doc: Add some more RCU list patterns in the kernel

=============================================

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

(``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-Side Action Taken Outside of Lock, No In-Place Updates

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

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::

implementation of ``audit_filter_task()`` might be as follows::

	static enum audit_state audit_filter_task(struct task_struct *tsk)

	{

		enum audit_state   state;

		read_lock(&auditsc_lock);

		/* Note: audit_netlink_sem held by caller. */

		/* 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);

		enum audit_state   state;

		rcu_read_lock();

		/* Note: audit_netlink_sem held by caller. */

		/* 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 AUDIT_BUILD_CONTEXT;

	}

The read_lock() and read_unlock() calls have become rcu_read_lock()

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

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::

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;

		/* Do not use the _rcu iterator here, since this is the only

		/* 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)) {

		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_netlink_sem, 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.  Normally, the write_lock() calls would

be converted into spin_lock() calls.

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.

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!

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 2: Handling In-Place Updates

Example 3: Handling In-Place Updates

------------------------------------

The system-call auditing code does not update auditing rules in place.

However, if it did, reader-writer-locked code to do so might look as

follows (presumably, the field_count is only permitted to decrease,

otherwise, the added fields would need to be filled in)::

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 newfield_count)

	{

		struct audit_entry *e;

		struct audit_newentry *ne;

		struct audit_entry *ne;

		write_lock(&auditsc_lock);

		/* Note: audit_netlink_sem held by caller. */

		/* 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;

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 doing a copy to perform an update, is what gives

RCU ("read-copy update") its name.  The RCU code is as follows::

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 newfield_count)

	{

		struct audit_entry *e;

		struct audit_newentry *ne;

		struct audit_entry *ne;

		list_for_each_entry(e, list, list) {

			if (!audit_compare_rule(rule, &e->rule)) {

		return -EFAULT;		/* No matching rule */

	}

Again, this assumes that the caller holds audit_netlink_sem.  Normally,

the reader-writer lock would become a spinlock in this sort of code.

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 3: Eliminating Stale Data

Example 4: Eliminating Stale Data

---------------------------------

The auditing examples above tolerate stale data, as do most algorithms

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 normally not a problem.

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 ipc_lock()

function in ipc/util.c).  This code checks a "deleted" flag under a

per-entry spinlock, and, if the "deleted" flag is set, pretends that the

function in ipc/util.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 ipc_lock() does in fact do.

return holding the per-entry lock, as ipc_lock() does in fact do.

.. _quick_quiz:

Quick Quiz:

	Why does the search function need to return holding the per-entry lock for

	this deleted-flag technique to be helpful?

	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 <answer_quick_quiz_list>`

: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::

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)

	{

	}

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().

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::

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;

		/* Do not need to use the _rcu iterator here, since this

		/* 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)) {

		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

-------

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

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.

.. _answer_quick_quiz_list:

.. _quick_quiz_answer:

Answer to Quick Quiz:

	Why does the search function need to return holding the per-entry

	lock for this deleted-flag technique to be helpful?

	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,

	*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>`