU read-side critical section
(thus read-holding rcu_gp_mutex), task B blocked
attempting to write-acquire rcu_gp_mutex, and
task C blocked in rcu_read_lock() attempting to
read_acquire rcu_gp_mutex.  Task A's RCU read-side
latency is holding up task C, albeit indirectly via
task B.

Realtime RCU implementations therefore use a counter-based
approach where tasks in RCU read-side critical sections
cannot be blocked by tasks executing synchronize_rcu().
”h]”(jË  )”}”(hŒAnswer:”h]”hŒAnswer:”…””}”(hjk  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÊ  h³hÃh´MEhjg  ubjÛ  )”}”(hhh]”(hÚ)”}”(hŒ*Consider the following sequence of events:”h]”hŒ*Consider the following sequence of events:”…””}”(hj|  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M hjy  ubj¾  )”}”(hhh]”(jÃ  )”}”(hŒgCPU 0 acquires some unrelated lock, call it
"problematic_lock", disabling irq via
spin_lock_irqsave().
”h]”hÚ)”}”(hŒfCPU 0 acquires some unrelated lock, call it
"problematic_lock", disabling irq via
spin_lock_irqsave().”h]”hŒjCPU 0 acquires some unrelated lock, call it
â€œproblematic_lockâ€, disabling irq via
spin_lock_irqsave().”…””}”(hj‘  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M"hj  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjŠ  ubjÃ  )”}”(hŒ>CPU 1 enters synchronize_rcu(), write-acquiring
rcu_gp_mutex.
”h]”hÚ)”}”(hŒ=CPU 1 enters synchronize_rcu(), write-acquiring
rcu_gp_mutex.”h]”hŒ=CPU 1 enters synchronize_rcu(), write-acquiring
rcu_gp_mutex.”…””}”(hj©  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M&hj¥  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjŠ  ubjÃ  )”}”(hŒNCPU 0 enters rcu_read_lock(), but must wait
because CPU 1 holds rcu_gp_mutex.
”h]”hÚ)”}”(hŒMCPU 0 enters rcu_read_lock(), but must wait
because CPU 1 holds rcu_gp_mutex.”h]”hŒMCPU 0 enters rcu_read_lock(), but must wait
because CPU 1 holds rcu_gp_mutex.”…””}”(hjÁ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M)hj½  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjŠ  ubjÃ  )”}”(hŒPCPU 1 is interrupted, and the irq handler
attempts to acquire problematic_lock.
”h]”hÚ)”}”(hŒOCPU 1 is interrupted, and the irq handler
attempts to acquire problematic_lock.”h]”hŒOCPU 1 is interrupted, and the irq handler
attempts to acquire problematic_lock.”…””}”(hjÙ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M,hjÕ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÂ  hjŠ  ubeh}”(h]”h ]”h"]”h$]”h&]”j  Œarabic”j  hj  j  uh1j½  hjy  ubhÚ)”}”(hŒThe system is now deadlocked.”h]”hŒThe system is now deadlocked.”…””}”(hjô  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M/hjy  ubhÚ)”}”(hX@  One way to avoid this deadlock is to use an approach like
that of CONFIG_PREEMPT_RT, where all normal spinlocks
become blocking locks, and all irq handlers execute in
the context of special tasks.  In this case, in step 4
above, the irq handler would block, allowing CPU 1 to
release rcu_gp_mutex, avoiding the deadlock.”h]”hX@  One way to avoid this deadlock is to use an approach like
that of CONFIG_PREEMPT_RT, where all normal spinlocks
become blocking locks, and all irq handlers execute in
the context of special tasks.  In this case, in step 4
above, the irq handler would block, allowing CPU 1 to
release rcu_gp_mutex, avoiding the deadlock.”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M1hjy  ubhÚ)”}”(hXÌ  Even in the absence of deadlock, this RCU implementation
allows latency to "bleed" from readers to other
readers through synchronize_rcu().  To see this,
consider task A in an RCU read-side critical section
(thus read-holding rcu_gp_mutex), task B blocked
attempting to write-acquire rcu_gp_mutex, and
task C blocked in rcu_read_lock() attempting to
read_acquire rcu_gp_mutex.  Task A's RCU read-side
latency is holding up task C, albeit indirectly via
task B.”h]”hXÒ  Even in the absence of deadlock, this RCU implementation
allows latency to â€œbleedâ€ from readers to other
readers through synchronize_rcu().  To see this,
consider task A in an RCU read-side critical section
(thus read-holding rcu_gp_mutex), task B blocked
