€•‘      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)”…””}”(hhŒparent”hubaŒ
attributes”}”(Œids”]”Œclasses”]”Œnames”]”Œdupnames”]”Œbackrefs”]”Œ	refdomain”Œstd”Œreftype”Œdoc”Œ	reftarget”Œ/translations/zh_CN/RCU/listRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuŒtagname”hhhubh)”}”(hhh]”hŒChinese (Traditional)”…””}”(hhhh2ubah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ/translations/zh_TW/RCU/listRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒItalian”…””}”(hhhhFubah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ/translations/it_IT/RCU/listRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒJapanese”…””}”(hhhhZubah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ/translations/ja_JP/RCU/listRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒKorean”…””}”(hhhhnubah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ/translations/ko_KR/RCU/listRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubh)”}”(hhh]”hŒSpanish”…””}”(hhhh‚ubah}”(h]”h ]”h"]”h$]”h&]”Œ	refdomain”h)Œreftype”h+Œ	reftarget”Œ/translations/sp_SP/RCU/listRCU”Œmodname”NŒ	classname”NŒrefexplicit”ˆuh1hhhubeh}”(h]”h ]”h"]”h$]”h&]”Œcurrent_language”ŒEnglish”uh1h
hhŒ	_document”hŒsource”NŒline”NubhŒtarget”“”)”}”(hŒ.. _list_rcu_doc:”h]”h}”(h]”h ]”h"]”h$]”h&]”Œrefid”Œlist-rcu-doc”uh1h¡h KhhhžhhŸŒ9/var/lib/git/docbuild/linux/Documentation/RCU/listRCU.rst”ubhŒsection”“”)”}”(hhh]”(hŒtitle”“”)”}”(hŒ-Using RCU to Protect Read-Mostly Linked Lists”h]”hŒ-Using RCU to Protect Read-Mostly Linked Lists”…””}”(hh¹hh·hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hµhh²hžhhŸh¯h KubhŒ	paragraph”“”)”}”(hX  One of the most common uses of RCU is protecting read-mostly linked lists
(``struct list_head`` in list.h).  One big advantage of this approach is
that all of the required memory ordering is provided by the list macros.
This document describes several list-based RCU use cases.”h]”(hŒKOne of the most common uses of RCU is protecting read-mostly linked lists
(”…””}”(hŒKOne of the most common uses of RCU is protecting read-mostly linked lists
(”hhÇhžhhŸNh NubhŒliteral”“”)”}”(hŒ``struct list_head``”h]”hŒstruct list_head”…””}”(hhhhÒhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhhÇubhŒ¶ in list.h).  One big advantage of this approach is
that all of the required memory ordering is provided by the list macros.
This document describes several list-based RCU use cases.”…””}”(hŒ¶ in list.h).  One big advantage of this approach is
that all of the required memory ordering is provided by the list macros.
This document describes several list-based RCU use cases.”hhÇhžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Khh²hžhubhÆ)”}”(hXø  When iterating a list while holding the rcu_read_lock(), writers may
modify the list.  The reader is guaranteed to see all of the elements
which were added to the list before they acquired the rcu_read_lock()
and are still on the list when they drop the rcu_read_unlock().
Elements which are added to, or removed from the list may or may not
be seen.  If the writer calls list_replace_rcu(), the reader may see
either the old element or the new element; they will not see both,
nor will they see neither.”h]”hXø  When iterating a list while holding the rcu_read_lock(), writers may
modify the list.  The reader is guaranteed to see all of the elements
which were added to the list before they acquired the rcu_read_lock()
and are still on the list when they drop the rcu_read_unlock().
Elements which are added to, or removed from the list may or may not
be seen.  If the writer calls list_replace_rcu(), the reader may see
either the old element or the new element; they will not see both,
nor will they see neither.”…””}”(hhíhhëhžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Khh²hžhubh±)”}”(hhh]”(h¶)”}”(hŒ1Example 1: Read-mostly list: Deferred Destruction”h]”hŒ1Example 1: Read-mostly list: Deferred Destruction”…””}”(hhþhhühžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hµhhùhžhhŸh¯h KubhÆ)”}”(hX   A widely used usecase for RCU lists in the kernel is lockless iteration over
all processes in the system. ``task_struct::tasks`` represents the list node that
links all the processes. The list can be traversed in parallel to any list
additions or removals.”h]”(hŒjA widely used usecase for RCU lists in the kernel is lockless iteration over
all processes in the system. ”…””}”(hŒjA widely used usecase for RCU lists in the kernel is lockless iteration over
all processes in the system. ”hj
  hžhhŸNh NubhÑ)”}”(hŒ``task_struct::tasks``”h]”hŒtask_struct::tasks”…””}”(hhhj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj
  ubhŒ€ represents the list node that
links all the processes. The list can be traversed in parallel to any list
additions or removals.”…””}”(hŒ€ represents the list node that
links all the processes. The list can be traversed in parallel to any list
additions or removals.”hj
  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KhhùhžhubhÆ)”}”(hŒaThe traversal of the list is done using ``for_each_process()`` which is defined
by the 2 macros::”h]”(hŒ(The traversal of the list is done using ”…””}”(hŒ(The traversal of the list is done using ”hj,  hžhhŸNh NubhÑ)”}”(hŒ``for_each_process()``”h]”hŒfor_each_process()”…””}”(hhhj5  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj,  ubhŒ" which is defined
by the 2 macros:”…””}”(hŒ" which is defined
by the 2 macros:”hj,  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KhhùhžhubhŒliteral_block”“”)”}”(hŒ»#define next_task(p) \
        list_entry_rcu((p)->tasks.next, struct task_struct, tasks)

#define for_each_process(p) \
        for (p = &init_task ; (p = next_task(p)) != &init_task ; )”h]”hŒ»#define next_task(p) \
        list_entry_rcu((p)->tasks.next, struct task_struct, tasks)

