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621934ee PM |
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 | |
4e87b2d7 | 19 | * Copyright (C) Fujitsu, 2012 |
621934ee PM |
20 | * |
21 | * Author: Paul McKenney <paulmck@us.ibm.com> | |
4e87b2d7 | 22 | * Lai Jiangshan <laijs@cn.fujitsu.com> |
621934ee PM |
23 | * |
24 | * For detailed explanation of Read-Copy Update mechanism see - | |
25 | * Documentation/RCU/ *.txt | |
26 | * | |
27 | */ | |
28 | ||
9984de1a | 29 | #include <linux/export.h> |
621934ee PM |
30 | #include <linux/mutex.h> |
31 | #include <linux/percpu.h> | |
32 | #include <linux/preempt.h> | |
33 | #include <linux/rcupdate.h> | |
34 | #include <linux/sched.h> | |
621934ee | 35 | #include <linux/smp.h> |
46fdb093 | 36 | #include <linux/delay.h> |
621934ee PM |
37 | #include <linux/srcu.h> |
38 | ||
931ea9d1 LJ |
39 | /* |
40 | * Initialize an rcu_batch structure to empty. | |
41 | */ | |
42 | static inline void rcu_batch_init(struct rcu_batch *b) | |
43 | { | |
44 | b->head = NULL; | |
45 | b->tail = &b->head; | |
46 | } | |
47 | ||
48 | /* | |
49 | * Enqueue a callback onto the tail of the specified rcu_batch structure. | |
50 | */ | |
51 | static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head) | |
52 | { | |
53 | *b->tail = head; | |
54 | b->tail = &head->next; | |
55 | } | |
56 | ||
57 | /* | |
58 | * Is the specified rcu_batch structure empty? | |
59 | */ | |
60 | static inline bool rcu_batch_empty(struct rcu_batch *b) | |
61 | { | |
62 | return b->tail == &b->head; | |
63 | } | |
64 | ||
65 | /* | |
66 | * Remove the callback at the head of the specified rcu_batch structure | |
67 | * and return a pointer to it, or return NULL if the structure is empty. | |
68 | */ | |
69 | static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b) | |
70 | { | |
71 | struct rcu_head *head; | |
72 | ||
73 | if (rcu_batch_empty(b)) | |
74 | return NULL; | |
75 | ||
76 | head = b->head; | |
77 | b->head = head->next; | |
78 | if (b->tail == &head->next) | |
79 | rcu_batch_init(b); | |
80 | ||
81 | return head; | |
82 | } | |
83 | ||
84 | /* | |
85 | * Move all callbacks from the rcu_batch structure specified by "from" to | |
86 | * the structure specified by "to". | |
87 | */ | |
88 | static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from) | |
89 | { | |
90 | if (!rcu_batch_empty(from)) { | |
91 | *to->tail = from->head; | |
92 | to->tail = from->tail; | |
93 | rcu_batch_init(from); | |
94 | } | |
95 | } | |
96 | ||
632ee200 PM |
97 | static int init_srcu_struct_fields(struct srcu_struct *sp) |
98 | { | |
99 | sp->completed = 0; | |
931ea9d1 LJ |
100 | spin_lock_init(&sp->queue_lock); |
101 | sp->running = false; | |
102 | rcu_batch_init(&sp->batch_queue); | |
103 | rcu_batch_init(&sp->batch_check0); | |
104 | rcu_batch_init(&sp->batch_check1); | |
105 | rcu_batch_init(&sp->batch_done); | |
106 | INIT_DELAYED_WORK(&sp->work, process_srcu); | |
632ee200 PM |
107 | sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array); |
108 | return sp->per_cpu_ref ? 0 : -ENOMEM; | |
109 | } | |
110 | ||
111 | #ifdef CONFIG_DEBUG_LOCK_ALLOC | |
112 | ||
113 | int __init_srcu_struct(struct srcu_struct *sp, const char *name, | |
114 | struct lock_class_key *key) | |
115 | { | |
632ee200 PM |
116 | /* Don't re-initialize a lock while it is held. */ |
117 | debug_check_no_locks_freed((void *)sp, sizeof(*sp)); | |
118 | lockdep_init_map(&sp->dep_map, name, key, 0); | |
632ee200 PM |
119 | return init_srcu_struct_fields(sp); |
120 | } | |
121 | EXPORT_SYMBOL_GPL(__init_srcu_struct); | |
122 | ||
123 | #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */ | |
124 | ||
621934ee PM |
125 | /** |
126 | * init_srcu_struct - initialize a sleep-RCU structure | |
