€•sI      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/rcu”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$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ/translations/zh_TW/RCU/rcu”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒItalian”…””}”hhFsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ/translations/it_IT/RCU/rcu”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒJapanese”…””}”hhZsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ/translations/ja_JP/RCU/rcu”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒKorean”…””}”hhnsbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ/translations/ko_KR/RCU/rcu”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒSpanish”…””}”hh‚sbah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ/translations/sp_SP/RCU/rcu”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubeh}”(h]”h ]”h"]”h$]”h&]”Œcurrent_language”ŒEnglish”uh1h
hhŒ	_document”hŒsource”NŒline”NubhŒtarget”“”)”}”(hŒ.. _rcu_doc:”h]”h}”(h]”h ]”h"]”h$]”h&]”Œrefid”Œrcu-doc”uh1h¡h KhhhžhhŸŒ5/var/lib/git/docbuild/linux/Documentation/RCU/rcu.rst”ubhŒsection”“”)”}”(hhh]”(hŒtitle”“”)”}”(hŒRCU Concepts”h]”hŒRCU Concepts”…””}”(hh·hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hµhh²hžhhŸh¯h KubhŒ	paragraph”“”)”}”(hXq  The basic idea behind RCU (read-copy update) is to split destructive
operations into two parts, one that prevents anyone from seeing the data
item being destroyed, and one that actually carries out the destruction.
A "grace period" must elapse between the two parts, and this grace period
must be long enough that any readers accessing the item being deleted have
since dropped their references.  For example, an RCU-protected deletion
from a linked list would first remove the item from the list, wait for
a grace period to elapse, then free the element.  See listRCU.rst for more
information on using RCU with linked lists.”h]”hXu  The basic idea behind RCU (read-copy update) is to split destructive
operations into two parts, one that prevents anyone from seeing the data
item being destroyed, and one that actually carries out the destruction.
A â€œgrace periodâ€ must elapse between the two parts, and this grace period
must be long enough that any readers accessing the item being deleted have
since dropped their references.  For example, an RCU-protected deletion
from a linked list would first remove the item from the list, wait for
a grace period to elapse, then free the element.  See listRCU.rst for more
information on using RCU with linked lists.”…””}”(hhÇhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Khh²hžhubh±)”}”(hhh]”(h¶)”}”(hŒFrequently Asked Questions”h]”hŒFrequently Asked Questions”…””}”(hhØhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hµhhÕhžhhŸh¯h KubhŒbullet_list”“”)”}”(hhh]”(hŒ	list_item”“”)”}”(hXà  Why would anyone want to use RCU?

The advantage of RCU's two-part approach is that RCU readers need
not acquire any locks, perform any atomic instructions, write to
shared memory, or (on CPUs other than Alpha) execute any memory
barriers.  The fact that these operations are quite expensive
on modern CPUs is what gives RCU its performance advantages
in read-mostly situations.  The fact that RCU readers need not
acquire locks can also greatly simplify deadlock-avoidance code.
”h]”(hÆ)”}”(hŒ!Why would anyone want to use RCU?”h]”hŒ!Why would anyone want to use RCU?”…””}”(hhñhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KhhíubhÆ)”}”(hX¼  The advantage of RCU's two-part approach is that RCU readers need
not acquire any locks, perform any atomic instructions, write to
shared memory, or (on CPUs other than Alpha) execute any memory
barriers.  The fact that these operations are quite expensive
on modern CPUs is what gives RCU its performance advantages
in read-mostly situations.  The fact that RCU readers need not
acquire locks can also greatly simplify deadlock-avoidance code.”h]”hX¾  The advantage of RCUâ€™s two-part approach is that RCU readers need
not acquire any locks, perform any atomic instructions, write to
shared memory, or (on CPUs other than Alpha) execute any memory
barriers.  The fact that these operations are quite expensive
on modern CPUs is what gives RCU its performance advantages
in read-mostly situations.  The fact that RCU readers need not
acquire locks can also greatly simplify deadlock-avoidance code.”…””}”(hhÿhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Khhíubeh}”(h]”h ]”h"]”h$]”h&]”uh1hëhhèhžhhŸh¯h Nubhì)”}”(hXÏ  How can the updater tell when a grace period has completed
if the RCU readers give no indication when they are done?