#define for_each_process(p) \
        for (p = &init_task ; (p = next_task(p)) != &init_task ; )”…””}”(hhhjP  ubah}”(h]”h ]”h"]”h$]”h&]”Œ	xml:space”Œpreserve”uh1jN  hŸh¯h K hhùhžhubhÆ)”}”(hŒDThe code traversing the list of all processes typically looks like::”h]”hŒCThe code traversing the list of all processes typically looks like:”…””}”(hŒCThe code traversing the list of all processes typically looks like:”hj`  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K&hhùhžhubjO  )”}”(hŒ]rcu_read_lock();
for_each_process(p) {
        /* Do something with p */
}
rcu_read_unlock();”h]”hŒ]rcu_read_lock();
for_each_process(p) {
        /* Do something with p */
}
rcu_read_unlock();”…””}”(hhhjo  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h K(hhùhžhubhÆ)”}”(hŒTThe simplified and heavily inlined code for removing a process from a
task list is::”h]”hŒSThe simplified and heavily inlined code for removing a process from a
task list is:”…””}”(hŒSThe simplified and heavily inlined code for removing a process from a
task list is:”hj}  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K.hhùhžhubjO  )”}”(hŒËvoid release_task(struct task_struct *p)
{
        write_lock(&tasklist_lock);
        list_del_rcu(&p->tasks);
        write_unlock(&tasklist_lock);
        call_rcu(&p->rcu, delayed_put_task_struct);
}”h]”hŒËvoid release_task(struct task_struct *p)
{
        write_lock(&tasklist_lock);
        list_del_rcu(&p->tasks);
        write_unlock(&tasklist_lock);
        call_rcu(&p->rcu, delayed_put_task_struct);
}”…””}”(hhhjŒ  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h K1hhùhžhubhÆ)”}”(hX›  When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)``
via __exit_signal() and __unhash_process() under ``tasklist_lock``
writer lock protection.  The list_del_rcu() invocation removes
the task from the list of all tasks. The ``tasklist_lock``
prevents concurrent list additions/removals from corrupting the
list. Readers using ``for_each_process()`` are not protected with the
``tasklist_lock``. To prevent readers from noticing changes in the list
pointers, the ``task_struct`` object is freed only after one or more
grace periods elapse, with the help of call_rcu(), which is invoked via
put_task_struct_rcu_user(). This deferring of destruction ensures that
any readers traversing the list will see valid ``p->tasks.next`` pointers
and deletion/freeing can happen in parallel with traversal of the list.
This pattern is also called an **existence lock**, since RCU refrains
from invoking the delayed_put_task_struct() callback function until
all existing readers finish, which guarantees that the ``task_struct``
object in question will remain in existence until after the completion
of all RCU readers that might possibly have a reference to that object.”h]”(hŒWhen a process exits, ”…””}”(hŒWhen a process exits, ”hjš  hžhhŸNh NubhÑ)”}”(hŒ``release_task()``”h]”hŒrelease_task()”…””}”(hhhj£  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjš  ubhŒ calls ”…””}”(hŒ calls ”hjš  hžhhŸNh NubhÑ)”}”(hŒ``list_del_rcu(&p->tasks)``”h]”hŒlist_del_rcu(&p->tasks)”…””}”(hhhj¶  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjš  ubhŒ2
via __exit_signal() and __unhash_process() under ”…””}”(hŒ2
via __exit_signal() and __unhash_process() under ”hjš  hžhhŸNh NubhÑ)”}”(hŒ``tasklist_lock``”h]”hŒtasklist_lock”…””}”(hhhjÉ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjš  ubhŒi
writer lock protection.  The list_del_rcu() invocation removes
the task from the list of all tasks. The ”…””}”(hŒi
writer lock protection.  The list_del_rcu() invocation removes
the task from the list of all tasks. The ”hjš  hžhhŸNh NubhÑ)”}”(hŒ``tasklist_lock``”h]”hŒtasklist_lock”…””}”(hhhjÜ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjš  ubhŒU
prevents concurrent list additions/removals from corrupting the
list. Readers using ”…””}”(hŒU
prevents concurrent list additions/removals from corrupting the
list. Readers using ”hjš  hžhhŸNh NubhÑ)”}”(hŒ``for_each_process()``”h]”hŒfor_each_process()”…””}”(hhhjï  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjš  ubhŒ are not protected with the
”…””}”(hŒ are not protected with the
”hjš  hžhhŸNh NubhÑ)”}”(hŒ``tasklist_lock``”h]”hŒtasklist_lock”…””}”(hhhj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjš  ubhŒE. To prevent readers from noticing changes in the list
pointers, the ”…””}”(hŒE. To prevent readers from noticing changes in the list
pointers, the ”hjš  hžhhŸNh NubhÑ)”}”(hŒ``task_struct``”h]”hŒtask_struct”…””}”(hhhj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjš  ubhŒæ object is freed only after one or more
grace periods elapse, with the help of call_rcu(), which is invoked via
put_task_struct_rcu_user(). This deferring of destruction ensures that
any readers traversing the list will see valid ”…””}”(hŒæ object is freed only after one or more
grace periods elapse, with the help of call_rcu(), which is invoked via
put_task_struct_rcu_user(). This deferring of destruction ensures that
any readers traversing the list will see valid ”hjš  hžhhŸNh NubhÑ)”}”(hŒ``p->tasks.next``”h]”hŒp->tasks.next”…””}”(hhhj(  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjš  ubhŒq pointers
and deletion/freeing can happen in parallel with traversal of the list.
This pattern is also called an ”…””}”(hŒq pointers
and deletion/freeing can happen in parallel with traversal of the list.
This pattern is also called an ”hjš  hžhhŸNh NubhŒstrong”“”)”}”(hŒ**existence lock**”h]”hŒexistence lock”…””}”(hhhj=  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j;  hjš  ubhŒ, since RCU refrains
from invoking the delayed_put_task_struct() callback function until
all existing readers finish, which guarantees that the ”…””}”(hŒ, since RCU refrains
from invoking the delayed_put_task_struct() callback function until
all existing readers finish, which guarantees that the ”hjš  hžhhŸNh NubhÑ)”}”(hŒ``task_struct``”h]”hŒtask_struct”…””}”(hhhjP  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjš  ubhŒ
object in question will remain in existence until after the completion
of all RCU readers that might possibly have a reference to that object.”…””}”(hŒ
object in question will remain in existence until after the completion
of all RCU readers that might possibly have a reference to that object.”hjš  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K9hhùhžhubeh}”(h]”Œ/example-1-read-mostly-list-deferred-destruction”ah ]”h"]”Œ1example 1: read-mostly list: deferred destruction”ah$]”h&]”uh1h°hh²hžhhŸh¯h Kubh±)”}”(hhh]”(h¶)”}”(hŒFExample 2: Read-Side Action Taken Outside of Lock: No In-Place Updates”h]”hŒFExample 2: Read-Side Action Taken Outside of Lock: No In-Place Updates”…””}”(hjv  hjt  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hµhjq  hžhhŸh¯h KMubhÆ)”}”(hX¶  Some reader-writer locking use cases compute a value while holding
the read-side lock, but continue to use that value after that lock is
released.  These use cases are often good candidates for conversion
to RCU.  One prominent example involves network packet routing.
Because the packet-routing data tracks the state of equipment outside
of the computer, it will at times contain stale data.  Therefore, once
the route has been computed, there is no need to hold the routing table
static during transmission of the packet.  After all, you can hold the
routing table static all you want, but that won't keep the external
Internet from changing, and it is the state of the external Internet
that really matters.  In addition, routing entries are typically added
or deleted, rather than being modified in place.  This is a rare example
of the finite speed of light and the non-zero size of atoms actually
helping make synchronization be lighter weight.”h]”hX¸  Some reader-writer locking use cases compute a value while holding
the read-side lock, but continue to use that value after that lock is
released.  These use cases are often good candidates for conversion
to RCU.  One prominent example involves network packet routing.
Because the packet-routing data tracks the state of equipment outside
of the computer, it will at times contain stale data.  Therefore, once
the route has been computed, there is no need to hold the routing table
static during transmission of the packet.  After all, you can hold the
routing table static all you want, but that wonâ€™t keep the external
Internet from changing, and it is the state of the external Internet
that really matters.  In addition, routing entries are typically added
or deleted, rather than being modified in place.  This is a rare example
of the finite speed of light and the non-zero size of atoms actually
helping make synchronization be lighter weight.”…””}”(hj„  hj‚  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KOhjq  hžhubhÆ)”}”(hŒÍA straightforward example of this type of RCU use case may be found in
the system-call auditing support.  For example, a reader-writer locked
implementation of ``audit_filter_task()`` might be as follows::”h]”(hŒ A straightforward example of this type of RCU use case may be found in
the system-call auditing support.  For example, a reader-writer locked
implementation of ”…””}”(hŒ A straightforward example of this type of RCU use case may be found in
the system-call auditing support.  For example, a reader-writer locked
implementation of ”hj  hžhhŸNh NubhÑ)”}”(hŒ``audit_filter_task()``”h]”hŒaudit_filter_task()”…””}”(hhhj™  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj  ubhŒ might be as follows:”…””}”(hŒ might be as follows:”hj  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K^hjq  hžhubjO  )”}”(hX±  static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
{
        struct audit_entry *e;
        enum audit_state   state;

        read_lock(&auditsc_lock);
        /* Note: audit_filter_mutex held by caller. */
        list_for_each_entry(e, &audit_tsklist, list) {
                if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
                        if (state == AUDIT_STATE_RECORD)
                                *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
                        read_unlock(&auditsc_lock);
                        return state;
                }
        }
        read_unlock(&auditsc_lock);
        return AUDIT_BUILD_CONTEXT;
}”h]”hX±  static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
{
        struct audit_entry *e;
        enum audit_state   state;

        read_lock(&auditsc_lock);
        /* Note: audit_filter_mutex held by caller. */
        list_for_each_entry(e, &audit_tsklist, list) {
                if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
                        if (state == AUDIT_STATE_RECORD)
                                *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
                        read_unlock(&auditsc_lock);
                        return state;
                }
        }
        read_unlock(&auditsc_lock);
        return AUDIT_BUILD_CONTEXT;
}”…””}”(hhhj²  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h Kbhjq  hžhubhÆ)”}”(hX#  Here the list is searched under the lock, but the lock is dropped before
the corresponding value is returned.  By the time that this value is acted
on, the list may well have been modified.  This makes sense, since if
you are turning auditing off, it is OK to audit a few extra system calls.”h]”hX#  Here the list is searched under the lock, but the lock is dropped before
the corresponding value is returned.  By the time that this value is acted
on, the list may well have been modified.  This makes sense, since if
you are turning auditing off, it is OK to audit a few extra system calls.”…””}”(hjÂ  hjÀ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Kuhjq  hžhubhÆ)”}”(hŒHThis means that RCU can be easily applied to the read side, as follows::”h]”hŒGThis means that RCU can be easily applied to the read side, as follows:”…””}”(hŒGThis means that RCU can be easily applied to the read side, as follows:”hjÎ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Kzhjq  hžhubjO  )”}”(hXš  static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
{
        struct audit_entry *e;
        enum audit_state   state;

