ia64/linux-2.6.18-xen.hg

view Documentation/RCU/listRCU.txt @ 854:950b9eb27661

usbback: fix urb interval value for interrupt urbs.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
author Keir Fraser <keir.fraser@citrix.com>
date Mon Apr 06 13:51:20 2009 +0100 (2009-04-06)
parents 831230e53067
children
line source
1 Using RCU to Protect Read-Mostly Linked Lists
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.
11 Example 1: Read-Side Action Taken Outside of Lock, No In-Place Updates
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.
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:
29 static enum audit_state audit_filter_task(struct task_struct *tsk)
30 {
31 struct audit_entry *e;
32 enum audit_state state;
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 }
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.
51 This means that RCU can be easily applied to the read side, as follows:
53 static enum audit_state audit_filter_task(struct task_struct *tsk)
54 {
55 struct audit_entry *e;
56 enum audit_state state;
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 }
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.
75 The changes to the update side are also straightforward. A reader-writer
76 lock might be used as follows for deletion and insertion:
78 static inline int audit_del_rule(struct audit_rule *rule,
79 struct list_head *list)
80 {
81 struct audit_entry *e;
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 }
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 }
109 Following are the RCU equivalents for these two functions:
111 static inline int audit_del_rule(struct audit_rule *rule,
112 struct list_head *list)
113 {
114 struct audit_entry *e;
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 }
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 }
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.
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.
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!
158 Example 2: Handling In-Place Updates
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):
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;
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 }
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:
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;
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 }
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.
220 Example 3: Eliminating Stale Data
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.
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.
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?
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:
242 static enum audit_state audit_filter_task(struct task_struct *tsk)
243 {
244 struct audit_entry *e;
245 enum audit_state state;
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 }
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().
270 The audit_del_rule() function would need to set the "deleted"
271 flag under the spinlock as follows:
273 static inline int audit_del_rule(struct audit_rule *rule,
274 struct list_head *list)
275 {
276 struct audit_entry *e;
278 /* Do not need to use the _rcu iterator here, since this
279 * is the only 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 }
294 Summary
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.
306 Answer to Quick Quiz
307 Why does the search function need to return holding the per-entry
308 lock for this deleted-flag technique to be helpful?
310 If the search function drops the per-entry lock before returning,
311 then the caller will be processing stale data in any case. If it
312 is really OK to be processing stale data, then you don't need a
313 "deleted" flag. If processing stale data really is a problem,
314 then you need to hold the per-entry lock across all of the code
315 that uses the value that was returned.