127 | * @sp: structure to initialize. | |
128 | * | |
129 | * Must invoke this on a given srcu_struct before passing that srcu_struct | |
130 | * to any other function. Each srcu_struct represents a separate domain | |
131 | * of SRCU protection. | |
132 | */ | |
e6a92013 | 133 | int init_srcu_struct(struct srcu_struct *sp) |
621934ee | 134 | { |
632ee200 | 135 | return init_srcu_struct_fields(sp); |
621934ee | 136 | } |
0cd397d3 | 137 | EXPORT_SYMBOL_GPL(init_srcu_struct); |
621934ee | 138 | |
632ee200 PM |
139 | #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */ |
140 | ||
b52ce066 LJ |
141 | /* |
142 | * Returns approximate total of the readers' ->seq[] values for the | |
143 | * rank of per-CPU counters specified by idx. | |
144 | */ | |
145 | static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx) | |
146 | { | |
147 | int cpu; | |
148 | unsigned long sum = 0; | |
149 | unsigned long t; | |
150 | ||
151 | for_each_possible_cpu(cpu) { | |
152 | t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]); | |
153 | sum += t; | |
154 | } | |
155 | return sum; | |
156 | } | |
157 | ||
621934ee | 158 | /* |
cef50120 | 159 | * Returns approximate number of readers active on the specified rank |
b52ce066 | 160 | * of the per-CPU ->c[] counters. |
621934ee | 161 | */ |
cef50120 PM |
162 | static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx) |
163 | { | |
164 | int cpu; | |
165 | unsigned long sum = 0; | |
166 | unsigned long t; | |
621934ee | 167 | |
cef50120 PM |
168 | for_each_possible_cpu(cpu) { |
169 | t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]); | |
170 | sum += t; | |
cef50120 | 171 | } |
b52ce066 | 172 | return sum; |
cef50120 PM |
173 | } |
174 | ||
175 | /* | |
b52ce066 LJ |
176 | * Return true if the number of pre-existing readers is determined to |
177 | * be stably zero. An example unstable zero can occur if the call | |
178 | * to srcu_readers_active_idx() misses an __srcu_read_lock() increment, | |
179 | * but due to task migration, sees the corresponding __srcu_read_unlock() | |
180 | * decrement. This can happen because srcu_readers_active_idx() takes | |
181 | * time to sum the array, and might in fact be interrupted or preempted | |
182 | * partway through the summation. | |
cef50120 PM |
183 | */ |
184 | static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx) | |
621934ee | 185 | { |
b52ce066 LJ |
186 | unsigned long seq; |
187 | ||
188 | seq = srcu_readers_seq_idx(sp, idx); | |
189 | ||
190 | /* | |
191 | * The following smp_mb() A pairs with the smp_mb() B located in | |
192 | * __srcu_read_lock(). This pairing ensures that if an | |
193 | * __srcu_read_lock() increments its counter after the summation | |
194 | * in srcu_readers_active_idx(), then the corresponding SRCU read-side | |
195 | * critical section will see any changes made prior to the start | |
196 | * of the current SRCU grace period. | |
197 | * | |
198 | * Also, if the above call to srcu_readers_seq_idx() saw the | |
199 | * increment of ->seq[], then the call to srcu_readers_active_idx() | |
200 | * must see the increment of ->c[]. | |
201 | */ | |
202 | smp_mb(); /* A */ | |
621934ee | 203 | |
cef50120 PM |
204 | /* |
205 | * Note that srcu_readers_active_idx() can incorrectly return | |
206 | * zero even though there is a pre-existing reader throughout. | |
207 | * To see this, suppose that task A is in a very long SRCU | |
208 | * read-side critical section that started on CPU 0, and that | |
b52ce066 | 209 | * no other reader exists, so that the sum of the counters |
cef50120 PM |
210 | * is equal to one. Then suppose that task B starts executing |
211 | * srcu_readers_active_idx(), summing up to CPU 1, and then that | |