        rcu_read_lock();
        /* Note: audit_filter_mutex held by caller. */
        list_for_each_entry_rcu(e, &audit_tsklist, list) {
                if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
                        if (state == AUDIT_STATE_RECORD)
                                *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
                        rcu_read_unlock();
                        return state;
                }
        }
        rcu_read_unlock();
        return AUDIT_BUILD_CONTEXT;
}”h]”hXš  static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
{
        struct audit_entry *e;
        enum audit_state   state;

        rcu_read_lock();
        /* Note: audit_filter_mutex held by caller. */
        list_for_each_entry_rcu(e, &audit_tsklist, list) {
                if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
                        if (state == AUDIT_STATE_RECORD)
                                *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
                        rcu_read_unlock();
                        return state;
                }
        }
        rcu_read_unlock();
        return AUDIT_BUILD_CONTEXT;
}”…””}”(hhhjÝ  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h K|hjq  hžhubhÆ)”}”(hX<  The read_lock() and read_unlock() calls have become rcu_read_lock()
and rcu_read_unlock(), respectively, and the list_for_each_entry()
has become list_for_each_entry_rcu().  The **_rcu()** list-traversal
primitives add READ_ONCE() and diagnostic checks for incorrect use
outside of an RCU read-side critical section.”h]”(hŒ²The read_lock() and read_unlock() calls have become rcu_read_lock()
and rcu_read_unlock(), respectively, and the list_for_each_entry()
has become list_for_each_entry_rcu().  The ”…””}”(hŒ²The read_lock() and read_unlock() calls have become rcu_read_lock()
and rcu_read_unlock(), respectively, and the list_for_each_entry()
has become list_for_each_entry_rcu().  The ”hjë  hžhhŸNh Nubj<  )”}”(hŒ
**_rcu()**”h]”hŒ_rcu()”…””}”(hhhjô  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j;  hjë  ubhŒ€ list-traversal
primitives add READ_ONCE() and diagnostic checks for incorrect use
outside of an RCU read-side critical section.”…””}”(hŒ€ list-traversal
primitives add READ_ONCE() and diagnostic checks for incorrect use
outside of an RCU read-side critical section.”hjë  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Khjq  hžhubhÆ)”}”(hŒÉThe changes to the update side are also straightforward. A reader-writer lock
might be used as follows for deletion and insertion in these simplified
versions of audit_del_rule() and audit_add_rule()::”h]”hŒÈThe changes to the update side are also straightforward. A reader-writer lock
might be used as follows for deletion and insertion in these simplified
versions of audit_del_rule() and audit_add_rule():”…””}”(hŒÈThe changes to the update side are also straightforward. A reader-writer lock
might be used as follows for deletion and insertion in these simplified
versions of audit_del_rule() and audit_add_rule():”hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K•hjq  hžhubjO  )”}”(hXÒ  static inline int audit_del_rule(struct audit_rule *rule,
                                 struct list_head *list)
{
        struct audit_entry *e;

        write_lock(&auditsc_lock);
        list_for_each_entry(e, list, list) {
                if (!audit_compare_rule(rule, &e->rule)) {
                        list_del(&e->list);
                        write_unlock(&auditsc_lock);
                        return 0;
                }
        }
        write_unlock(&auditsc_lock);
        return -EFAULT;         /* No matching rule */
}

static inline int audit_add_rule(struct audit_entry *entry,
                                 struct list_head *list)
{
        write_lock(&auditsc_lock);
        if (entry->rule.flags & AUDIT_PREPEND) {
                entry->rule.flags &= ~AUDIT_PREPEND;
                list_add(&entry->list, list);
        } else {
                list_add_tail(&entry->list, list);
        }
        write_unlock(&auditsc_lock);
        return 0;
}”h]”hXÒ  static inline int audit_del_rule(struct audit_rule *rule,
                                 struct list_head *list)
{
        struct audit_entry *e;

        write_lock(&auditsc_lock);
        list_for_each_entry(e, list, list) {
                if (!audit_compare_rule(rule, &e->rule)) {
                        list_del(&e->list);
                        write_unlock(&auditsc_lock);
                        return 0;
                }
        }
        write_unlock(&auditsc_lock);
        return -EFAULT;         /* No matching rule */
}

static inline int audit_add_rule(struct audit_entry *entry,
                                 struct list_head *list)
{
        write_lock(&auditsc_lock);
        if (entry->rule.flags & AUDIT_PREPEND) {
                entry->rule.flags &= ~AUDIT_PREPEND;
                list_add(&entry->list, list);
        } else {
                list_add_tail(&entry->list, list);
        }
        write_unlock(&auditsc_lock);
        return 0;
}”…””}”(hhhj  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h K™hjq  hžhubhÆ)”}”(hŒ;Following are the RCU equivalents for these two functions::”h]”hŒ:Following are the RCU equivalents for these two functions:”…””}”(hŒ:Following are the RCU equivalents for these two functions:”hj*  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K¸hjq  hžhubjO  )”}”(hX¾  static inline int audit_del_rule(struct audit_rule *rule,
                                 struct list_head *list)
{
        struct audit_entry *e;

        /* No need to use the _rcu iterator here, since this is the only
         * deletion routine. */
        list_for_each_entry(e, list, list) {
                if (!audit_compare_rule(rule, &e->rule)) {
                        list_del_rcu(&e->list);
                        call_rcu(&e->rcu, audit_free_rule);
                        return 0;
                }
        }
        return -EFAULT;         /* No matching rule */
}

static inline int audit_add_rule(struct audit_entry *entry,
                                 struct list_head *list)
{
        if (entry->rule.flags & AUDIT_PREPEND) {
                entry->rule.flags &= ~AUDIT_PREPEND;
                list_add_rcu(&entry->list, list);
        } else {
                list_add_tail_rcu(&entry->list, list);
        }
        return 0;
}”h]”hX¾  static inline int audit_del_rule(struct audit_rule *rule,
                                 struct list_head *list)
{
        struct audit_entry *e;

        /* No need to use the _rcu iterator here, since this is the only
         * deletion routine. */
        list_for_each_entry(e, list, list) {
                if (!audit_compare_rule(rule, &e->rule)) {
                        list_del_rcu(&e->list);
                        call_rcu(&e->rcu, audit_free_rule);
                        return 0;
                }
        }
        return -EFAULT;         /* No matching rule */
}

