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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 : : * Copyright (C) Fujitsu, 2012
20 : : *
21 : : * Author: Paul McKenney <paulmck@us.ibm.com>
22 : : * Lai Jiangshan <laijs@cn.fujitsu.com>
23 : : *
24 : : * For detailed explanation of Read-Copy Update mechanism see -
25 : : * Documentation/RCU/ *.txt
26 : : *
27 : : */
28 : :
29 : : #include <linux/export.h>
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>
35 : : #include <linux/smp.h>
36 : : #include <linux/delay.h>
37 : : #include <linux/srcu.h>
38 : :
39 : : #include <trace/events/rcu.h>
40 : :
41 : : #include "rcu.h"
42 : :
43 : : /*
44 : : * Initialize an rcu_batch structure to empty.
45 : : */
46 : : static inline void rcu_batch_init(struct rcu_batch *b)
47 : : {
48 : 111 : b->head = NULL;
49 : 111 : b->tail = &b->head;
50 : : }
51 : :
52 : : /*
53 : : * Enqueue a callback onto the tail of the specified rcu_batch structure.
54 : : */
55 : : static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
56 : : {
57 : 37 : *b->tail = head;
58 : 37 : b->tail = &head->next;
59 : : }
60 : :
61 : : /*
62 : : * Is the specified rcu_batch structure empty?
63 : : */
64 : : static inline bool rcu_batch_empty(struct rcu_batch *b)
65 : : {
66 : 481 : return b->tail == &b->head;
67 : : }
68 : :
69 : : /*
70 : : * Remove the callback at the head of the specified rcu_batch structure
71 : : * and return a pointer to it, or return NULL if the structure is empty.
72 : : */
73 : : static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
74 : : {
75 : : struct rcu_head *head;
76 : :
77 [ # # ][ + - ]: 37 : if (rcu_batch_empty(b))
78 : : return NULL;
79 : :
80 : 0 : head = b->head;
81 : 37 : b->head = head->next;
82 [ # # ][ + - ]: 37 : if (b->tail == &head->next)
83 : : rcu_batch_init(b);
84 : :
85 : : return head;
86 : : }
87 : :
88 : : /*
89 : : * Move all callbacks from the rcu_batch structure specified by "from" to
90 : : * the structure specified by "to".
91 : : */
92 : : static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
93 : : {
94 [ - + ][ + - ]: 111 : if (!rcu_batch_empty(from)) {
[ + - # # ]
95 : 74 : *to->tail = from->head;
96 : 74 : to->tail = from->tail;
97 : : rcu_batch_init(from);
98 : : }
99 : : }
100 : :
101 : 0 : static int init_srcu_struct_fields(struct srcu_struct *sp)
102 : : {
103 : 0 : sp->completed = 0;
104 : 0 : spin_lock_init(&sp->queue_lock);
105 : 0 : sp->running = false;
106 : : rcu_batch_init(&sp->batch_queue);
107 : : rcu_batch_init(&sp->batch_check0);
108 : : rcu_batch_init(&sp->batch_check1);
109 : : rcu_batch_init(&sp->batch_done);
110 : 0 : INIT_DELAYED_WORK(&sp->work, process_srcu);
111 : 0 : sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
112 [ # # ]: 0 : return sp->per_cpu_ref ? 0 : -ENOMEM;
113 : : }
114 : :
115 : : #ifdef CONFIG_DEBUG_LOCK_ALLOC
116 : :
117 : : int __init_srcu_struct(struct srcu_struct *sp, const char *name,
118 : : struct lock_class_key *key)
119 : : {
120 : : /* Don't re-initialize a lock while it is held. */
121 : : debug_check_no_locks_freed((void *)sp, sizeof(*sp));
122 : : lockdep_init_map(&sp->dep_map, name, key, 0);
123 : : return init_srcu_struct_fields(sp);
124 : : }
125 : : EXPORT_SYMBOL_GPL(__init_srcu_struct);
126 : :
127 : : #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
128 : :
129 : : /**
130 : : * init_srcu_struct - initialize a sleep-RCU structure
131 : : * @sp: structure to initialize.
132 : : *
133 : : * Must invoke this on a given srcu_struct before passing that srcu_struct
134 : : * to any other function. Each srcu_struct represents a separate domain
135 : : * of SRCU protection.
136 : : */
137 : 0 : int init_srcu_struct(struct srcu_struct *sp)
138 : : {
139 : 0 : return init_srcu_struct_fields(sp);
140 : : }
141 : : EXPORT_SYMBOL_GPL(init_srcu_struct);
142 : :
143 : : #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
144 : :
145 : : /*
146 : : * Returns approximate total of the readers' ->seq[] values for the
147 : : * rank of per-CPU counters specified by idx.