212 | * task C starts reading on CPU 0, so that its increment is not | |
213 | * summed, but finishes reading on CPU 2, so that its decrement | |
214 | * -is- summed. Then when task B completes its sum, it will | |
215 | * incorrectly get zero, despite the fact that task A has been | |
216 | * in its SRCU read-side critical section the whole time. | |
217 | * | |
218 | * We therefore do a validation step should srcu_readers_active_idx() | |
219 | * return zero. | |
220 | */ | |
221 | if (srcu_readers_active_idx(sp, idx) != 0) | |
222 | return false; | |
223 | ||
224 | /* | |
b52ce066 LJ |
225 | * The remainder of this function is the validation step. |
226 | * The following smp_mb() D pairs with the smp_mb() C in | |
227 | * __srcu_read_unlock(). If the __srcu_read_unlock() was seen | |
228 | * by srcu_readers_active_idx() above, then any destructive | |
229 | * operation performed after the grace period will happen after | |
230 | * the corresponding SRCU read-side critical section. | |
cef50120 | 231 | * |
b52ce066 LJ |
232 | * Note that there can be at most NR_CPUS worth of readers using |
233 | * the old index, which is not enough to overflow even a 32-bit | |
234 | * integer. (Yes, this does mean that systems having more than | |
235 | * a billion or so CPUs need to be 64-bit systems.) Therefore, | |
236 | * the sum of the ->seq[] counters cannot possibly overflow. | |
237 | * Therefore, the only way that the return values of the two | |
238 | * calls to srcu_readers_seq_idx() can be equal is if there were | |
239 | * no increments of the corresponding rank of ->seq[] counts | |
240 | * in the interim. But the missed-increment scenario laid out | |
241 | * above includes an increment of the ->seq[] counter by | |
242 | * the corresponding __srcu_read_lock(). Therefore, if this | |
243 | * scenario occurs, the return values from the two calls to | |
244 | * srcu_readers_seq_idx() will differ, and thus the validation | |
245 | * step below suffices. | |
cef50120 | 246 | */ |
b52ce066 LJ |
247 | smp_mb(); /* D */ |
248 | ||
249 | return srcu_readers_seq_idx(sp, idx) == seq; | |
621934ee PM |
250 | } |
251 | ||
252 | /** | |
253 | * srcu_readers_active - returns approximate number of readers. | |
254 | * @sp: which srcu_struct to count active readers (holding srcu_read_lock). | |
255 | * | |
256 | * Note that this is not an atomic primitive, and can therefore suffer | |
257 | * severe errors when invoked on an active srcu_struct. That said, it | |
258 | * can be useful as an error check at cleanup time. | |
259 | */ | |
bb695170 | 260 | static int srcu_readers_active(struct srcu_struct *sp) |
621934ee | 261 | { |
dc879175 LJ |
262 | int cpu; |
263 | unsigned long sum = 0; | |
264 | ||
265 | for_each_possible_cpu(cpu) { | |
266 | sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]); | |
267 | sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]); | |
268 | } | |
269 | return sum; | |
621934ee PM |
270 | } |
271 | ||
272 | /** | |
273 | * cleanup_srcu_struct - deconstruct a sleep-RCU structure | |
274 | * @sp: structure to clean up. | |
275 | * | |
276 | * Must invoke this after you are finished using a given srcu_struct that | |
277 | * was initialized via init_srcu_struct(), else you leak memory. | |
278 | */ | |
279 | void cleanup_srcu_struct(struct srcu_struct *sp) | |
280 | { | |
281 | int sum; | |
282 | ||
283 | sum = srcu_readers_active(sp); | |
284 | WARN_ON(sum); /* Leakage unless caller handles error. */ | |
285 | if (sum != 0) | |
286 | return; | |
287 | free_percpu(sp->per_cpu_ref); | |
288 | sp->per_cpu_ref = NULL; | |
289 | } | |
0cd397d3 | 290 | EXPORT_SYMBOL_GPL(cleanup_srcu_struct); |
621934ee | 291 | |