static inline int audit_add_rule(struct audit_entry *entry,
                                 struct list_head *list)
{
        if (entry->rule.flags & AUDIT_PREPEND) {
                entry->rule.flags &= ~AUDIT_PREPEND;
                list_add_rcu(&entry->list, list);
        } else {
                list_add_tail_rcu(&entry->list, list);
        }
        return 0;
}”…””}”(hhhj9  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h Kºhjq  hžhubhÆ)”}”(hX9  Normally, the write_lock() and write_unlock() would be replaced by a
spin_lock() and a spin_unlock(). But in this case, all callers hold
``audit_filter_mutex``, so no additional locking is required. The
auditsc_lock can therefore be eliminated, since use of RCU eliminates the
need for writers to exclude readers.”h]”(hŒ‰Normally, the write_lock() and write_unlock() would be replaced by a
spin_lock() and a spin_unlock(). But in this case, all callers hold
”…””}”(hŒ‰Normally, the write_lock() and write_unlock() would be replaced by a
spin_lock() and a spin_unlock(). But in this case, all callers hold
”hjG  hžhhŸNh NubhÑ)”}”(hŒ``audit_filter_mutex``”h]”hŒaudit_filter_mutex”…””}”(hhhjP  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjG  ubhŒš, so no additional locking is required. The
auditsc_lock can therefore be eliminated, since use of RCU eliminates the
need for writers to exclude readers.”…””}”(hŒš, so no additional locking is required. The
auditsc_lock can therefore be eliminated, since use of RCU eliminates the
need for writers to exclude readers.”hjG  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h K×hjq  hžhubhÆ)”}”(hX‚  The list_del(), list_add(), and list_add_tail() primitives have been
replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu().
The **_rcu()** list-manipulation primitives add memory barriers that are
needed on weakly ordered CPUs.  The list_del_rcu() primitive omits the
pointer poisoning debug-assist code that would otherwise cause concurrent
readers to fail spectacularly.”h]”(hŒŽThe list_del(), list_add(), and list_add_tail() primitives have been
replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu().
The ”…””}”(hŒŽThe list_del(), list_add(), and list_add_tail() primitives have been
replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu().
The ”hji  hžhhŸNh Nubj<  )”}”(hŒ
**_rcu()**”h]”hŒ_rcu()”…””}”(hhhjr  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j;  hji  ubhŒê list-manipulation primitives add memory barriers that are
needed on weakly ordered CPUs.  The list_del_rcu() primitive omits the
pointer poisoning debug-assist code that would otherwise cause concurrent
readers to fail spectacularly.”…””}”(hŒê list-manipulation primitives add memory barriers that are
needed on weakly ordered CPUs.  The list_del_rcu() primitive omits the
pointer poisoning debug-assist code that would otherwise cause concurrent
readers to fail spectacularly.”hji  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h KÝhjq  hžhubhÆ)”}”(hŒ‘So, when readers can tolerate stale data and when entries are either added or
deleted, without in-place modification, it is very easy to use RCU!”h]”hŒ‘So, when readers can tolerate stale data and when entries are either added or
deleted, without in-place modification, it is very easy to use RCU!”…””}”(hj  hj‹  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Kähjq  hžhubeh}”(h]”ŒDexample-2-read-side-action-taken-outside-of-lock-no-in-place-updates”ah ]”h"]”ŒFexample 2: read-side action taken outside of lock: no in-place updates”ah$]”h&]”uh1h°hh²hžhhŸh¯h KMubh±)”}”(hhh]”(h¶)”}”(hŒ$Example 3: Handling In-Place Updates”h]”hŒ$Example 3: Handling In-Place Updates”…””}”(hj¦  hj¤  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hµhj¡  hžhhŸh¯h KéubhÆ)”}”(hŒýThe system-call auditing code does not update auditing rules in place.  However,
if it did, the reader-writer-locked code to do so might look as follows
(assuming only ``field_count`` is updated, otherwise, the added fields would
need to be filled in)::”h]”(hŒ¨The system-call auditing code does not update auditing rules in place.  However,
if it did, the reader-writer-locked code to do so might look as follows
(assuming only ”…””}”(hŒ¨The system-call auditing code does not update auditing rules in place.  However,
if it did, the reader-writer-locked code to do so might look as follows
(assuming only ”hj²  hžhhŸNh NubhÑ)”}”(hŒ``field_count``”h]”hŒfield_count”…””}”(hhhj»  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj²  ubhŒE is updated, otherwise, the added fields would
need to be filled in):”…””}”(hŒE is updated, otherwise, the added fields would
need to be filled in):”hj²  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Këhj¡  hžhubjO  )”}”(hX"  static inline int audit_upd_rule(struct audit_rule *rule,
                                 struct list_head *list,
                                 __u32 newaction,
                                 __u32 newfield_count)
{
        struct audit_entry *e;
        struct audit_entry *ne;

        write_lock(&auditsc_lock);
        /* Note: audit_filter_mutex held by caller. */
        list_for_each_entry(e, list, list) {
                if (!audit_compare_rule(rule, &e->rule)) {
                        e->rule.action = newaction;
                        e->rule.field_count = newfield_count;
                        write_unlock(&auditsc_lock);
                        return 0;
                }
        }
        write_unlock(&auditsc_lock);
        return -EFAULT;         /* No matching rule */
}”h]”hX"  static inline int audit_upd_rule(struct audit_rule *rule,
                                 struct list_head *list,
                                 __u32 newaction,
                                 __u32 newfield_count)
{
        struct audit_entry *e;
        struct audit_entry *ne;

        write_lock(&auditsc_lock);
        /* Note: audit_filter_mutex held by caller. */
        list_for_each_entry(e, list, list) {
                if (!audit_compare_rule(rule, &e->rule)) {
                        e->rule.action = newaction;
                        e->rule.field_count = newfield_count;
                        write_unlock(&auditsc_lock);
                        return 0;
                }
        }
        write_unlock(&auditsc_lock);
        return -EFAULT;         /* No matching rule */
}”…””}”(hhhjÔ  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h Kðhj¡  hžhubhÆ)”}”(hŒûThe RCU version creates a copy, updates the copy, then replaces the old
entry with the newly updated entry.  This sequence of actions, allowing
concurrent reads while making a copy to perform an update, is what gives
RCU (*read-copy update*) its name.”h]”(hŒÞThe RCU version creates a copy, updates the copy, then replaces the old
entry with the newly updated entry.  This sequence of actions, allowing
concurrent reads while making a copy to perform an update, is what gives
RCU (”…””}”(hŒÞThe RCU version creates a copy, updates the copy, then replaces the old
entry with the newly updated entry.  This sequence of actions, allowing
concurrent reads while making a copy to perform an update, is what gives
RCU (”hjâ  hžhhŸNh NubhŒemphasis”“”)”}”(hŒ*read-copy update*”h]”hŒread-copy update”…””}”(hhhjí  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jë  hjâ  ubhŒ) its name.”…””}”(hŒ) its name.”hjâ  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Mhj¡  hžhubhÆ)”}”(hŒ3The RCU version of audit_upd_rule() is as follows::”h]”hŒ2The RCU version of audit_upd_rule() is as follows:”…””}”(hŒ2The RCU version of audit_upd_rule() is as follows:”hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Mhj¡  hžhubjO  )”}”(hXÃ  static inline int audit_upd_rule(struct audit_rule *rule,
                                 struct list_head *list,
                                 __u32 newaction,
                                 __u32 newfield_count)
{
        struct audit_entry *e;
        struct audit_entry *ne;

        list_for_each_entry(e, list, list) {
                if (!audit_compare_rule(rule, &e->rule)) {
                        ne = kmalloc(sizeof(*entry), GFP_ATOMIC);
                        if (ne == NULL)
                                return -ENOMEM;
                        audit_copy_rule(&ne->rule, &e->rule);
                        ne->rule.action = newaction;
                        ne->rule.field_count = newfield_count;
                        list_replace_rcu(&e->list, &ne->list);
                        call_rcu(&e->rcu, audit_free_rule);
                        return 0;
                }
        }
        return -EFAULT;         /* No matching rule */
}”h]”hXÃ  static inline int audit_upd_rule(struct audit_rule *rule,
                                 struct list_head *list,
                                 __u32 newaction,
                                 __u32 newfield_count)
{
        struct audit_entry *e;
        struct audit_entry *ne;

