Documentation/RCU/listRCU.txt

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-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_netlink_sem 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_netlink_sem 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;

		/* Do not 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, e);
				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_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.

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 2: 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):

	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_newentry *ne;

		write_lock(&auditsc_lock);
		/* Note: audit_netlink_sem held by caller. */
		list_for_each_entry(e, list, list) {
			if (!audit_compare_rule(rule, &e->rule)) {
				e->rule.action = newaction;
				e->rule.file_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 doing 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_newentry *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.file_count = newfield_count;
				list_replace_rcu(e, ne);
				call_rcu(&e->rcu, audit_free_rule, e);
				return 0;
			}
		}
		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.


Example 3: Eliminating Stale Data

The auditing examples above tolerate 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.

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
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.

Quick Quiz:  Why does the search function need to return holding the
per-entry lock for this deleted-flag technique to be helpful?

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;

		/* Do not 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, e);
				return 0;
			}
		}
		return -EFAULT;		/* No matching rule */
	}


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.


Answer to Quick Quiz

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 looked up.
1	Using RCU to Protect Read-Mostly Linked Lists
2
3
4	One of the best applications of RCU is to protect read-mostly linked lists
5	("struct list_head" in list.h). One big advantage of this approach
6	is that all of the required memory barriers are included for you in
7	the list macros. This document describes several applications of RCU,
8	with the best fits first.
9
10
11	Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates
12
13	The best applications are cases where, if reader-writer locking were
14	used, the read-side lock would be dropped before taking any action
15	based on the results of the search. The most celebrated example is
16	the routing table. Because the routing table is tracking the state of
17	equipment outside of the computer, it will at times contain stale data.
18	Therefore, once the route has been computed, there is no need to hold
19	the routing table static during transmission of the packet. After all,
20	you can hold the routing table static all you want, but that won't keep
21	the external Internet from changing, and it is the state of the external
22	Internet that really matters. In addition, routing entries are typically
23	added or deleted, rather than being modified in place.
24
25	A straightforward example of this use of RCU may be found in the
26	system-call auditing support. For example, a reader-writer locked
27	implementation of audit_filter_task() might be as follows:
28
29	static enum audit_state audit_filter_task(struct task_struct *tsk)
30	{
31	struct audit_entry *e;
32	enum audit_state state;
33
34	read_lock(&auditsc_lock);
35	/* Note: audit_netlink_sem held by caller. */
36	list_for_each_entry(e, &audit_tsklist, list) {
37	if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
38	read_unlock(&auditsc_lock);
39	return state;
40	}
41	}
42	read_unlock(&auditsc_lock);
43	return AUDIT_BUILD_CONTEXT;
44	}
45
46	Here the list is searched under the lock, but the lock is dropped before
47	the corresponding value is returned. By the time that this value is acted
48	on, the list may well have been modified. This makes sense, since if
49	you are turning auditing off, it is OK to audit a few extra system calls.
50
51	This means that RCU can be easily applied to the read side, as follows:
52
53	static enum audit_state audit_filter_task(struct task_struct *tsk)
54	{
55	struct audit_entry *e;
56	enum audit_state state;
57
58	rcu_read_lock();
59	/* Note: audit_netlink_sem held by caller. */
60	list_for_each_entry_rcu(e, &audit_tsklist, list) {
