Documentation/RCU/listRCU.rst - linux - Git at Google

 .. _list_rcu_doc:

 Using RCU to Protect Read-Mostly Linked Lists
 =============================================

 One of the best applications of RCU is to protect read-mostly linked lists
 (``struct list_head`` in list.h).  One big advantage of this approach
 is that all of the required memory barriers are included for you in
 the list macros.  This document describes several applications of RCU,
 with the best fits first.


 Example 1: Read-mostly list: Deferred Destruction
 -------------------------------------------------

 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.

 The traversal of the list is done using ``for_each_process()`` which is defined
 by the 2 macros::

 	#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 ; )

 The code traversing the list of all processes typically looks like::

 	rcu_read_lock();
 	for_each_process(p) {
 		/* Do something with p */
 	}
 	rcu_read_unlock();

 The simplified code for removing a process from a task list is::

 	void 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);
 	}

 When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)`` under
 ``tasklist_lock`` writer lock protection, to remove 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()). 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 pins the object in
 memory until all existing readers finish.


 Example 2: Read-Side Action Taken Outside of Lock: No In-Place Updates
 ----------------------------------------------------------------------

 The best applications are cases where, if reader-writer locking were
 used, the read-side lock would be dropped before taking any action
 based on the results of the search.  The most celebrated example is
 the routing table.  Because the routing table is tracking 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.

 A straightforward example of this use of RCU may be found in the
 system-call auditing support.  For example, a reader-writer locked
 implementation of ``audit_filter_task()`` might be as follows::

 	static enum audit_state audit_filter_task(struct task_struct *tsk)
 	{
 		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)) {
 				read_unlock(&auditsc_lock);
 				return state;
 			}
 		}
 		read_unlock(&auditsc_lock);
 		return AUDIT_BUILD_CONTEXT;
 	}

 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.

 This means that RCU can be easily applied to the read side, as follows::

 	static enum audit_state audit_filter_task(struct task_struct *tsk)
 	{
 		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)) {
 				rcu_read_unlock();
 				return state;
 			}
 		}
 		rcu_read_unlock();
 		return AUDIT_BUILD_CONTEXT;
 	}

 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
 insert the read-side memory barriers that are required on DEC Alpha CPUs.

 The changes to the update side are also straightforward. A reader-writer lock
 might be used as follows for deletion and insertion::

 	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;
 	}

 Following are the RCU equivalents for these two functions::

 	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;
 	}

 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.

 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 (most of them!).  The list_del_rcu() primitive omits the
 pointer poisoning debug-assist code that would otherwise cause concurrent
 readers to fail spectacularly.

 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!


 Example 3: Handling In-Place Updates
 ------------------------------------

 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 ``field_count`` is updated, otherwise, the added fields would
 need to be filled in)::

 	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 */
 	}

 The 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.  The RCU code is as follows::

 	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 */
 	}

 Again, this assumes that the caller holds ``audit_filter_mutex``.  Normally, the
 writer lock would become a spinlock in this sort of code.

 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().


 Example 4: Eliminating Stale Data
 ---------------------------------

 The auditing example above tolerates stale data, as do most algorithms
 that are tracking external state.  Because there is a delay from the
 time the external state changes before Linux becomes aware of the change,
 additional RCU-induced staleness is generally not a problem.

 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.

 .. _quick_quiz:

 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?

 :ref:`Answer to Quick Quiz <quick_quiz_answer>`

 If 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::

 	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();
 				return state;
 			}
 		}
 		rcu_read_unlock();
 		return AUDIT_BUILD_CONTEXT;
 	}

 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 additional memory barriers to ensure that the list_add_rcu() was really
 executed before the list_del_rcu().

 The ``audit_del_rule()`` function would need to set the ``deleted`` flag under the
 spinlock as follows::

 	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 */
 	}

 This too assumes that the caller holds ``audit_filter_mutex``.


 Example 5: Skipping Stale Objects
 ---------------------------------

 For some usecases, reader performance can be improved by skipping stale objects
 during read-side list traversal if the object in concern is pending destruction
 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 to expire are woken up 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()``::

 	static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
 	{
 		spin_lock(&ctx->cancel_lock);
 		if ((ctx->clockid == CLOCK_REALTIME &&
 		    (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);
 			}
 		}
 		spin_unlock(&ctx->cancel_lock);
 	}

 When 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::

 	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);

 		hrtimer_cancel(&ctx->t.tmr);
 		kfree_rcu(ctx, rcu);
 		return 0;
 	}

 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::

 	void timerfd_clock_was_set(void)
 	{
 		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 != ktime_mono_to_real(0)) {
 				ctx->moffs = KTIME_MAX;
 				ctx->ticks++;
 				wake_up_locked_poll(&ctx->wqh, EPOLLIN);
 			}
 			spin_unlock_irqrestore(&ctx->wqh.lock, flags);
 		}
 		rcu_read_unlock();
 	}

 The key point here is, because RCU-traversal of the ``cancel_list`` happens
 while objects are being added and removed to the list, sometimes the traversal
 can step on an object that has been removed from the list. In this example, it
 is seen that it is better to skip such objects using a flag.


 Summary
 -------

 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.

 .. _quick_quiz_answer:

 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.