Just as with spinlocks, RCU readers are not permitted to
block, switch to user-mode execution, or enter the idle loop.
Therefore, as soon as a CPU is seen passing through any of these
three states, we know that that CPU has exited any previous RCU
read-side critical sections.  So, if we remove an item from a
linked list, and then wait until all CPUs have switched context,
executed in user mode, or executed in the idle loop, we can
safely free up that item.

Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
same effect, but require that the readers manipulate CPU-local
counters.  These counters allow limited types of blocking within
RCU read-side critical sections.  SRCU also uses CPU-local
counters, and permits general blocking within RCU read-side
critical sections.  These variants of RCU detect grace periods
by sampling these counters.
”h]”(hÆ)”}”(hŒtHow can the updater tell when a grace period has completed
if the RCU readers give no indication when they are done?”h]”hŒtHow can the updater tell when a grace period has completed
if the RCU readers give no indication when they are done?”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Khj  ubhÆ)”}”(hXÌ  Just as with spinlocks, RCU readers are not permitted to
block, switch to user-mode execution, or enter the idle loop.
Therefore, as soon as a CPU is seen passing through any of these
three states, we know that that CPU has exited any previous RCU
read-side critical sections.  So, if we remove an item from a
linked list, and then wait until all CPUs have switched context,
executed in user mode, or executed in the idle loop, we can
safely free up that item.”h]”hXÌ  Just as with spinlocks, RCU readers are not permitted to
block, switch to user-mode execution, or enter the idle loop.
Therefore, as soon as a CPU is seen passing through any of these
three states, we know that that CPU has exited any previous RCU
read-side critical sections.  So, if we remove an item from a
linked list, and then wait until all CPUs have switched context,
executed in user mode, or executed in the idle loop, we can
safely free up that item.”…””}”(hj%  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K hj  ubhÆ)”}”(hXŠ  Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
same effect, but require that the readers manipulate CPU-local
counters.  These counters allow limited types of blocking within
RCU read-side critical sections.  SRCU also uses CPU-local
counters, and permits general blocking within RCU read-side
critical sections.  These variants of RCU detect grace periods
by sampling these counters.”h]”hXŠ  Preemptible variants of RCU (CONFIG_PREEMPT_RCU) get the
same effect, but require that the readers manipulate CPU-local
counters.  These counters allow limited types of blocking within
RCU read-side critical sections.  SRCU also uses CPU-local
counters, and permits general blocking within RCU read-side
critical sections.  These variants of RCU detect grace periods
by sampling these counters.”…””}”(hj3  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K)hj  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1hëhhèhžhhŸh¯h Nubhì)”}”(hŒ™If I am running on a uniprocessor kernel, which can only do one
thing at a time, why should I wait for a grace period?

See UP.rst for more information.
”h]”(hÆ)”}”(hŒvIf I am running on a uniprocessor kernel, which can only do one
thing at a time, why should I wait for a grace period?”h]”hŒvIf I am running on a uniprocessor kernel, which can only do one
thing at a time, why should I wait for a grace period?”…””}”(hjK  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K1hjG  ubhÆ)”}”(hŒ See UP.rst for more information.”h]”hŒ See UP.rst for more information.”…””}”(hjY  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K4hjG  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1hëhhèhžhhŸh¯h Nubhì)”}”(hX”  How can I see where RCU is currently used in the Linux kernel?

Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
"rcu_read_lock_bh", "rcu_read_unlock_bh", "srcu_read_lock",
"srcu_read_unlock", "synchronize_rcu", "synchronize_net",
"synchronize_srcu", and the other RCU primitives.  Or grab one
of the cscope databases from:

(http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html).
”h]”(hÆ)”}”(hŒ>How can I see where RCU is currently used in the Linux kernel?”h]”hŒ>How can I see where RCU is currently used in the Linux kernel?”…””}”(hjq  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K6hjm  ubhÆ)”}”(hX  Search for "rcu_read_lock", "rcu_read_unlock", "call_rcu",
"rcu_read_lock_bh", "rcu_read_unlock_bh", "srcu_read_lock",
"srcu_read_unlock", "synchronize_rcu", "synchronize_net",
"synchronize_srcu", and the other RCU primitives.  Or grab one
of the cscope databases from:”h]”hX5  Search for â€œrcu_read_lockâ€, â€œrcu_read_unlockâ€, â€œcall_rcuâ€,
â€œrcu_read_lock_bhâ€, â€œrcu_read_unlock_bhâ€, â€œsrcu_read_lockâ€,
â€œsrcu_read_unlockâ€, â€œsynchronize_rcuâ€, â€œsynchronize_netâ€,
â€œsynchronize_srcuâ€, and the other RCU primitives.  Or grab one
of the cscope databases from:”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K8hjm  ubhÆ)”}”(hŒD(http://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html).”h]”(hŒ(”…””}”(hj  hžhhŸNh NubhŒ	reference”“”)”}”(hŒAhttp://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html”h]”hŒAhttp://www.rdrop.com/users/paulmck/RCU/linuxusage/rculocktab.html”…””}”(hj—  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j™  uh1j•  hj  ubhŒ).”…””}”(hj  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K>hjm  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1hëhhèhžhhŸh¯h Nubhì)”}”(hŒUWhat guidelines should I follow when writing code that uses RCU?

See checklist.rst.
”h]”(hÆ)”}”(hŒ@What guidelines should I follow when writing code that uses RCU?”h]”hŒ@What guidelines should I follow when writing code that uses RCU?”…””}”(hjº  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K@hj¶  ubhÆ)”}”(hŒSee checklist.rst.”h]”hŒSee checklist.rst.”…””}”(hjÈ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KBhj¶  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1hëhhèhžhhŸh¯h Nubhì)”}”(hŒ£Why the name "RCU"?

"RCU" stands for "read-copy update".
listRCU.rst has more information on where this name came from, search
for "read-copy update" to find it.
”h]”(hÆ)”}”(hŒWhy the name "RCU"?”h]”hŒWhy the name â€œRCUâ€?”…””}”(hjà  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KDhjÜ  ubhÆ)”}”(hŒ"RCU" stands for "read-copy update".
listRCU.rst has more information on where this name came from, search
for "read-copy update" to find it.”h]”hŒ™â€œRCUâ€ stands for â€œread-copy updateâ€.
listRCU.rst has more information on where this name came from, search
for â€œread-copy updateâ€ to find it.”…””}”(hjî  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KFhjÜ  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1hëhhèhžhhŸh¯h Nubhì)”}”(hXº  I hear that RCU is patented?  What is with that?

Yes, it is.  There are several known patents related to RCU,
search for the string "Patent" in Documentation/RCU/RTFP.txt to find them.
Of these, one was allowed to lapse by the assignee, and the
others have been contributed to the Linux kernel under GPL.
Many (but not all) have long since expired.
There are now also LGPL implementations of user-level RCU
available (https://liburcu.org/).
”h]”(hÆ)”}”(hŒ0I hear that RCU is patented?  What is with that?”h]”hŒ0I hear that RCU is patented?  What is with that?”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KJhj  ubhÆ)”}”(hX‡  Yes, it is.  There are several known patents related to RCU,
search for the string "Patent" in Documentation/RCU/RTFP.txt to find them.
Of these, one was allowed to lapse by the assignee, and the
others have been contributed to the Linux kernel under GPL.
Many (but not all) have long since expired.
There are now also LGPL implementations of user-level RCU
available (https://liburcu.org/).”h]”(hXu  Yes, it is.  There are several known patents related to RCU,
search for the string â€œPatentâ€ in Documentation/RCU/RTFP.txt to find them.
Of these, one was allowed to lapse by the assignee, and the
others have been contributed to the Linux kernel under GPL.
Many (but not all) have long since expired.
There are now also LGPL implementations of user-level RCU
available (”…””}”(hj  hžhhŸNh Nubj–  )”}”(hŒhttps://liburcu.org/”h]”hŒhttps://liburcu.org/”…””}”(hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j  uh1j•  hj  ubhŒ).”…””}”(hj  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KLhj  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1hëhhèhžhhŸh¯h Nubhì)”}”(hŒžI hear that RCU needs work in order to support realtime kernels?