attempting to write-acquire rcu_gp_mutex, and
task C blocked in rcu_read_lock() attempting to
read_acquire rcu_gp_mutex.  Task Aâ€™s RCU read-side
latency is holding up task C, albeit indirectly via
task B.”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M8hjy  ubhÚ)”}”(hŒªRealtime RCU implementations therefore use a counter-based
approach where tasks in RCU read-side critical sections
cannot be blocked by tasks executing synchronize_rcu().”h]”hŒªRealtime RCU implementations therefore use a counter-based
approach where tasks in RCU read-side critical sections
cannot be blocked by tasks executing synchronize_rcu().”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MChjy  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÚ  hjg  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  h³hÃh´MEhj5  h²hubeh}”(h]”h ]”h"]”h$]”h&]”uh1j¿  hj$  h²hh³hÃh´NubhÚ)”}”(hŒ%:ref:`Back to Quick Quiz #1 <quiz_1>`”h]”h)”}”(hj@  h]”jí  )”}”(hj@  h]”hŒBack to Quick Quiz #1”…””}”(hjE  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hjB  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”jO  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œquiz_1”uh1hh³hÃh´MGhj>  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MGhj$  h²hubjÀ  )”}”(hhh]”(jÅ  )”}”(hŒWQuick Quiz #2:
Give an example where Classic RCU's read-side
overhead is **negative**.
”h]”(jË  )”}”(hŒQuick Quiz #2:”h]”hŒQuick Quiz #2:”…””}”(hjn  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÊ  h³hÃh´MKhjj  ubjÛ  )”}”(hhh]”hÚ)”}”(hŒGGive an example where Classic RCU's read-side
overhead is **negative**.”h]”(hŒ<Give an example where Classic RCUâ€™s read-side
overhead is ”…””}”(hj  h²hh³Nh´Nubjí  )”}”(hŒ**negative**”h]”hŒnegative”…””}”(hj‡  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jì  hj  ubhŒ.”…””}”(hj  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MJhj|  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÚ  hjj  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  h³hÃh´MKhjg  ubjÅ  )”}”(hXá  Answer:
Imagine a single-CPU system with a non-CONFIG_PREEMPTION
kernel where a routing table is used by process-context
code, but can be updated by irq-context code (for example,
by an "ICMP REDIRECT" packet).  The usual way of handling
this would be to have the process-context code disable
interrupts while searching the routing table.  Use of
RCU allows such interrupt-disabling to be dispensed with.
Thus, without RCU, you pay the cost of disabling interrupts,
and with RCU you don't.

One can argue that the overhead of RCU in this
case is negative with respect to the single-CPU
interrupt-disabling approach.  Others might argue that
the overhead of RCU is merely zero, and that replacing
the positive overhead of the interrupt-disabling scheme
with the zero-overhead RCU scheme does not constitute
negative overhead.

In real life, of course, things are more complex.  But
even the theoretical possibility of negative overhead for
a synchronization primitive is a bit unexpected.  ;-)
”h]”(jË  )”}”(hŒAnswer:”h]”hŒAnswer:”…””}”(hj¯  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÊ  h³hÃh´Mbhj«  ubjÛ  )”}”(hhh]”(hÚ)”}”(hXá  Imagine a single-CPU system with a non-CONFIG_PREEMPTION
kernel where a routing table is used by process-context
code, but can be updated by irq-context code (for example,
by an "ICMP REDIRECT" packet).  The usual way of handling
this would be to have the process-context code disable
interrupts while searching the routing table.  Use of
RCU allows such interrupt-disabling to be dispensed with.
Thus, without RCU, you pay the cost of disabling interrupts,
and with RCU you don't.”h]”hXç  Imagine a single-CPU system with a non-CONFIG_PREEMPTION
kernel where a routing table is used by process-context
code, but can be updated by irq-context code (for example,
by an â€œICMP REDIRECTâ€ packet).  The usual way of handling
this would be to have the process-context code disable
interrupts while searching the routing table.  Use of
RCU allows such interrupt-disabling to be dispensed with.