        list_for_each_entry(e, list, list) {
                if (!audit_compare_rule(rule, &e->rule)) {
                        ne = kmalloc(sizeof(*entry), GFP_ATOMIC);
                        if (ne == NULL)
                                return -ENOMEM;
                        audit_copy_rule(&ne->rule, &e->rule);
                        ne->rule.action = newaction;
                        ne->rule.field_count = newfield_count;
                        list_replace_rcu(&e->list, &ne->list);
                        call_rcu(&e->rcu, audit_free_rule);
                        return 0;
                }
        }
        return -EFAULT;         /* No matching rule */
}”…””}”(hhhj  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h Mhj¡  hžhubhÆ)”}”(hŒŠAgain, this assumes that the caller holds ``audit_filter_mutex``.  Normally, the
writer lock would become a spinlock in this sort of code.”h]”(hŒ*Again, this assumes that the caller holds ”…””}”(hŒ*Again, this assumes that the caller holds ”hj#  hžhhŸNh NubhÑ)”}”(hŒ``audit_filter_mutex``”h]”hŒaudit_filter_mutex”…””}”(hhhj,  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj#  ubhŒJ.  Normally, the
writer lock would become a spinlock in this sort of code.”…””}”(hŒJ.  Normally, the
writer lock would become a spinlock in this sort of code.”hj#  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h M%hj¡  hžhubhÆ)”}”(hŒpThe update_lsm_rule() does something very similar, for those who would
prefer to look at real Linux-kernel code.”h]”hŒpThe update_lsm_rule() does something very similar, for those who would
prefer to look at real Linux-kernel code.”…””}”(hjG  hjE  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h M(hj¡  hžhubhÆ)”}”(hXl  Another use of this pattern can be found in the openswitch driver's *connection
tracking table* code in ``ct_limit_set()``.  The table holds connection tracking
entries and has a limit on the maximum entries.  There is one such table
per-zone and hence one *limit* per zone.  The zones are mapped to their limits
through a hashtable using an RCU-managed hlist for the hash chains. When a new
limit is set, a new limit object is allocated and ``ct_limit_set()`` is called
to replace the old limit object with the new one using list_replace_rcu().
The old limit object is then freed after a grace period using kfree_rcu().”h]”(hŒFAnother use of this pattern can be found in the openswitch driverâ€™s ”…””}”(hŒDAnother use of this pattern can be found in the openswitch driver's ”hjS  hžhhŸNh Nubjì  )”}”(hŒ*connection
tracking table*”h]”hŒconnection
tracking table”…””}”(hhhj\  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jë  hjS  ubhŒ	 code in ”…””}”(hŒ	 code in ”hjS  hžhhŸNh NubhÑ)”}”(hŒ``ct_limit_set()``”h]”hŒct_limit_set()”…””}”(hhhjo  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjS  ubhŒ‡.  The table holds connection tracking
entries and has a limit on the maximum entries.  There is one such table
per-zone and hence one ”…””}”(hŒ‡.  The table holds connection tracking
entries and has a limit on the maximum entries.  There is one such table
per-zone and hence one ”hjS  hžhhŸNh Nubjì  )”}”(hŒ*limit*”h]”hŒlimit”…””}”(hhhj‚  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jë  hjS  ubhŒ² per zone.  The zones are mapped to their limits
through a hashtable using an RCU-managed hlist for the hash chains. When a new
limit is set, a new limit object is allocated and ”…””}”(hŒ² per zone.  The zones are mapped to their limits
through a hashtable using an RCU-managed hlist for the hash chains. When a new
limit is set, a new limit object is allocated and ”hjS  hžhhŸNh NubhÑ)”}”(hŒ``ct_limit_set()``”h]”hŒct_limit_set()”…””}”(hhhj•  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjS  ubhŒ  is called
to replace the old limit object with the new one using list_replace_rcu().
The old limit object is then freed after a grace period using kfree_rcu().”…””}”(hŒ  is called
to replace the old limit object with the new one using list_replace_rcu().
The old limit object is then freed after a grace period using kfree_rcu().”hjS  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h M+hj¡  hžhubeh}”(h]”Œ#example-3-handling-in-place-updates”ah ]”h"]”Œ$example 3: handling in-place updates”ah$]”h&]”uh1h°hh²hžhhŸh¯h Kéubh±)”}”(hhh]”(h¶)”}”(hŒ!Example 4: Eliminating Stale Data”h]”hŒ!Example 4: Eliminating Stale Data”…””}”(hj»  hj¹  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hµhj¶  hžhhŸh¯h M6ubhÆ)”}”(hXH  The auditing example above tolerates stale data, as do most algorithms
that are tracking external state.  After all, given there is a delay
from the time the external state changes before Linux becomes aware
of the change, and so as noted earlier, a small quantity of additional
RCU-induced staleness is generally not a problem.”h]”hXH  The auditing example above tolerates stale data, as do most algorithms
that are tracking external state.  After all, given there is a delay
from the time the external state changes before Linux becomes aware
of the change, and so as noted earlier, a small quantity of additional
RCU-induced staleness is generally not a problem.”…””}”(hjÉ  hjÇ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h M8hj¶  hžhubhÆ)”}”(hX¨  However, there are many examples where stale data cannot be tolerated.
One example in the Linux kernel is the System V IPC (see the shm_lock()
function in ipc/shm.c).  This code checks a *deleted* flag under a
per-entry spinlock, and, if the *deleted* flag is set, pretends that the
entry does not exist.  For this to be helpful, the search function must
return holding the per-entry spinlock, as shm_lock() does in fact do.”h]”(hŒ»However, there are many examples where stale data cannot be tolerated.
One example in the Linux kernel is the System V IPC (see the shm_lock()
function in ipc/shm.c).  This code checks a ”…””}”(hŒ»However, there are many examples where stale data cannot be tolerated.
One example in the Linux kernel is the System V IPC (see the shm_lock()
function in ipc/shm.c).  This code checks a ”hjÕ  hžhhŸNh Nubjì  )”}”(hŒ	*deleted*”h]”hŒdeleted”…””}”(hhhjÞ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jë  hjÕ  ubhŒ. flag under a
per-entry spinlock, and, if the ”…””}”(hŒ. flag under a
per-entry spinlock, and, if the ”hjÕ  hžhhŸNh Nubjì  )”}”(hŒ	*deleted*”h]”hŒdeleted”…””}”(hhhjñ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jë  hjÕ  ubhŒ­ flag is set, pretends that the
entry does not exist.  For this to be helpful, the search function must
return holding the per-entry spinlock, as shm_lock() does in fact do.”…””}”(hŒ­ flag is set, pretends that the
entry does not exist.  For this to be helpful, the search function must
return holding the per-entry spinlock, as shm_lock() does in fact do.”hjÕ  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h M>hj¶  hžhubh¢)”}”(hŒ.. _quick_quiz:”h]”h}”(h]”h ]”h"]”h$]”h&]”h­Œ
quick-quiz”uh1h¡h MEhj¶  hžhhŸh¯ubhŒdefinition_list”“”)”}”(hhh]”hŒdefinition_list_item”“”)”}”(hŒ“Quick Quiz:
For the deleted-flag technique to be helpful, why is it necessary
to hold the per-entry lock while returning from the search function?
”h]”(hŒterm”“”)”}”(hŒQuick Quiz:”h]”hŒQuick Quiz:”…””}”(hj$  hj"  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j   hŸh¯h MIhj  ubhŒ
definition”“”)”}”(hhh]”hÆ)”}”(hŒ†For the deleted-flag technique to be helpful, why is it necessary
to hold the per-entry lock while returning from the search function?”h]”hŒ†For the deleted-flag technique to be helpful, why is it necessary
to hold the per-entry lock while returning from the search function?”…””}”(hj7  hj5  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h MHhj2  ubah}”(h]”h ]”h"]”h$]”h&]”uh1j0  hj  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1j  hŸh¯h MIhj  ubah}”(h]”j  ah ]”h"]”Œ
quick_quiz”ah$]”h&]”uh1j  hj¶  hžhhŸh¯h NŒexpect_referenced_by_name”}”jS  j
  sŒexpect_referenced_by_id”}”j  j
  subhÆ)”}”(hŒ/:ref:`Answer to Quick Quiz <quick_quiz_answer>`”h]”h)”}”(hj\  h]”hŒinline”“”)”}”(hj\  h]”hŒAnswer to Quick Quiz”…””}”(hhhjc  hžhhŸNh Nubah}”(h]”h ]”(Œxref”Œstd”Œstd-ref”eh"]”h$]”h&]”uh1ja  hj^  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”ŒRCU/listRCU”Œ	refdomain”jn  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆŒ	reftarget”Œquick_quiz_answer”uh1hhŸh¯h MKhjZ  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h MKhj¶  hžhubhÆ)”}”(hŒëIf the system-call audit module were to ever need to reject stale data, one way
to accomplish this would be to add a ``deleted`` flag and a ``lock`` spinlock to the
``audit_entry`` structure, and modify audit_filter_task() as follows::”h]”(hŒuIf the system-call audit module were to ever need to reject stale data, one way
to accomplish this would be to add a ”…””}”(hŒuIf the system-call audit module were to ever need to reject stale data, one way
to accomplish this would be to add a ”hjˆ  hžhhŸNh NubhÑ)”}”(hŒ``deleted``”h]”hŒdeleted”…””}”(hhhj‘  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjˆ  ubhŒ flag and a ”…””}”(hŒ flag and a ”hjˆ  hžhhŸNh NubhÑ)”}”(hŒ``lock``”h]”hŒlock”…””}”(hhhj¤  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjˆ  ubhŒ spinlock to the
”…””}”(hŒ spinlock to the
”hjˆ  hžhhŸNh NubhÑ)”}”(hŒ``audit_entry``”h]”hŒaudit_entry”…””}”(hhhj·  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjˆ  ubhŒ6 structure, and modify audit_filter_task() as follows:”…””}”(hŒ6 structure, and modify audit_filter_task() as follows:”hjˆ  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h MMhj¶  hžhubjO  )”}”(hXn  static enum audit_state audit_filter_task(struct task_struct *tsk)
{
        struct audit_entry *e;
        enum audit_state   state;