632ee200 | 292 | /* |
621934ee PM |
293 | * Counts the new reader in the appropriate per-CPU element of the |
294 | * srcu_struct. Must be called from process context. | |
295 | * Returns an index that must be passed to the matching srcu_read_unlock(). | |
296 | */ | |
632ee200 | 297 | int __srcu_read_lock(struct srcu_struct *sp) |
621934ee PM |
298 | { |
299 | int idx; | |
300 | ||
301 | preempt_disable(); | |
cef50120 PM |
302 | idx = rcu_dereference_index_check(sp->completed, |
303 | rcu_read_lock_sched_held()) & 0x1; | |
b52ce066 | 304 | ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1; |
cef50120 | 305 | smp_mb(); /* B */ /* Avoid leaking the critical section. */ |
b52ce066 | 306 | ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1; |
621934ee PM |
307 | preempt_enable(); |
308 | return idx; | |
309 | } | |
632ee200 | 310 | EXPORT_SYMBOL_GPL(__srcu_read_lock); |
621934ee | 311 | |
632ee200 | 312 | /* |
621934ee PM |
313 | * Removes the count for the old reader from the appropriate per-CPU |
314 | * element of the srcu_struct. Note that this may well be a different | |
315 | * CPU than that which was incremented by the corresponding srcu_read_lock(). | |
316 | * Must be called from process context. | |
317 | */ | |
632ee200 | 318 | void __srcu_read_unlock(struct srcu_struct *sp, int idx) |
621934ee PM |
319 | { |
320 | preempt_disable(); | |
cef50120 | 321 | smp_mb(); /* C */ /* Avoid leaking the critical section. */ |
440253c1 | 322 | ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) -= 1; |
621934ee PM |
323 | preempt_enable(); |
324 | } | |
632ee200 | 325 | EXPORT_SYMBOL_GPL(__srcu_read_unlock); |
621934ee | 326 | |
c072a388 PM |
327 | /* |
328 | * We use an adaptive strategy for synchronize_srcu() and especially for | |
329 | * synchronize_srcu_expedited(). We spin for a fixed time period | |
330 | * (defined below) to allow SRCU readers to exit their read-side critical | |
331 | * sections. If there are still some readers after 10 microseconds, | |
332 | * we repeatedly block for 1-millisecond time periods. This approach | |
333 | * has done well in testing, so there is no need for a config parameter. | |
334 | */ | |
931ea9d1 | 335 | #define SRCU_RETRY_CHECK_DELAY 5 |
d9792edd LJ |
336 | #define SYNCHRONIZE_SRCU_TRYCOUNT 2 |
337 | #define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12 | |
cef50120 | 338 | |
18108ebf | 339 | /* |
931ea9d1 | 340 | * @@@ Wait until all pre-existing readers complete. Such readers |
18108ebf | 341 | * will have used the index specified by "idx". |
931ea9d1 LJ |
342 | * the caller should ensures the ->completed is not changed while checking |
343 | * and idx = (->completed & 1) ^ 1 | |
18108ebf | 344 | */ |
931ea9d1 | 345 | static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount) |
cef50120 | 346 | { |
931ea9d1 LJ |
347 | for (;;) { |
348 | if (srcu_readers_active_idx_check(sp, idx)) | |
349 | return true; | |
350 | if (--trycount <= 0) | |
351 | return false; | |
352 | udelay(SRCU_RETRY_CHECK_DELAY); | |
cef50120 | 353 | } |
cef50120 | 354 | } |
c072a388 | 355 | |
931ea9d1 LJ |
356 | /* |
357 | * Increment the ->completed counter so that future SRCU readers will | |
358 | * use the other rank of the ->c[] and ->seq[] arrays. This allows | |
359 | * us to wait for pre-existing readers in a starvation-free manner. | |
360 | */ | |
18108ebf | 361 | static void srcu_flip(struct srcu_struct *sp) |
944ce9af | 362 | { |
18108ebf | 363 | sp->completed++; |
944ce9af LJ |
364 | } |
365 | ||
931ea9d1 LJ |
366 | /* |
367 | * Enqueue an SRCU callback on the specified srcu_struct structure, | |
368 | * initiating grace-period processing if it is not already running. | |
369 | */ | |