61	if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
62	rcu_read_unlock();
63	return state;
64	}
65	}
66	rcu_read_unlock();
67	return AUDIT_BUILD_CONTEXT;
68	}
69
70	The read_lock() and read_unlock() calls have become rcu_read_lock()
71	and rcu_read_unlock(), respectively, and the list_for_each_entry() has
72	become list_for_each_entry_rcu(). The _rcu() list-traversal primitives
73	insert the read-side memory barriers that are required on DEC Alpha CPUs.
74
75	The changes to the update side are also straightforward. A reader-writer
76	lock might be used as follows for deletion and insertion:
77
78	static inline int audit_del_rule(struct audit_rule *rule,
79	struct list_head *list)
80	{
81	struct audit_entry *e;
82
83	write_lock(&auditsc_lock);
84	list_for_each_entry(e, list, list) {
85	if (!audit_compare_rule(rule, &e->rule)) {
86	list_del(&e->list);
87	write_unlock(&auditsc_lock);
88	return 0;
89	}
90	}
91	write_unlock(&auditsc_lock);
92	return -EFAULT; /* No matching rule */
93	}
94
95	static inline int audit_add_rule(struct audit_entry *entry,
96	struct list_head *list)
97	{
98	write_lock(&auditsc_lock);
99	if (entry->rule.flags & AUDIT_PREPEND) {
100	entry->rule.flags &= ~AUDIT_PREPEND;
101	list_add(&entry->list, list);
102	} else {
103	list_add_tail(&entry->list, list);
104	}
105	write_unlock(&auditsc_lock);
106	return 0;
107	}
108
109	Following are the RCU equivalents for these two functions:
110
111	static inline int audit_del_rule(struct audit_rule *rule,
112	struct list_head *list)
113	{
114	struct audit_entry *e;
115
116	/* Do not use the _rcu iterator here, since this is the only
117	* deletion routine. */
118	list_for_each_entry(e, list, list) {
119	if (!audit_compare_rule(rule, &e->rule)) {
120	list_del_rcu(&e->list);
121	call_rcu(&e->rcu, audit_free_rule, e);
122	return 0;
123	}
124	}
125	return -EFAULT; /* No matching rule */
126	}
127
128	static inline int audit_add_rule(struct audit_entry *entry,
129	struct list_head *list)
130	{
131	if (entry->rule.flags & AUDIT_PREPEND) {
132	entry->rule.flags &= ~AUDIT_PREPEND;
133	list_add_rcu(&entry->list, list);
134	} else {
135	list_add_tail_rcu(&entry->list, list);
136	}
137	return 0;
138	}
139
140	Normally, the write_lock() and write_unlock() would be replaced by
141	a spin_lock() and a spin_unlock(), but in this case, all callers hold
142	audit_netlink_sem, so no additional locking is required. The auditsc_lock
143	can therefore be eliminated, since use of RCU eliminates the need for
144	writers to exclude readers. Normally, the write_lock() calls would
145	be converted into spin_lock() calls.
146
147	The list_del(), list_add(), and list_add_tail() primitives have been
148	replaced by list_del_rcu(), list_add_rcu(), and list_add_tail_rcu().
149	The _rcu() list-manipulation primitives add memory barriers that are
150	needed on weakly ordered CPUs (most of them!). The list_del_rcu()
151	primitive omits the pointer poisoning debug-assist code that would
152	otherwise cause concurrent readers to fail spectacularly.
153
154	So, when readers can tolerate stale data and when entries are either added
155	or deleted, without in-place modification, it is very easy to use RCU!
156
157
158	Example 2: Handling In-Place Updates
159
160	The system-call auditing code does not update auditing rules in place.
161	However, if it did, reader-writer-locked code to do so might look as
162	follows (presumably, the field_count is only permitted to decrease,
163	otherwise, the added fields would need to be filled in):
164
165	static inline int audit_upd_rule(struct audit_rule *rule,
166	struct list_head *list,
167	__u32 newaction,
168	__u32 newfield_count)
169	{
170	struct audit_entry *e;
171	struct audit_newentry *ne;
172
173	write_lock(&auditsc_lock);
174	/* Note: audit_netlink_sem held by caller. */
175	list_for_each_entry(e, list, list) {
176	if (!audit_compare_rule(rule, &e->rule)) {
177	e->rule.action = newaction;
178	e->rule.file_count = newfield_count;
179	write_unlock(&auditsc_lock);
180	return 0;
181	}
182	}
183	write_unlock(&auditsc_lock);
184	return -EFAULT; /* No matching rule */
185	}
186
187	The RCU version creates a copy, updates the copy, then replaces the old
188	entry with the newly updated entry. This sequence of actions, allowing
189	concurrent reads while doing a copy to perform an update, is what gives
190	RCU ("read-copy update") its name. The RCU code is as follows:
191
192	static inline int audit_upd_rule(struct audit_rule *rule,