        rcu_read_lock();
        list_for_each_entry_rcu(e, &audit_tsklist, list) {
                if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
                        spin_lock(&e->lock);
                        if (e->deleted) {
                                spin_unlock(&e->lock);
                                rcu_read_unlock();
                                return AUDIT_BUILD_CONTEXT;
                        }
                        rcu_read_unlock();
                        if (state == AUDIT_STATE_RECORD)
                                *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
                        return state;
                }
        }
        rcu_read_unlock();
        return AUDIT_BUILD_CONTEXT;
}”h]”hXn  static enum audit_state audit_filter_task(struct task_struct *tsk)
{
        struct audit_entry *e;
        enum audit_state   state;

        rcu_read_lock();
        list_for_each_entry_rcu(e, &audit_tsklist, list) {
                if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
                        spin_lock(&e->lock);
                        if (e->deleted) {
                                spin_unlock(&e->lock);
                                rcu_read_unlock();
                                return AUDIT_BUILD_CONTEXT;
                        }
                        rcu_read_unlock();
                        if (state == AUDIT_STATE_RECORD)
                                *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
                        return state;
                }
        }
        rcu_read_unlock();
        return AUDIT_BUILD_CONTEXT;
}”…””}”(hhhjÐ  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h MQhj¶  hžhubhÆ)”}”(hŒhThe ``audit_del_rule()`` function would need to set the ``deleted`` flag under the
spinlock as follows::”h]”(hŒThe ”…””}”(hŒThe ”hjÞ  hžhhŸNh NubhÑ)”}”(hŒ``audit_del_rule()``”h]”hŒaudit_del_rule()”…””}”(hhhjç  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjÞ  ubhŒ  function would need to set the ”…””}”(hŒ  function would need to set the ”hjÞ  hžhhŸNh NubhÑ)”}”(hŒ``deleted``”h]”hŒdeleted”…””}”(hhhjú  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjÞ  ubhŒ$ flag under the
spinlock as follows:”…””}”(hŒ$ flag under the
spinlock as follows:”hjÞ  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Mihj¶  hžhubjO  )”}”(hXÌ  static inline int audit_del_rule(struct audit_rule *rule,
                                 struct list_head *list)
{
        struct audit_entry *e;

        /* No need to use the _rcu iterator here, since this
         * is the only deletion routine. */
        list_for_each_entry(e, list, list) {
                if (!audit_compare_rule(rule, &e->rule)) {
                        spin_lock(&e->lock);
                        list_del_rcu(&e->list);
                        e->deleted = 1;
                        spin_unlock(&e->lock);
                        call_rcu(&e->rcu, audit_free_rule);
                        return 0;
                }
        }
        return -EFAULT;         /* No matching rule */
}”h]”hXÌ  static inline int audit_del_rule(struct audit_rule *rule,
                                 struct list_head *list)
{
        struct audit_entry *e;

        /* No need to use the _rcu iterator here, since this
         * is the only deletion routine. */
        list_for_each_entry(e, list, list) {
                if (!audit_compare_rule(rule, &e->rule)) {
                        spin_lock(&e->lock);
                        list_del_rcu(&e->list);
                        e->deleted = 1;
                        spin_unlock(&e->lock);
                        call_rcu(&e->rcu, audit_free_rule);
                        return 0;
                }
        }
        return -EFAULT;         /* No matching rule */
}”…””}”(hhhj  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h Mlhj¶  hžhubhÆ)”}”(hŒ>This too assumes that the caller holds ``audit_filter_mutex``.”h]”(hŒ'This too assumes that the caller holds ”…””}”(hŒ'This too assumes that the caller holds ”hj!  hžhhŸNh NubhÑ)”}”(hŒ``audit_filter_mutex``”h]”hŒaudit_filter_mutex”…””}”(hhhj*  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj!  ubhŒ.”…””}”(hŒ.”hj!  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h M€hj¶  hžhubhÆ)”}”(hXL  Note that this example assumes that entries are only added and deleted.
Additional mechanism is required to deal correctly with the update-in-place
performed by audit_upd_rule().  For one thing, audit_upd_rule() would
need to hold the locks of both the old ``audit_entry`` and its replacement
while executing the list_replace_rcu().”h]”(hX  Note that this example assumes that entries are only added and deleted.
Additional mechanism is required to deal correctly with the update-in-place
performed by audit_upd_rule().  For one thing, audit_upd_rule() would
need to hold the locks of both the old ”…””}”(hX  Note that this example assumes that entries are only added and deleted.
Additional mechanism is required to deal correctly with the update-in-place
performed by audit_upd_rule().  For one thing, audit_upd_rule() would
need to hold the locks of both the old ”hjC  hžhhŸNh NubhÑ)”}”(hŒ``audit_entry``”h]”hŒaudit_entry”…””}”(hhhjL  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjC  ubhŒ< and its replacement
while executing the list_replace_rcu().”…””}”(hŒ< and its replacement
while executing the list_replace_rcu().”hjC  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h M‚hj¶  hžhubeh}”(h]”Œ example-4-eliminating-stale-data”ah ]”h"]”Œ!example 4: eliminating stale data”ah$]”h&]”uh1h°hh²hžhhŸh¯h M6ubh±)”}”(hhh]”(h¶)”}”(hŒ!Example 5: Skipping Stale Objects”h]”hŒ!Example 5: Skipping Stale Objects”…””}”(hjr  hjp  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hµhjm  hžhhŸh¯h MŠubhÆ)”}”(hX‰  For some use cases, reader performance can be improved by skipping
stale objects during read-side list traversal, where stale objects
are those that will be removed and destroyed after one or more grace
periods. One such example can be found in the timerfd subsystem. When a
``CLOCK_REALTIME`` clock is reprogrammed (for example due to setting
of the system time) then all programmed ``timerfds`` that depend on
this clock get triggered and processes waiting on them are awakened in
advance of their scheduled expiry. To facilitate this, all such timers
are added to an RCU-managed ``cancel_list`` when they are setup in
``timerfd_setup_cancel()``::”h]”(hX  For some use cases, reader performance can be improved by skipping
stale objects during read-side list traversal, where stale objects
are those that will be removed and destroyed after one or more grace
periods. One such example can be found in the timerfd subsystem. When a
”…””}”(hX  For some use cases, reader performance can be improved by skipping
stale objects during read-side list traversal, where stale objects
are those that will be removed and destroyed after one or more grace
periods. One such example can be found in the timerfd subsystem. When a
”hj~  hžhhŸNh NubhÑ)”}”(hŒ``CLOCK_REALTIME``”h]”hŒCLOCK_REALTIME”…””}”(hhhj‡  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj~  ubhŒ[ clock is reprogrammed (for example due to setting
of the system time) then all programmed ”…””}”(hŒ[ clock is reprogrammed (for example due to setting
of the system time) then all programmed ”hj~  hžhhŸNh NubhÑ)”}”(hŒ``timerfds``”h]”hŒtimerfds”…””}”(hhhjš  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj~  ubhŒº that depend on
this clock get triggered and processes waiting on them are awakened in
advance of their scheduled expiry. To facilitate this, all such timers
are added to an RCU-managed ”…””}”(hŒº that depend on
this clock get triggered and processes waiting on them are awakened in
advance of their scheduled expiry. To facilitate this, all such timers
are added to an RCU-managed ”hj~  hžhhŸNh NubhÑ)”}”(hŒ``cancel_list``”h]”hŒcancel_list”…””}”(hhhj­  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj~  ubhŒ when they are setup in
”…””}”(hŒ when they are setup in
”hj~  hžhhŸNh NubhÑ)”}”(hŒ``timerfd_setup_cancel()``”h]”hŒtimerfd_setup_cancel()”…””}”(hhhjÀ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj~  ubhŒ:”…””}”(hŒ:”hj~  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h MŒhjm  hžhubjO  )”}”(hX§  static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
{
        spin_lock(&ctx->cancel_lock);
        if ((ctx->clockid == CLOCK_REALTIME ||
             ctx->clockid == CLOCK_REALTIME_ALARM) &&
            (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) {
                if (!ctx->might_cancel) {
                        ctx->might_cancel = true;
                        spin_lock(&cancel_lock);
                        list_add_rcu(&ctx->clist, &cancel_list);
                        spin_unlock(&cancel_lock);
                }
        } else {
                __timerfd_remove_cancel(ctx);
        }
        spin_unlock(&ctx->cancel_lock);
}”h]”hX§  static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
{
        spin_lock(&ctx->cancel_lock);
        if ((ctx->clockid == CLOCK_REALTIME ||
             ctx->clockid == CLOCK_REALTIME_ALARM) &&
            (flags & TFD_TIMER_ABSTIME) && (flags & TFD_TIMER_CANCEL_ON_SET)) {
                if (!ctx->might_cancel) {
                        ctx->might_cancel = true;
                        spin_lock(&cancel_lock);
                        list_add_rcu(&ctx->clist, &cancel_list);
                        spin_unlock(&cancel_lock);
                }
        } else {
                __timerfd_remove_cancel(ctx);
        }
        spin_unlock(&ctx->cancel_lock);
}”…””}”(hhhjÙ  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h M—hjm  hžhubhÆ)”}”(hŒêWhen a timerfd is freed (fd is closed), then the ``might_cancel``
flag of the timerfd object is cleared, the object removed from the
``cancel_list`` and destroyed, as shown in this simplified and inlined
version of timerfd_release()::”h]”(hŒ1When a timerfd is freed (fd is closed), then the ”…””}”(hŒ1When a timerfd is freed (fd is closed), then the ”hjç  hžhhŸNh NubhÑ)”}”(hŒ``might_cancel``”h]”hŒmight_cancel”…””}”(hhhjð  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjç  ubhŒD
flag of the timerfd object is cleared, the object removed from the
”…””}”(hŒD
flag of the timerfd object is cleared, the object removed from the
”hjç  hžhhŸNh NubhÑ)”}”(hŒ``cancel_list``”h]”hŒcancel_list”…””}”(hhhj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhjç  ubhŒU and destroyed, as shown in this simplified and inlined
version of timerfd_release():”…””}”(hŒU and destroyed, as shown in this simplified and inlined
version of timerfd_release():”hjç  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h M©hjm  hžhubjO  )”}”(hXJ  int timerfd_release(struct inode *inode, struct file *file)
{
        struct timerfd_ctx *ctx = file->private_data;