370 | void call_srcu(struct srcu_struct *sp, struct rcu_head *head, | |
371 | void (*func)(struct rcu_head *head)) | |
372 | { | |
373 | unsigned long flags; | |
374 | ||
375 | head->next = NULL; | |
376 | head->func = func; | |
377 | spin_lock_irqsave(&sp->queue_lock, flags); | |
378 | rcu_batch_queue(&sp->batch_queue, head); | |
379 | if (!sp->running) { | |
380 | sp->running = true; | |
3b07e9ca | 381 | schedule_delayed_work(&sp->work, 0); |
931ea9d1 LJ |
382 | } |
383 | spin_unlock_irqrestore(&sp->queue_lock, flags); | |
384 | } | |
385 | EXPORT_SYMBOL_GPL(call_srcu); | |
386 | ||
387 | struct rcu_synchronize { | |
388 | struct rcu_head head; | |
389 | struct completion completion; | |
390 | }; | |
391 | ||
392 | /* | |
393 | * Awaken the corresponding synchronize_srcu() instance now that a | |
394 | * grace period has elapsed. | |
395 | */ | |
396 | static void wakeme_after_rcu(struct rcu_head *head) | |
397 | { | |
398 | struct rcu_synchronize *rcu; | |
399 | ||
400 | rcu = container_of(head, struct rcu_synchronize, head); | |
401 | complete(&rcu->completion); | |
402 | } | |
403 | ||
404 | static void srcu_advance_batches(struct srcu_struct *sp, int trycount); | |
405 | static void srcu_reschedule(struct srcu_struct *sp); | |
406 | ||
0cd397d3 PM |
407 | /* |
408 | * Helper function for synchronize_srcu() and synchronize_srcu_expedited(). | |
621934ee | 409 | */ |
d9792edd | 410 | static void __synchronize_srcu(struct srcu_struct *sp, int trycount) |
621934ee | 411 | { |
931ea9d1 LJ |
412 | struct rcu_synchronize rcu; |
413 | struct rcu_head *head = &rcu.head; | |
414 | bool done = false; | |
18108ebf | 415 | |
fe15d706 PM |
416 | rcu_lockdep_assert(!lock_is_held(&sp->dep_map) && |
417 | !lock_is_held(&rcu_bh_lock_map) && | |
418 | !lock_is_held(&rcu_lock_map) && | |
419 | !lock_is_held(&rcu_sched_lock_map), | |
420 | "Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section"); | |
421 | ||
931ea9d1 LJ |
422 | init_completion(&rcu.completion); |
423 | ||
424 | head->next = NULL; | |
425 | head->func = wakeme_after_rcu; | |
426 | spin_lock_irq(&sp->queue_lock); | |
427 | if (!sp->running) { | |
428 | /* steal the processing owner */ | |
429 | sp->running = true; | |
430 | rcu_batch_queue(&sp->batch_check0, head); | |
431 | spin_unlock_irq(&sp->queue_lock); | |
432 | ||
433 | srcu_advance_batches(sp, trycount); | |
434 | if (!rcu_batch_empty(&sp->batch_done)) { | |
435 | BUG_ON(sp->batch_done.head != head); | |
436 | rcu_batch_dequeue(&sp->batch_done); | |
437 | done = true; | |
438 | } | |
439 | /* give the processing owner to work_struct */ | |
440 | srcu_reschedule(sp); | |
441 | } else { | |
442 | rcu_batch_queue(&sp->batch_queue, head); | |
443 | spin_unlock_irq(&sp->queue_lock); | |
444 | } | |
944ce9af | 445 | |
931ea9d1 LJ |
446 | if (!done) |
447 | wait_for_completion(&rcu.completion); | |
621934ee PM |
448 | } |
449 | ||
0cd397d3 PM |
450 | /** |
451 | * synchronize_srcu - wait for prior SRCU read-side critical-section completion | |
452 | * @sp: srcu_struct with which to synchronize. | |
453 | * | |
454 | * Flip the completed counter, and wait for the old count to drain to zero. | |
455 | * As with classic RCU, the updater must use some separate means of | |
456 | * synchronizing concurrent updates. Can block; must be called from | |
457 | * process context. | |
458 | * | |
459 | * Note that it is illegal to call synchronize_srcu() from the corresponding | |
460 | * SRCU read-side critical section; doing so will result in deadlock. | |
461 | * However, it is perfectly legal to call synchronize_srcu() on one | |
462 | * srcu_struct from some other srcu_struct's read-side critical section. | |
463 | */ | |
464 | void synchronize_srcu(struct srcu_struct *sp) | |