Thus, without RCU, you pay the cost of disabling interrupts,
and with RCU you donâ€™t.”…””}”(hjÀ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MNhj½  ubhÚ)”}”(hXM  One can argue that the overhead of RCU in this
case is negative with respect to the single-CPU
interrupt-disabling approach.  Others might argue that
the overhead of RCU is merely zero, and that replacing
the positive overhead of the interrupt-disabling scheme
with the zero-overhead RCU scheme does not constitute
negative overhead.”h]”hXM  One can argue that the overhead of RCU in this
case is negative with respect to the single-CPU
interrupt-disabling approach.  Others might argue that
the overhead of RCU is merely zero, and that replacing
the positive overhead of the interrupt-disabling scheme
with the zero-overhead RCU scheme does not constitute
negative overhead.”…””}”(hjÎ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´MXhj½  ubhÚ)”}”(hŒ¦In real life, of course, things are more complex.  But
even the theoretical possibility of negative overhead for
a synchronization primitive is a bit unexpected.  ;-)”h]”hŒ¦In real life, of course, things are more complex.  But
even the theoretical possibility of negative overhead for
a synchronization primitive is a bit unexpected.  ;-)”…””}”(hjÜ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M`hj½  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÚ  hj«  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  h³hÃh´Mbhjg  h²hubeh}”(h]”h ]”h"]”h$]”h&]”uh1j¿  hj$  h²hh³hÃh´NubhÚ)”}”(hŒ%:ref:`Back to Quick Quiz #2 <quiz_2>`”h]”h)”}”(hjþ  h]”jí  )”}”(hjþ  h]”hŒBack to Quick Quiz #2”…””}”(hj  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hj   ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”j  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œquiz_2”uh1hh³hÃh´Mdhjü  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mdhj$  h²hubjÀ  )”}”(hhh]”(jÅ  )”}”(hŒ¡Quick Quiz #3:
If it is illegal to block in an RCU read-side
critical section, what the heck do you do in
CONFIG_PREEMPT_RT, where normal spinlocks can block???
”h]”(jË  )”}”(hŒQuick Quiz #3:”h]”hŒQuick Quiz #3:”…””}”(hj,  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÊ  h³hÃh´Mihj(  ubjÛ  )”}”(hhh]”hÚ)”}”(hŒ‘If it is illegal to block in an RCU read-side
critical section, what the heck do you do in
CONFIG_PREEMPT_RT, where normal spinlocks can block???”h]”hŒ‘If it is illegal to block in an RCU read-side
critical section, what the heck do you do in
CONFIG_PREEMPT_RT, where normal spinlocks can block???”…””}”(hj=  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mghj:  ubah}”(h]”h ]”h"]”h$]”h&]”uh1jÚ  hj(  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  h³hÃh´Mihj%  ubjÅ  )”}”(hX;  Answer:
Just as CONFIG_PREEMPT_RT permits preemption of spinlock
critical sections, it permits preemption of RCU
read-side critical sections.  It also permits
spinlocks blocking while in RCU read-side critical
sections.

Why the apparent inconsistency?  Because it is
possible to use priority boosting to keep the RCU
grace periods short if need be (for example, if running
short of memory).  In contrast, if blocking waiting
for (say) network reception, there is no way to know
what should be boosted.  Especially given that the
process we need to boost might well be a human being
who just went out for a pizza or something.  And although
a computer-operated cattle prod might arouse serious
interest, it might also provoke serious objections.
Besides, how does the computer know what pizza parlor
the human being went to???
”h]”(jË  )”}”(hŒAnswer:”h]”hŒAnswer:”…””}”(hj[  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jÊ  h³hÃh´M}hjW  ubjÛ  )”}”(hhh]”(hÚ)”}”(hŒÓJust as CONFIG_PREEMPT_RT permits preemption of spinlock
critical sections, it permits preemption of RCU
read-side critical sections.  It also permits
spinlocks blocking while in RCU read-side critical
sections.”h]”hŒÓJust as CONFIG_PREEMPT_RT permits preemption of spinlock
critical sections, it permits preemption of RCU
read-side critical sections.  It also permits
spinlocks blocking while in RCU read-side critical
sections.”…””}”(hjl  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mlhji  ubhÚ)”}”(hX]  Why the apparent inconsistency?  Because it is
possible to use priority boosting to keep the RCU
grace periods short if need be (for example, if running
short of memory).  In contrast, if blocking waiting
for (say) network reception, there is no way to know
what should be boosted.  Especially given that the
process we need to boost might well be a human being
who just went out for a pizza or something.  And although
a computer-operated cattle prod might arouse serious
interest, it might also provoke serious objections.