        spin_lock(&ctx->cancel_lock);
        if (ctx->might_cancel) {
                ctx->might_cancel = false;
                spin_lock(&cancel_lock);
                list_del_rcu(&ctx->clist);
                spin_unlock(&cancel_lock);
        }
        spin_unlock(&ctx->cancel_lock);

        if (isalarm(ctx))
                alarm_cancel(&ctx->t.alarm);
        else
                hrtimer_cancel(&ctx->t.tmr);
        kfree_rcu(ctx, rcu);
        return 0;
}”h]”hXJ  int timerfd_release(struct inode *inode, struct file *file)
{
        struct timerfd_ctx *ctx = file->private_data;

        spin_lock(&ctx->cancel_lock);
        if (ctx->might_cancel) {
                ctx->might_cancel = false;
                spin_lock(&cancel_lock);
                list_del_rcu(&ctx->clist);
                spin_unlock(&cancel_lock);
        }
        spin_unlock(&ctx->cancel_lock);

        if (isalarm(ctx))
                alarm_cancel(&ctx->t.alarm);
        else
                hrtimer_cancel(&ctx->t.tmr);
        kfree_rcu(ctx, rcu);
        return 0;
}”…””}”(hhhj  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h M®hjm  hžhubhÆ)”}”(hX0  If the ``CLOCK_REALTIME`` clock is set, for example by a time server, the
hrtimer framework calls ``timerfd_clock_was_set()`` which walks the
``cancel_list`` and wakes up processes waiting on the timerfd. While iterating
the ``cancel_list``, the ``might_cancel`` flag is consulted to skip stale
objects::”h]”(hŒIf the ”…””}”(hŒIf the ”hj*  hžhhŸNh NubhÑ)”}”(hŒ``CLOCK_REALTIME``”h]”hŒCLOCK_REALTIME”…””}”(hhhj3  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj*  ubhŒI clock is set, for example by a time server, the
hrtimer framework calls ”…””}”(hŒI clock is set, for example by a time server, the
hrtimer framework calls ”hj*  hžhhŸNh NubhÑ)”}”(hŒ``timerfd_clock_was_set()``”h]”hŒtimerfd_clock_was_set()”…””}”(hhhjF  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj*  ubhŒ which walks the
”…””}”(hŒ which walks the
”hj*  hžhhŸNh NubhÑ)”}”(hŒ``cancel_list``”h]”hŒcancel_list”…””}”(hhhjY  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj*  ubhŒD and wakes up processes waiting on the timerfd. While iterating
the ”…””}”(hŒD and wakes up processes waiting on the timerfd. While iterating
the ”hj*  hžhhŸNh NubhÑ)”}”(hŒ``cancel_list``”h]”hŒcancel_list”…””}”(hhhjl  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj*  ubhŒ, the ”…””}”(hŒ, the ”hj*  hžhhŸNh NubhÑ)”}”(hŒ``might_cancel``”h]”hŒmight_cancel”…””}”(hhhj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj*  ubhŒ) flag is consulted to skip stale
objects:”…””}”(hŒ) flag is consulted to skip stale
objects:”hj*  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h MÃhjm  hžhubjO  )”}”(hX£  void timerfd_clock_was_set(void)
{
        ktime_t moffs = ktime_mono_to_real(0);
        struct timerfd_ctx *ctx;
        unsigned long flags;

        rcu_read_lock();
        list_for_each_entry_rcu(ctx, &cancel_list, clist) {
                if (!ctx->might_cancel)
                        continue;
                spin_lock_irqsave(&ctx->wqh.lock, flags);
                if (ctx->moffs != moffs) {
                        ctx->moffs = KTIME_MAX;
                        ctx->ticks++;
                        wake_up_locked_poll(&ctx->wqh, EPOLLIN);
                }
                spin_unlock_irqrestore(&ctx->wqh.lock, flags);
        }
        rcu_read_unlock();
}”h]”hX£  void timerfd_clock_was_set(void)
{
        ktime_t moffs = ktime_mono_to_real(0);
        struct timerfd_ctx *ctx;
        unsigned long flags;