Realtime-friendly RCU are enabled via the CONFIG_PREEMPTION
kernel configuration parameter.
”h]”(hÆ)”}”(hŒ@I hear that RCU needs work in order to support realtime kernels?”h]”hŒ@I hear that RCU needs work in order to support realtime kernels?”…””}”(hj?  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KThj;  ubhÆ)”}”(hŒ[Realtime-friendly RCU are enabled via the CONFIG_PREEMPTION
kernel configuration parameter.”h]”hŒ[Realtime-friendly RCU are enabled via the CONFIG_PREEMPTION
kernel configuration parameter.”…””}”(hjM  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KVhj;  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1hëhhèhžhhŸh¯h Nubhì)”}”(hX*  Where can I find more information on RCU?

See the Documentation/RCU/RTFP.txt file.
Or point your browser at (https://docs.google.com/document/d/1X0lThx8OK0ZgLMqVoXiR4ZrGURHrXK6NyLRbeXe3Xac/edit)
or (https://docs.google.com/document/d/1GCdQC8SDbb54W1shjEXqGZ0Rq8a6kIeYutdSIajfpLA/edit?usp=sharing).”h]”(hÆ)”}”(hŒ)Where can I find more information on RCU?”h]”hŒ)Where can I find more information on RCU?”…””}”(hje  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KYhja  ubhÆ)”}”(hŒÿSee the Documentation/RCU/RTFP.txt file.
Or point your browser at (https://docs.google.com/document/d/1X0lThx8OK0ZgLMqVoXiR4ZrGURHrXK6NyLRbeXe3Xac/edit)
or (https://docs.google.com/document/d/1GCdQC8SDbb54W1shjEXqGZ0Rq8a6kIeYutdSIajfpLA/edit?usp=sharing).”h]”(hŒCSee the Documentation/RCU/RTFP.txt file.
Or point your browser at (”…””}”(hjs  hžhhŸNh Nubj–  )”}”(hŒThttps://docs.google.com/document/d/1X0lThx8OK0ZgLMqVoXiR4ZrGURHrXK6NyLRbeXe3Xac/edit”h]”hŒThttps://docs.google.com/document/d/1X0lThx8OK0ZgLMqVoXiR4ZrGURHrXK6NyLRbeXe3Xac/edit”…””}”(hj{  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j}  uh1j•  hjs  ubhŒ)
or (”…””}”(hjs  hžhhŸNh Nubj–  )”}”(hŒ`https://docs.google.com/document/d/1GCdQC8SDbb54W1shjEXqGZ0Rq8a6kIeYutdSIajfpLA/edit?usp=sharing”h]”hŒ`https://docs.google.com/document/d/1GCdQC8SDbb54W1shjEXqGZ0Rq8a6kIeYutdSIajfpLA/edit?usp=sharing”…””}”(hjŽ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”Œrefuri”j  uh1j•  hjs  ubhŒ).”…””}”(hjs  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K[hja  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1hëhhèhžhhŸh¯h Nubeh}”(h]”h ]”h"]”h$]”h&]”Œbullet”Œ-”uh1hæhŸh¯h KhhÕhžhubeh}”(h]”Œfrequently-asked-questions”ah ]”h"]”Œfrequently asked questions”ah$]”h&]”uh1h°hh²hžhhŸh¯h Kubeh}”(h]”(Œrcu-concepts”h®eh ]”h"]”(Œrcu concepts”Œrcu_doc”eh$]”h&]”uh1h°hhhžhhŸh¯h KŒexpect_referenced_by_name”}”jÃ  h£sŒexpect_referenced_by_id”}”h®h£subeh}”(h]”h ]”h"]”h$]”h&]”Œsource”h¯uh1hŒcurrent_source”NŒcurrent_line”NŒsettings”Œdocutils.frontend”ŒValues”“”)”}”(hµNŒ	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
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