465 | { | |
d9792edd | 466 | __synchronize_srcu(sp, SYNCHRONIZE_SRCU_TRYCOUNT); |
0cd397d3 PM |
467 | } |
468 | EXPORT_SYMBOL_GPL(synchronize_srcu); | |
469 | ||
470 | /** | |
236fefaf | 471 | * synchronize_srcu_expedited - Brute-force SRCU grace period |
0cd397d3 PM |
472 | * @sp: srcu_struct with which to synchronize. |
473 | * | |
cef50120 PM |
474 | * Wait for an SRCU grace period to elapse, but be more aggressive about |
475 | * spinning rather than blocking when waiting. | |
0cd397d3 | 476 | * |
236fefaf | 477 | * Note that it is illegal to call this function while holding any lock |
cef50120 | 478 | * that is acquired by a CPU-hotplug notifier. It is also illegal to call |
236fefaf PM |
479 | * synchronize_srcu_expedited() from the corresponding SRCU read-side |
480 | * critical section; doing so will result in deadlock. However, it is | |
481 | * perfectly legal to call synchronize_srcu_expedited() on one srcu_struct | |
482 | * from some other srcu_struct's read-side critical section, as long as | |
483 | * the resulting graph of srcu_structs is acyclic. | |
0cd397d3 PM |
484 | */ |
485 | void synchronize_srcu_expedited(struct srcu_struct *sp) | |
486 | { | |
d9792edd | 487 | __synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT); |
0cd397d3 PM |
488 | } |
489 | EXPORT_SYMBOL_GPL(synchronize_srcu_expedited); | |
490 | ||
931ea9d1 LJ |
491 | /** |
492 | * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete. | |
493 | */ | |
494 | void srcu_barrier(struct srcu_struct *sp) | |
495 | { | |
496 | synchronize_srcu(sp); | |
497 | } | |
498 | EXPORT_SYMBOL_GPL(srcu_barrier); | |
499 | ||
621934ee PM |
500 | /** |
501 | * srcu_batches_completed - return batches completed. | |
502 | * @sp: srcu_struct on which to report batch completion. | |
503 | * | |
504 | * Report the number of batches, correlated with, but not necessarily | |
505 | * precisely the same as, the number of grace periods that have elapsed. | |
506 | */ | |
621934ee PM |
507 | long srcu_batches_completed(struct srcu_struct *sp) |
508 | { | |
509 | return sp->completed; | |
510 | } | |
621934ee | 511 | EXPORT_SYMBOL_GPL(srcu_batches_completed); |
931ea9d1 LJ |
512 | |
513 | #define SRCU_CALLBACK_BATCH 10 | |
514 | #define SRCU_INTERVAL 1 | |
515 | ||
516 | /* | |
517 | * Move any new SRCU callbacks to the first stage of the SRCU grace | |
518 | * period pipeline. | |
519 | */ | |
520 | static void srcu_collect_new(struct srcu_struct *sp) | |
521 | { | |
522 | if (!rcu_batch_empty(&sp->batch_queue)) { | |
523 | spin_lock_irq(&sp->queue_lock); | |
524 | rcu_batch_move(&sp->batch_check0, &sp->batch_queue); | |
525 | spin_unlock_irq(&sp->queue_lock); | |
526 | } | |
527 | } | |
528 | ||
529 | /* | |
530 | * Core SRCU state machine. Advance callbacks from ->batch_check0 to | |
531 | * ->batch_check1 and then to ->batch_done as readers drain. | |
532 | */ | |
533 | static void srcu_advance_batches(struct srcu_struct *sp, int trycount) | |
534 | { | |
535 | int idx = 1 ^ (sp->completed & 1); | |
536 | ||
537 | /* | |
538 | * Because readers might be delayed for an extended period after | |
539 | * fetching ->completed for their index, at any point in time there | |
540 | * might well be readers using both idx=0 and idx=1. We therefore | |
541 | * need to wait for readers to clear from both index values before | |
542 | * invoking a callback. | |
543 | */ | |
544 | ||
545 | if (rcu_batch_empty(&sp->batch_check0) && | |
546 | rcu_batch_empty(&sp->batch_check1)) | |
547 | return; /* no callbacks need to be advanced */ | |
548 | ||
549 | if (!try_check_zero(sp, idx, trycount)) | |
550 | return; /* failed to advance, will try after SRCU_INTERVAL */ | |
551 | ||
552 | /* | |
