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1 | /* |
2 | * Sleepable Read-Copy Update mechanism for mutual exclusion. |
3 | * |
4 | * This program is free software; you can redistribute it and/or modify |
5 | * it under the terms of the GNU General Public License as published by |
6 | * the Free Software Foundation; either version 2 of the License, or |
7 | * (at your option) any later version. |
8 | * |
9 | * This program is distributed in the hope that it will be useful, |
10 | * but WITHOUT ANY WARRANTY; without even the implied warranty of |
11 | * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
12 | * GNU General Public License for more details. |
13 | * |
14 | * You should have received a copy of the GNU General Public License |
15 | * along with this program; if not, write to the Free Software |
16 | * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. |
17 | * |
18 | * Copyright (C) IBM Corporation, 2006 |
19 | * |
20 | * Author: Paul McKenney <paulmck@us.ibm.com> |
21 | * |
22 | * For detailed explanation of Read-Copy Update mechanism see - |
23 | * Documentation/RCU/ *.txt |
24 | * |
25 | */ |
26 | |
27 | #include <linux/module.h> |
28 | #include <linux/mutex.h> |
29 | #include <linux/percpu.h> |
30 | #include <linux/preempt.h> |
31 | #include <linux/rcupdate.h> |
32 | #include <linux/sched.h> |
33 | #include <linux/smp.h> |
34 | #include <linux/srcu.h> |
35 | |
36 | static int init_srcu_struct_fields(struct srcu_struct *sp) |
37 | { |
38 | sp->completed = 0; |
39 | mutex_init(&sp->mutex); |
40 | sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array); |
41 | return sp->per_cpu_ref ? 0 : -ENOMEM; |
42 | } |
43 | |
44 | #ifdef CONFIG_DEBUG_LOCK_ALLOC |
45 | |
46 | int __init_srcu_struct(struct srcu_struct *sp, const char *name, |
47 | struct lock_class_key *key) |
48 | { |
49 | #ifdef CONFIG_DEBUG_LOCK_ALLOC |
50 | /* Don't re-initialize a lock while it is held. */ |
51 | debug_check_no_locks_freed((void *)sp, sizeof(*sp)); |
52 | lockdep_init_map(&sp->dep_map, name, key, 0); |
53 | #endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ |
54 | return init_srcu_struct_fields(sp); |
55 | } |
56 | EXPORT_SYMBOL_GPL(__init_srcu_struct); |
57 | |
58 | #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ |
59 | |
60 | /** |
61 | * init_srcu_struct - initialize a sleep-RCU structure |
62 | * @sp: structure to initialize. |
63 | * |
64 | * Must invoke this on a given srcu_struct before passing that srcu_struct |
65 | * to any other function. Each srcu_struct represents a separate domain |
66 | * of SRCU protection. |
67 | */ |
68 | int init_srcu_struct(struct srcu_struct *sp) |
69 | { |
70 | return init_srcu_struct_fields(sp); |
71 | } |
72 | EXPORT_SYMBOL_GPL(init_srcu_struct); |
73 | |
74 | #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */ |
75 | |
76 | /* |
77 | * srcu_readers_active_idx -- returns approximate number of readers |
78 | * active on the specified rank of per-CPU counters. |
79 | */ |
80 | |
81 | static int srcu_readers_active_idx(struct srcu_struct *sp, int idx) |
82 | { |
83 | int cpu; |
84 | int sum; |
85 | |
86 | sum = 0; |
87 | for_each_possible_cpu(cpu) |
88 | sum += per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]; |
89 | return sum; |
90 | } |
91 | |
92 | /** |
93 | * srcu_readers_active - returns approximate number of readers. |
94 | * @sp: which srcu_struct to count active readers (holding srcu_read_lock). |
95 | * |
96 | * Note that this is not an atomic primitive, and can therefore suffer |
97 | * severe errors when invoked on an active srcu_struct. That said, it |
98 | * can be useful as an error check at cleanup time. |
99 | */ |
100 | static int srcu_readers_active(struct srcu_struct *sp) |
101 | { |
102 | return srcu_readers_active_idx(sp, 0) + srcu_readers_active_idx(sp, 1); |
103 | } |
104 | |
105 | /** |
106 | * cleanup_srcu_struct - deconstruct a sleep-RCU structure |
107 | * @sp: structure to clean up. |
108 | * |
109 | * Must invoke this after you are finished using a given srcu_struct that |