        rcu_read_lock();
        list_for_each_entry_rcu(ctx, &cancel_list, clist) {
                if (!ctx->might_cancel)
                        continue;
                spin_lock_irqsave(&ctx->wqh.lock, flags);
                if (ctx->moffs != moffs) {
                        ctx->moffs = KTIME_MAX;
                        ctx->ticks++;
                        wake_up_locked_poll(&ctx->wqh, EPOLLIN);
                }
                spin_unlock_irqrestore(&ctx->wqh.lock, flags);
        }
        rcu_read_unlock();
}”…””}”(hhhj˜  ubah}”(h]”h ]”h"]”h$]”h&]”j^  j_  uh1jN  hŸh¯h MÉhjm  hžhubhÆ)”}”(hX  The key point is that because RCU-protected traversal of the
``cancel_list`` happens concurrently with object addition and removal,
sometimes the traversal can access an object that has been removed from
the list. In this example, a flag is used to skip such objects.”h]”(hŒ=The key point is that because RCU-protected traversal of the
”…””}”(hŒ=The key point is that because RCU-protected traversal of the
”hj¦  hžhhŸNh NubhÑ)”}”(hŒ``cancel_list``”h]”hŒcancel_list”…””}”(hhhj¯  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÐhj¦  ubhŒ¿ happens concurrently with object addition and removal,
sometimes the traversal can access an object that has been removed from
the list. In this example, a flag is used to skip such objects.”…””}”(hŒ¿ happens concurrently with object addition and removal,
sometimes the traversal can access an object that has been removed from
the list. In this example, a flag is used to skip such objects.”hj¦  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h MÞhjm  hžhubeh}”(h]”Œ example-5-skipping-stale-objects”ah ]”h"]”Œ!example 5: skipping stale objects”ah$]”h&]”uh1h°hh²hžhhŸh¯h MŠubh±)”}”(hhh]”(h¶)”}”(hŒSummary”h]”hŒSummary”…””}”(hjÕ  hjÓ  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hµhjÐ  hžhhŸh¯h MåubhÆ)”}”(hX  Read-mostly list-based data structures that can tolerate stale data are
the most amenable to use of RCU.  The simplest case is where entries are
either added or deleted from the data structure (or atomically modified
in place), but non-atomic in-place modifications can be handled by making
a copy, updating the copy, then replacing the original with the copy.
If stale data cannot be tolerated, then a *deleted* flag may be used
in conjunction with a per-entry spinlock in order to allow the search
function to reject newly deleted data.”h]”(hX“  Read-mostly list-based data structures that can tolerate stale data are
the most amenable to use of RCU.  The simplest case is where entries are
either added or deleted from the data structure (or atomically modified
in place), but non-atomic in-place modifications can be handled by making
a copy, updating the copy, then replacing the original with the copy.
If stale data cannot be tolerated, then a ”…””}”(hX“  Read-mostly list-based data structures that can tolerate stale data are
the most amenable to use of RCU.  The simplest case is where entries are
either added or deleted from the data structure (or atomically modified
in place), but non-atomic in-place modifications can be handled by making
a copy, updating the copy, then replacing the original with the copy.
If stale data cannot be tolerated, then a ”hjá  hžhhŸNh Nubjì  )”}”(hŒ	*deleted*”h]”hŒdeleted”…””}”(hhhjê  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jë  hjá  ubhŒ~ flag may be used
in conjunction with a per-entry spinlock in order to allow the search
function to reject newly deleted data.”…””}”(hŒ~ flag may be used
in conjunction with a per-entry spinlock in order to allow the search
function to reject newly deleted data.”hjá  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h MçhjÐ  hžhubh¢)”}”(hŒ.. _quick_quiz_answer:”h]”h}”(h]”h ]”h"]”h$]”h&]”h­Œquick-quiz-answer”uh1h¡h MðhjÐ  hžhhŸh¯ubj  )”}”(hhh]”j  )”}”(hX  Answer to Quick Quiz:
For the deleted-flag technique to be helpful, why is it necessary
to hold the per-entry lock while returning from the search function?

If the search function drops the per-entry lock before returning,
then the caller will be processing stale data in any case.  If it
is really OK to be processing stale data, then you don't need a
*deleted* flag.  If processing stale data really is a problem,
then you need to hold the per-entry lock across all of the code
that uses the value that was returned.
”h]”(j!  )”}”(hŒAnswer to Quick Quiz:”h]”hŒAnswer to Quick Quiz:”…””}”(hj  hj  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1j   hŸh¯h Mûhj  ubj1  )”}”(hhh]”(hÆ)”}”(hŒ†For the deleted-flag technique to be helpful, why is it necessary
to hold the per-entry lock while returning from the search function?”h]”hŒ†For the deleted-flag technique to be helpful, why is it necessary
to hold the per-entry lock while returning from the search function?”…””}”(hj(  hj&  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Móhj#  ubhÆ)”}”(hXi  If the search function drops the per-entry lock before returning,
then the caller will be processing stale data in any case.  If it
is really OK to be processing stale data, then you don't need a
*deleted* flag.  If processing stale data really is a problem,
then you need to hold the per-entry lock across all of the code
that uses the value that was returned.”h]”(hŒÆIf the search function drops the per-entry lock before returning,
then the caller will be processing stale data in any case.  If it
is really OK to be processing stale data, then you donâ€™t need a
”…””}”(hŒÄIf the search function drops the per-entry lock before returning,
then the caller will be processing stale data in any case.  If it
is really OK to be processing stale data, then you don't need a
”hj4  hžhhŸNh Nubjì  )”}”(hŒ	*deleted*”h]”hŒdeleted”…””}”(hhhj=  hžhhŸNh Nubah}”(h]”h ]”h"]”h$]”h&]”uh1jë  hj4  ubhŒœ flag.  If processing stale data really is a problem,
then you need to hold the per-entry lock across all of the code
that uses the value that was returned.”…””}”(hŒœ flag.  If processing stale data really is a problem,
then you need to hold the per-entry lock across all of the code
that uses the value that was returned.”•E      hj4  hžhhŸNh Nubeh}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h Möhj#  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1j0  hj  ubeh}”(h]”h ]”h"]”h$]”h&]”uh1j  hŸh¯h Mûhj  ubah}”(h]”j  ah ]”h"]”Œquick_quiz_answer”ah$]”h&]”uh1j  hjÐ  hžhhŸh¯h NjV  }”jf  j  sjX  }”j  j  subhÆ)”}”(hŒ&:ref:`Back to Quick Quiz <quick_quiz>`”h]”h)”}”(hjm  h]”jb  )”}”(hjm  h]”hŒBack to Quick Quiz”…””}”(hhhjr  hžhhŸNh Nubah}”(h]”h ]”(jm  Œstd”Œstd-ref”eh"]”h$]”h&]”uh1ja  hjo  ubah}”(h]”h ]”h"]”h$]”h&]”Œrefdoc”jz  Œ	refdomain”j|  Œreftype”Œref”Œrefexplicit”ˆŒrefwarn”ˆj€  Œ
quick_quiz”uh1hhŸh¯h Mýhjk  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhŸh¯h MýhjÐ  hžhubeh}”(h]”Œsummary”ah ]”h"]”Œsummary”ah$]”h&]”uh1h°hh²hžhhŸh¯h Måubeh}”(h]”(Œ-using-rcu-to-protect-read-mostly-linked-lists”h®eh ]”h"]”(Œ-using rcu to protect read-mostly linked lists”Œlist_rcu_doc”eh$]”h&]”uh1h°hhhžhhŸh¯h KjV  }”j¢  h£sjX  }”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
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”jÊ  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”h¯Œ_destination”NŒ_config_files”]”Œ7/var/lib/git/docbuild/linux/Documentation/docutils.conf”aŒ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file_insertion_enabled”ˆŒraw_enabled”KŒline_length_limit”M'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”‰Œembed_images”‰Œenv”NubŒreporter”NŒindirect_targets”]”Œsubstitution_defs”}”Œsubstitution_names”}”Œrefnames”}”Œrefids”}”(h®]”h£aj  ]”j
  aj  ]”j  auŒnameids”}”(j¢  h®j¡  jž  jn  jk  jž  j›  j³  j°  jj  jg  jS  j  jÍ  jÊ  j™  j–  jf  j  uŒ	nametypes”}”(j¢  ˆj¡  Njn  Njž  Nj³  Njj  NjS  ˆjÍ  Nj™  Njf  ˆuh}”(h®h²jž  h²jk  hùj›  jq  j°  j¡  jg  j¶  j  j  jÊ  jm  j–  jÐ  j  j  uŒfootnote_refs”}”Œcitation_refs”}”Œautofootnotes”]”Œautofootnote_refs”]”Œsymbol_footnotes”]”Œsymbol_footnote_refs”]”Œ	footnotes”]”Œ	citations”]”Œautofootnote_start”KŒsymbol_footnote_start”K Œ
id_counter”Œcollections”ŒCounter”“”}”…”R”Œparse_messages”]”Œtransform_messages”]”(hŒsystem_message”“”)”}”(hhh]”hÆ)”}”(hhh]”hŒ2Hyperlink target "list-rcu-doc" is not referenced.”…””}”(hhhj7	  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhj4	  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”ŒINFO”Œsource”h¯Œline”Kuh1j2	  ubj3	  )”}”(hhh]”hÆ)”}”(hhh]”hŒ0Hyperlink target "quick-quiz" is not referenced.”…””}”(hhhjR	  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhjO	  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jL	  Œsource”h¯Œline”MEuh1j2	  ubj3	  )”}”(hhh]”hÆ)”}”(hhh]”hŒ7Hyperlink target "quick-quiz-answer" is not referenced.”…””}”(hhhjl	  ubah}”(h]”h ]”h"]”h$]”h&]”uh1hÅhji	  ubah}”(h]”h ]”h"]”h$]”h&]”Œlevel”KŒtype”jL	  Œsource”h¯Œline”Mðuh1j2	  ubeŒtransformer”NŒinclude_log”]”Œ
decoration”Nhžhub.