193	struct list_head *list,
194	__u32 newaction,
195	__u32 newfield_count)
196	{
197	struct audit_entry *e;
198	struct audit_newentry *ne;
199
200	list_for_each_entry(e, list, list) {
201	if (!audit_compare_rule(rule, &e->rule)) {
202	ne = kmalloc(sizeof(*entry), GFP_ATOMIC);
203	if (ne == NULL)
204	return -ENOMEM;
205	audit_copy_rule(&ne->rule, &e->rule);
206	ne->rule.action = newaction;
207	ne->rule.file_count = newfield_count;
208	list_replace_rcu(e, ne);
209	call_rcu(&e->rcu, audit_free_rule, e);
210	return 0;
211	}
212	}
213	return -EFAULT; /* No matching rule */
214	}
215
216	Again, this assumes that the caller holds audit_netlink_sem. Normally,
217	the reader-writer lock would become a spinlock in this sort of code.
218
219
220	Example 3: Eliminating Stale Data
221
222	The auditing examples above tolerate stale data, as do most algorithms
223	that are tracking external state. Because there is a delay from the
224	time the external state changes before Linux becomes aware of the change,
225	additional RCU-induced staleness is normally not a problem.
226
227	However, there are many examples where stale data cannot be tolerated.
228	One example in the Linux kernel is the System V IPC (see the ipc_lock()
229	function in ipc/util.c). This code checks a "deleted" flag under a
230	per-entry spinlock, and, if the "deleted" flag is set, pretends that the
231	entry does not exist. For this to be helpful, the search function must
232	return holding the per-entry spinlock, as ipc_lock() does in fact do.
233
234	Quick Quiz: Why does the search function need to return holding the
235	per-entry lock for this deleted-flag technique to be helpful?
236
237	If the system-call audit module were to ever need to reject stale data,
238	one way to accomplish this would be to add a "deleted" flag and a "lock"
239	spinlock to the audit_entry structure, and modify audit_filter_task()
240	as follows:
241
242	static enum audit_state audit_filter_task(struct task_struct *tsk)
243	{
244	struct audit_entry *e;
245	enum audit_state state;
246
247	rcu_read_lock();
248	list_for_each_entry_rcu(e, &audit_tsklist, list) {
249	if (audit_filter_rules(tsk, &e->rule, NULL, &state)) {
250	spin_lock(&e->lock);
251	if (e->deleted) {
252	spin_unlock(&e->lock);
253	rcu_read_unlock();
254	return AUDIT_BUILD_CONTEXT;
255	}
256	rcu_read_unlock();
257	return state;
258	}
259	}
260	rcu_read_unlock();
261	return AUDIT_BUILD_CONTEXT;
262	}
263
264	Note that this example assumes that entries are only added and deleted.
265	Additional mechanism is required to deal correctly with the
266	update-in-place performed by audit_upd_rule(). For one thing,
267	audit_upd_rule() would need additional memory barriers to ensure
268	that the list_add_rcu() was really executed before the list_del_rcu().
269
270	The audit_del_rule() function would need to set the "deleted"
271	flag under the spinlock as follows:
272
273	static inline int audit_del_rule(struct audit_rule *rule,
274	struct list_head *list)
275	{
276	struct audit_entry *e;
277
278	/* Do not use the _rcu iterator here, since this is the only
279	* deletion routine. */
280	list_for_each_entry(e, list, list) {
281	if (!audit_compare_rule(rule, &e->rule)) {
282	spin_lock(&e->lock);
283	list_del_rcu(&e->list);
284	e->deleted = 1;
285	spin_unlock(&e->lock);
286	call_rcu(&e->rcu, audit_free_rule, e);
287	return 0;
288	}
289	}
290	return -EFAULT; /* No matching rule */
291	}
292
293
294	Summary
295
296	Read-mostly list-based data structures that can tolerate stale data are
297	the most amenable to use of RCU. The simplest case is where entries are
298	either added or deleted from the data structure (or atomically modified
299	in place), but non-atomic in-place modifications can be handled by making
300	a copy, updating the copy, then replacing the original with the copy.
301	If stale data cannot be tolerated, then a "deleted" flag may be used
302	in conjunction with a per-entry spinlock in order to allow the search
303	function to reject newly deleted data.
304
305
306	Answer to Quick Quiz
307
308	If the search function drops the per-entry lock before returning, then
309	the caller will be processing stale data in any case. If it is really
310	OK to be processing stale data, then you don't need a "deleted" flag.
311	If processing stale data really is a problem, then you need to hold the
312	per-entry lock across all of the code that uses the value looked up.