 :ref:`Back to Quick Quiz <quick_quiz>`
	.. _list_rcu_doc:

	Using RCU to Protect Read-Mostly Linked Lists
	=============================================

	One of the best applications of RCU is to protect read-mostly linked lists
	(``struct list_head`` in list.h). One big advantage of this approach
	is that all of the required memory barriers are included for you in
	the list macros. This document describes several applications of RCU,
	with the best fits first.


	Example 1: Read-mostly list: Deferred Destruction
	-------------------------------------------------

	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.

	The traversal of the list is done using ``for_each_process()`` which is defined
	by the 2 macros::

	#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 ; )

	The code traversing the list of all processes typically looks like::

	rcu_read_lock();
	for_each_process(p) {
	/* Do something with p */
	}
	rcu_read_unlock();

	The simplified code for removing a process from a task list is::

	void 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);
	}

	When a process exits, ``release_task()`` calls ``list_del_rcu(&p->tasks)`` under
	``tasklist_lock`` writer lock protection, to remove 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()). 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 pins the object in
	memory until all existing readers finish.


	Example 2: Read-Side Action Taken Outside of Lock: No In-Place Updates
	----------------------------------------------------------------------

	The best applications are cases where, if reader-writer locking were
	used, the read-side lock would be dropped before taking any action
	based on the results of the search. The most celebrated example is
	the routing table. Because the routing table is tracking 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.

	A straightforward example of this use of RCU may be found in the
	system-call auditing support. For example, a reader-writer locked
	implementation of ``audit_filter_task()`` might be as follows::

	static enum audit_state audit_filter_task(struct task_struct *tsk)
	{
	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)) {
	read_unlock(&auditsc_lock);
	return state;
	}
	}
	read_unlock(&auditsc_lock);
	return AUDIT_BUILD_CONTEXT;
	}

	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.

	This means that RCU can be easily applied to the read side, as follows::

	static enum audit_state audit_filter_task(struct task_struct *tsk)
	{
	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)) {
	rcu_read_unlock();
	return state;
	}
	}
	rcu_read_unlock();
	return AUDIT_BUILD_CONTEXT;
	}

	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
	insert the read-side memory barriers that are required on DEC Alpha CPUs.

	The changes to the update side are also straightforward. A reader-writer lock
	might be used as follows for deletion and insertion::

	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;
	}

	Following are the RCU equivalents for these two functions::

	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;
	}

	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.

	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 (most of them!). The list_del_rcu() primitive omits the
	pointer poisoning debug-assist code that would otherwise cause concurrent
	readers to fail spectacularly.

	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!


	Example 3: Handling In-Place Updates
	------------------------------------

	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 ``field_count`` is updated, otherwise, the added fields would
	need to be filled in)::

	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 */
	}

	The 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. The RCU code is as follows::

	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 */
	}

	Again, this assumes that the caller holds ``audit_filter_mutex``. Normally, the
	writer lock would become a spinlock in this sort of code.

	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().


	Example 4: Eliminating Stale Data
	---------------------------------

	The auditing example above tolerates stale data, as do most algorithms
	that are tracking external state. Because there is a delay from the
	time the external state changes before Linux becomes aware of the change,
	additional RCU-induced staleness is generally not a problem.

	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.

	.. _quick_quiz:

	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?

	:ref:`Answer to Quick Quiz <quick_quiz_answer>`

	If 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::

	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();
	return state;
	}
	}
	rcu_read_unlock();
	return AUDIT_BUILD_CONTEXT;
	}

	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 additional memory barriers to ensure that the list_add_rcu() was really
	executed before the list_del_rcu().

	The ``audit_del_rule()`` function would need to set the ``deleted`` flag under the
	spinlock as follows::

	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 */
	}

	This too assumes that the caller holds ``audit_filter_mutex``.


	Example 5: Skipping Stale Objects
	---------------------------------

	For some usecases, reader performance can be improved by skipping stale objects
	during read-side list traversal if the object in concern is pending destruction
	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 to expire are woken up 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()``::

	static void timerfd_setup_cancel(struct timerfd_ctx *ctx, int flags)
	{
	spin_lock(&ctx->cancel_lock);
	if ((ctx->clockid == CLOCK_REALTIME &&
	(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);
	}
	}
	spin_unlock(&ctx->cancel_lock);
	}

	When 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::

	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);

	hrtimer_cancel(&ctx->t.tmr);
	kfree_rcu(ctx, rcu);
	return 0;
	}

	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::

	void timerfd_clock_was_set(void)
	{
	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 != ktime_mono_to_real(0)) {
	ctx->moffs = KTIME_MAX;
	ctx->ticks++;
	wake_up_locked_poll(&ctx->wqh, EPOLLIN);
	}
	spin_unlock_irqrestore(&ctx->wqh.lock, flags);
	}
	rcu_read_unlock();
	}

	The key point here is, because RCU-traversal of the ``cancel_list`` happens
	while objects are being added and removed to the list, sometimes the traversal
	can step on an object that has been removed from the list. In this example, it
	is seen that it is better to skip such objects using a flag.


	Summary
	-------

	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.

	.. _quick_quiz_answer:

	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.

	:ref:`Back to Quick Quiz <quick_quiz>`