Besides, how does the computer know what pizza parlor
the human being went to???”h]”hX]  Why the apparent inconsistency?  Because it is
possible to use priority boosting to keep the RCU
grace periods short if need be (for example, if running
short of memory).  In contrast, if blocking waiting
for (say) network reception, there is no way to know
what should be boosted.  Especially given that the
process we need to boost might well be a human being
who just went out for a pizza or something.  And although
a computer-operated cattle prod might arouse serious
interest, it might also provoke serious objections.
Besides, how does the computer know what pizza parlor
the human being went to???”…””}”(hjz  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mrhji  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÚ  hjW  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1jÄ  h³hÃh´M}hj%  h²hubeh}”(h]”h ]”h"]”h$]”h&]”uh1j¿  hj$  h²hh³hÃh´NubhÚ)”}”(hŒ%:ref:`Back to Quick Quiz #3 <quiz_3>`”h]”h)”}”(hjœ  h]”jí  )”}”(hjœ  h]”hŒBack to Quick Quiz #3”…””}”(hj¡  h²hh³Nh´Nubah}”(h]”h ]”(jø  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1jì  hjž  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”j  Œ	refdomain”j«  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj  Œquiz_3”uh1hh³hÃh´Mhjš  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj$  h²hubhÚ)”}”(hŒACKNOWLEDGEMENTS”h]”hŒACKNOWLEDGEMENTS”…””}”(hjÃ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mhj$  h²hubhÚ)”}”(hŒŽMy thanks to the people who helped make this human-readable, including
Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.”h]”hŒŽMy thanks to the people who helped make this human-readable, including
Jon Walpole, Josh Triplett, Serge Hallyn, Suzanne Wood, and Alan Stern.”…””}”(hjÑ  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´Mƒhj$  h²hubhÚ)”}”(hŒAFor more information, see http://www.rdrop.com/users/paulmck/RCU.”h]”(hŒFor more information, see ”…””}”(hjß  h²hh³Nh´Nubhø)”}”(hŒ&http://www.rdrop.com/users/paulmck/RCU”h]”hŒ&http://www.rdrop.com/users/paulmck/RCU”…””}”(hjç  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”jé  uh1h÷hjß  ubhŒ.”…””}”(hjß  h²hh³Nh´Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÙh³hÃh´M‡hj$  h²hubeh}”(h]”(Œanswers-to-quick-quizzes”j  eh ]”h"]”(Œ9. answers to quick quizzes”Œ9_whatisrcu”eh$]”h&]”uh1hÄhhÆh²hh³hÃh´MjU  }”j  j  sjW  }”j  j  subeh}”(h]”(Œwhat-is-rcu-read-copy-update”hÂeh ]”h"]”(Œ$what is rcu? -- "read, copy, update"”Œwhatisrcu_doc”eh$]”h&]”uh1hÄhhh²hh³hÃh´KjU  }”j  h·sjW  }”hÂh·subeh}”(h]”h ]”h"]”h$]”h&]”Œsource”hÃuh1hŒcurrent_source”NŒcurrent_line”NŒsettings”Œdocutils.frontend”ŒValues”“”)”}”(hÉNŒ	generator”NŒ	datestamp”NŒsource_link”NŒ
source_url”NŒtoc_backlinks”Œentry”Œfootnote_backlinks”KŒsectnum_xform”KŒstrip_comments”NŒstrip_elements_with_classes”NŒstrip_classes”NŒreport_level”KŒ
halt_level”KŒexit_status_level”KŒdebug”NŒwarning_stream”NŒ	traceback”ˆŒinput_encoding”Œ	utf-8-sig”Œinput_encoding_error_handler”Œstrict”Œoutput_encoding”Œutf-8”Œoutput_encoding_error_handler”j9  Œerror_encoding”Œutf-8”Œerror_encoding_error_handler”Œbackslashreplace”Œlanguage_code”Œen”Œrecord_dependencies”NŒconfig”NŒ	id_prefix”hŒauto_id_prefix”Œid”Œdump_settings”NŒdump_internals”NŒdump_transforms”NŒdump_pseudo_xml”NŒexpose_internals”NŒstrict_visitor”NŒ_disable_config”NŒ_source”hÃŒ_destination”NŒ_config_files”]”Œ7/var/lib/git/docbuild/linux/Documentation/docutils.conf”aŒfile_insertion_enabled”ˆŒraw_enabled”KŒline_length_limit”M'Œpep_references”NŒpep_base_url”Œhttps://peps.python.org/”Œpep_file_url_template”Œpep-%04d”Œrfc_references”NŒrfc_base_url”Œ&https://datatracker.ietf.org/doc/html/”Œ	tab_width”KŒtrim_footnote_reference_space”‰Œsyntax_highlight”Œlong”Œsmart_quotes”ˆŒsmartquotes_locales”]”Œcharacter_level_inline_markup”‰Œdoctitle_xform”‰Œdocinfo_xform”KŒsectsubtitle_xform”‰Œimage_loading”Œlink”Œembed_stylesheet”‰Œcloak_email_addresses”ˆŒsection_self_link”‰Œenv”NubŒreporter”NŒindirect_targets”]”Œsubstitution_defs”}”Œsubstitution_names”}”Œrefnames”}”(Œ1”]”jz  aŒ2”]”jÌ  auŒrefids”}”(hÂ]”h·aj  ]”jw  ajK  ]”jA  aj>