110 | * was initialized via init_srcu_struct(), else you leak memory. |
111 | */ |
112 | void cleanup_srcu_struct(struct srcu_struct *sp) |
113 | { |
114 | int sum; |
115 | |
116 | sum = srcu_readers_active(sp); |
117 | WARN_ON(sum); /* Leakage unless caller handles error. */ |
118 | if (sum != 0) |
119 | return; |
120 | free_percpu(sp->per_cpu_ref); |
121 | sp->per_cpu_ref = NULL; |
122 | } |
123 | EXPORT_SYMBOL_GPL(cleanup_srcu_struct); |
124 | |
125 | /* |
126 | * Counts the new reader in the appropriate per-CPU element of the |
127 | * srcu_struct. Must be called from process context. |
128 | * Returns an index that must be passed to the matching srcu_read_unlock(). |
129 | */ |
130 | int __srcu_read_lock(struct srcu_struct *sp) |
131 | { |
132 | int idx; |
133 | |
134 | preempt_disable(); |
135 | idx = sp->completed & 0x1; |
136 | barrier(); /* ensure compiler looks -once- at sp->completed. */ |
137 | per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]++; |
138 | srcu_barrier(); /* ensure compiler won't misorder critical section. */ |
139 | preempt_enable(); |
140 | return idx; |
141 | } |
142 | EXPORT_SYMBOL_GPL(__srcu_read_lock); |
143 | |
144 | /* |
145 | * Removes the count for the old reader from the appropriate per-CPU |
146 | * element of the srcu_struct. Note that this may well be a different |
147 | * CPU than that which was incremented by the corresponding srcu_read_lock(). |
148 | * Must be called from process context. |
149 | */ |
150 | void __srcu_read_unlock(struct srcu_struct *sp, int idx) |
151 | { |
152 | preempt_disable(); |
153 | srcu_barrier(); /* ensure compiler won't misorder critical section. */ |
154 | per_cpu_ptr(sp->per_cpu_ref, smp_processor_id())->c[idx]--; |
155 | preempt_enable(); |
156 | } |
157 | EXPORT_SYMBOL_GPL(__srcu_read_unlock); |
158 | |
159 | /* |
160 | * Helper function for synchronize_srcu() and synchronize_srcu_expedited(). |
161 | */ |
162 | static void __synchronize_srcu(struct srcu_struct *sp, void (*sync_func)(void)) |
163 | { |
164 | int idx; |
165 | |
166 | idx = sp->completed; |
167 | mutex_lock(&sp->mutex); |
168 | |
169 | /* |
170 | * Check to see if someone else did the work for us while we were |
171 | * waiting to acquire the lock. We need -two- advances of |
172 | * the counter, not just one. If there was but one, we might have |
173 | * shown up -after- our helper's first synchronize_sched(), thus |
174 | * having failed to prevent CPU-reordering races with concurrent |
175 | * srcu_read_unlock()s on other CPUs (see comment below). So we |
176 | * either (1) wait for two or (2) supply the second ourselves. |
177 | */ |
178 | |
179 | if ((sp->completed - idx) >= 2) { |
180 | mutex_unlock(&sp->mutex); |
181 | return; |
182 | } |
183 | |
184 | sync_func(); /* Force memory barrier on all CPUs. */ |
185 | |
186 | /* |
187 | * The preceding synchronize_sched() ensures that any CPU that |
188 | * sees the new value of sp->completed will also see any preceding |
189 | * changes to data structures made by this CPU. This prevents |
190 | * some other CPU from reordering the accesses in its SRCU |
191 | * read-side critical section to precede the corresponding |
192 | * srcu_read_lock() -- ensuring that such references will in |
193 | * fact be protected. |
194 | * |
195 | * So it is now safe to do the flip. |
196 | */ |
197 | |
198 | idx = sp->completed & 0x1; |
199 | sp->completed++; |
200 | |
201 | sync_func(); /* Force memory barrier on all CPUs. */ |
202 | |
203 | /* |
204 | * At this point, because of the preceding synchronize_sched(), |
205 | * all srcu_read_lock() calls using the old counters have completed. |
206 | * Their corresponding critical sections might well be still |
207 | * executing, but the srcu_read_lock() primitives themselves |
208 | * will have finished executing. |
209 | */ |
210 | |
211 | while (srcu_readers_active_idx(sp, idx)) |
212 | schedule_timeout_interruptible(1); |
213 | |
214 | sync_func(); /* Force memory barrier on all CPUs. */ |