553 | * The callbacks in ->batch_check1 have already done with their | |
554 | * first zero check and flip back when they were enqueued on | |
555 | * ->batch_check0 in a previous invocation of srcu_advance_batches(). | |
556 | * (Presumably try_check_zero() returned false during that | |
557 | * invocation, leaving the callbacks stranded on ->batch_check1.) | |
558 | * They are therefore ready to invoke, so move them to ->batch_done. | |
559 | */ | |
560 | rcu_batch_move(&sp->batch_done, &sp->batch_check1); | |
561 | ||
562 | if (rcu_batch_empty(&sp->batch_check0)) | |
563 | return; /* no callbacks need to be advanced */ | |
564 | srcu_flip(sp); | |
565 | ||
566 | /* | |
567 | * The callbacks in ->batch_check0 just finished their | |
568 | * first check zero and flip, so move them to ->batch_check1 | |
569 | * for future checking on the other idx. | |
570 | */ | |
571 | rcu_batch_move(&sp->batch_check1, &sp->batch_check0); | |
572 | ||
573 | /* | |
574 | * SRCU read-side critical sections are normally short, so check | |
575 | * at least twice in quick succession after a flip. | |
576 | */ | |
577 | trycount = trycount < 2 ? 2 : trycount; | |
578 | if (!try_check_zero(sp, idx^1, trycount)) | |
579 | return; /* failed to advance, will try after SRCU_INTERVAL */ | |
580 | ||
581 | /* | |
582 | * The callbacks in ->batch_check1 have now waited for all | |
583 | * pre-existing readers using both idx values. They are therefore | |
584 | * ready to invoke, so move them to ->batch_done. | |
585 | */ | |
586 | rcu_batch_move(&sp->batch_done, &sp->batch_check1); | |
587 | } | |
588 | ||
589 | /* | |
590 | * Invoke a limited number of SRCU callbacks that have passed through | |
591 | * their grace period. If there are more to do, SRCU will reschedule | |
592 | * the workqueue. | |
593 | */ | |
594 | static void srcu_invoke_callbacks(struct srcu_struct *sp) | |
595 | { | |
596 | int i; | |
597 | struct rcu_head *head; | |
598 | ||
599 | for (i = 0; i < SRCU_CALLBACK_BATCH; i++) { | |
600 | head = rcu_batch_dequeue(&sp->batch_done); | |
601 | if (!head) | |
602 | break; | |
603 | local_bh_disable(); | |
604 | head->func(head); | |
605 | local_bh_enable(); | |
606 | } | |
607 | } | |
608 | ||
609 | /* | |
610 | * Finished one round of SRCU grace period. Start another if there are | |
611 | * more SRCU callbacks queued, otherwise put SRCU into not-running state. | |
612 | */ | |
613 | static void srcu_reschedule(struct srcu_struct *sp) | |
614 | { | |
615 | bool pending = true; | |
616 | ||
617 | if (rcu_batch_empty(&sp->batch_done) && | |
618 | rcu_batch_empty(&sp->batch_check1) && | |
619 | rcu_batch_empty(&sp->batch_check0) && | |
620 | rcu_batch_empty(&sp->batch_queue)) { | |
621 | spin_lock_irq(&sp->queue_lock); | |
622 | if (rcu_batch_empty(&sp->batch_done) && | |
623 | rcu_batch_empty(&sp->batch_check1) && | |
624 | rcu_batch_empty(&sp->batch_check0) && | |
625 | rcu_batch_empty(&sp->batch_queue)) { | |
626 | sp->running = false; | |
627 | pending = false; | |
628 | } | |
629 | spin_unlock_irq(&sp->queue_lock); | |
630 | } | |
631 | ||
632 | if (pending) | |
3b07e9ca | 633 | schedule_delayed_work(&sp->work, SRCU_INTERVAL); |
931ea9d1 LJ |
634 | } |
635 | ||
636 | /* | |
637 | * This is the work-queue function that handles SRCU grace periods. | |
638 | */ | |
f2ebfbc9 | 639 | void process_srcu(struct work_struct *work) |
931ea9d1 LJ |
640 | { |
641 | struct srcu_struct *sp; | |
642 | ||
643 | sp = container_of(work, struct srcu_struct, work.work); | |
644 | ||
645 | srcu_collect_new(sp); | |
646 | srcu_advance_batches(sp, 1); | |
647 | srcu_invoke_callbacks(sp); | |
648 | srcu_reschedule(sp); | |
649 | } | |
f2ebfbc9 | 650 | EXPORT_SYMBOL_GPL(process_srcu); |