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  j`  jl  jÃ  jl  jÀ  jÌ  já  jÌ  j-  j  j¾  jÁ  jÙ  j3  jæ  jç  jg  jh  jÖ  jê  j†  jê  jƒ  j  jû  j  jø  j  j  j  j  j$  j  j$  uŒfootnote_refs”}”(jy  ]”jz  aj{  ]”jÌ  auŒcitation_refs”}”Œautofootnotes”]”Œautofootnote_refs”]”Œsymbol_footnotes”]”Œsymbol_footnote_refs”]”Œ	footnotes”]”(jî  j	  eŒ	citations”]”Œautofootnote_start”KŒsymbol_footnote_start”K Œ
id_counter”Œcollections”ŒCounter”“”}”jG  Ks…”R”Œparse_messages”]”hŒsystem_message”“”)”}”(hhh]”hÚ)”}”(hŒ`Blank line missing before literal block (after the "::")? Interpreted as a definition list item.”h]”hŒdBlank line missing before literal block (after the â€œ::â€)? Interpreted as a definition list item.”…””}”(hj³  h²hh³Nh´Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhj°  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”ŒINFO”Œline”M°Œsource”hÃuh1j®  hjl  ubaŒtransform_messages”]”(j¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ3Hyperlink target "whatisrcu-doc" is not referenced.”…””}”hjÑ  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhjÎ  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”Kuh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ/Hyperlink target "whatisrcu" is not referenced.”…””}”hjë  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhjè  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”KJuh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ)Hyperlink target "id1" is not referenced.”…””}”hj  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhj  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”Kuh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ)Hyperlink target "id6" is not referenced.”…””}”hj  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhj  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”M²uh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ)Hyperlink target "id7" is not referenced.”…””}”hj9  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhj6  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”Muh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ)Hyperlink target "id8" is not referenced.”…””}”hjS  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhjP  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”Muuh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ,Hyperlink target "quiz-1" is not referenced.”…””}”hjm  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhjj  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”MÐuh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ,Hyperlink target "quiz-2" is not referenced.”…””}”hj‡  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhj„  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”Muh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ,Hyperlink target "quiz-3" is not referenced.”…””}”hj¡  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhjž  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”Muh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ)Hyperlink target "id9" is not referenced.”…””}”hj»  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhj¸  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”Muh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ*Hyperlink target "id10" is not referenced.”…””}”hjÕ  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhjÒ  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”M˜uh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ*Hyperlink target "id11" is not referenced.”…””}”hjï  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhjì  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”Mñuh1j®  ubj¯  )”}”(hhh]”hÚ)”}”(hhh]”hŒ*Hyperlink target "id12" is not referenced.”…””}”hj	  sbah}”(h]”h ]”h"]”h$]”h&]”uh1hÙhj  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jÉ  Œsource”hÃŒline”Muh1j®  ubeŒtransformer”NŒinclude_log”]”Œ
decoration”Nh²hub.