215 | |
216 | /* |
217 | * The preceding synchronize_sched() forces all srcu_read_unlock() |
218 | * primitives that were executing concurrently with the preceding |
219 | * for_each_possible_cpu() loop to have completed by this point. |
220 | * More importantly, it also forces the corresponding SRCU read-side |
221 | * critical sections to have also completed, and the corresponding |
222 | * references to SRCU-protected data items to be dropped. |
223 | * |
224 | * Note: |
225 | * |
226 | * Despite what you might think at first glance, the |
227 | * preceding synchronize_sched() -must- be within the |
228 | * critical section ended by the following mutex_unlock(). |
229 | * Otherwise, a task taking the early exit can race |
230 | * with a srcu_read_unlock(), which might have executed |
231 | * just before the preceding srcu_readers_active() check, |
232 | * and whose CPU might have reordered the srcu_read_unlock() |
233 | * with the preceding critical section. In this case, there |
234 | * is nothing preventing the synchronize_sched() task that is |
235 | * taking the early exit from freeing a data structure that |
236 | * is still being referenced (out of order) by the task |
237 | * doing the srcu_read_unlock(). |
238 | * |
239 | * Alternatively, the comparison with "2" on the early exit |
240 | * could be changed to "3", but this increases synchronize_srcu() |
241 | * latency for bulk loads. So the current code is preferred. |
242 | */ |
243 | |
244 | mutex_unlock(&sp->mutex); |
245 | } |
246 | |
247 | /** |
248 | * synchronize_srcu - wait for prior SRCU read-side critical-section completion |
249 | * @sp: srcu_struct with which to synchronize. |
250 | * |
251 | * Flip the completed counter, and wait for the old count to drain to zero. |
252 | * As with classic RCU, the updater must use some separate means of |
253 | * synchronizing concurrent updates. Can block; must be called from |
254 | * process context. |
255 | * |
256 | * Note that it is illegal to call synchronize_srcu() from the corresponding |
257 | * SRCU read-side critical section; doing so will result in deadlock. |
258 | * However, it is perfectly legal to call synchronize_srcu() on one |
259 | * srcu_struct from some other srcu_struct's read-side critical section. |
260 | */ |
261 | void synchronize_srcu(struct srcu_struct *sp) |
262 | { |
263 | __synchronize_srcu(sp, synchronize_sched); |
264 | } |
265 | EXPORT_SYMBOL_GPL(synchronize_srcu); |
266 | |
267 | /** |
268 | * synchronize_srcu_expedited - like synchronize_srcu, but less patient |
269 | * @sp: srcu_struct with which to synchronize. |
270 | * |
271 | * Flip the completed counter, and wait for the old count to drain to zero. |
272 | * As with classic RCU, the updater must use some separate means of |
273 | * synchronizing concurrent updates. Can block; must be called from |
274 | * process context. |
275 | * |
276 | * Note that it is illegal to call synchronize_srcu_expedited() |
277 | * from the corresponding SRCU read-side critical section; doing so |
278 | * will result in deadlock. However, it is perfectly legal to call |
279 | * synchronize_srcu_expedited() on one srcu_struct from some other |
280 | * srcu_struct's read-side critical section. |
281 | */ |
282 | void synchronize_srcu_expedited(struct srcu_struct *sp) |
283 | { |
284 | __synchronize_srcu(sp, synchronize_sched_expedited); |
285 | } |
286 | EXPORT_SYMBOL_GPL(synchronize_srcu_expedited); |
287 | |
288 | /** |
289 | * srcu_batches_completed - return batches completed. |
290 | * @sp: srcu_struct on which to report batch completion. |
291 | * |
292 | * Report the number of batches, correlated with, but not necessarily |
293 | * precisely the same as, the number of grace periods that have elapsed. |
294 | */ |
295 | |
296 | long srcu_batches_completed(struct srcu_struct *sp) |
297 | { |
298 | return sp->completed; |
299 | } |
300 | EXPORT_SYMBOL_GPL(srcu_batches_completed); |
301 |
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