148 : : */
149 : 148 : static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
150 : : {
151 : : int cpu;
152 : : unsigned long sum = 0;
153 : : unsigned long t;
154 : :
155 [ + + ]: 888 : for_each_possible_cpu(cpu) {
156 : 740 : t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
157 : 740 : sum += t;
158 : : }
159 : 148 : return sum;
160 : : }
161 : :
162 : : /*
163 : : * Returns approximate number of readers active on the specified rank
164 : : * of the per-CPU ->c[] counters.
165 : : */
166 : 74 : static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
167 : : {
168 : : int cpu;
169 : : unsigned long sum = 0;
170 : : unsigned long t;
171 : :
172 [ + + ]: 444 : for_each_possible_cpu(cpu) {
173 : 370 : t = ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
174 : 370 : sum += t;
175 : : }
176 : 74 : return sum;
177 : : }
178 : :
179 : : /*
180 : : * Return true if the number of pre-existing readers is determined to
181 : : * be stably zero. An example unstable zero can occur if the call
182 : : * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
183 : : * but due to task migration, sees the corresponding __srcu_read_unlock()
184 : : * decrement. This can happen because srcu_readers_active_idx() takes
185 : : * time to sum the array, and might in fact be interrupted or preempted
186 : : * partway through the summation.
187 : : */
188 : 0 : static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
189 : : {
190 : : unsigned long seq;
191 : :
192 : 74 : seq = srcu_readers_seq_idx(sp, idx);
193 : :
194 : : /*
195 : : * The following smp_mb() A pairs with the smp_mb() B located in
196 : : * __srcu_read_lock(). This pairing ensures that if an
197 : : * __srcu_read_lock() increments its counter after the summation
198 : : * in srcu_readers_active_idx(), then the corresponding SRCU read-side
199 : : * critical section will see any changes made prior to the start
200 : : * of the current SRCU grace period.
201 : : *
202 : : * Also, if the above call to srcu_readers_seq_idx() saw the
203 : : * increment of ->seq[], then the call to srcu_readers_active_idx()
204 : : * must see the increment of ->c[].
205 : : */
206 : 74 : smp_mb(); /* A */
207 : :
208 : : /*
209 : : * Note that srcu_readers_active_idx() can incorrectly return
210 : : * zero even though there is a pre-existing reader throughout.
211 : : * To see this, suppose that task A is in a very long SRCU
212 : : * read-side critical section that started on CPU 0, and that
213 : : * no other reader exists, so that the sum of the counters
214 : : * is equal to one. Then suppose that task B starts executing
215 : : * srcu_readers_active_idx(), summing up to CPU 1, and then that
216 : : * task C starts reading on CPU 0, so that its increment is not
217 : : * summed, but finishes reading on CPU 2, so that its decrement
218 : : * -is- summed. Then when task B completes its sum, it will
219 : : * incorrectly get zero, despite the fact that task A has been
220 : : * in its SRCU read-side critical section the whole time.
221 : : *
222 : : * We therefore do a validation step should srcu_readers_active_idx()
223 : : * return zero.
224 : : */
225 [ + - ]: 74 : if (srcu_readers_active_idx(sp, idx) != 0)
226 : : return false;
227 : :
228 : : /*
229 : : * The remainder of this function is the validation step.
230 : : * The following smp_mb() D pairs with the smp_mb() C in
231 : : * __srcu_read_unlock(). If the __srcu_read_unlock() was seen
232 : : * by srcu_readers_active_idx() above, then any destructive
233 : : * operation performed after the grace period will happen after
234 : : * the corresponding SRCU read-side critical section.
235 : : *
236 : : * Note that there can be at most NR_CPUS worth of readers using
237 : : * the old index, which is not enough to overflow even a 32-bit
238 : : * integer. (Yes, this does mean that systems having more than
239 : : * a billion or so CPUs need to be 64-bit systems.) Therefore,
240 : : * the sum of the ->seq[] counters cannot possibly overflow.
241 : : * Therefore, the only way that the return values of the two
242 : : * calls to srcu_readers_seq_idx() can be equal is if there were
243 : : * no increments of the corresponding rank of ->seq[] counts
244 : : * in the interim. But the missed-increment scenario laid out
245 : : * above includes an increment of the ->seq[] counter by
246 : : * the corresponding __srcu_read_lock(). Therefore, if this
247 : : * scenario occurs, the return values from the two calls to
248 : : * srcu_readers_seq_idx() will differ, and thus the validation
249 : : * step below suffices.
250 : : */
251 : 74 : smp_mb(); /* D */
252 : :
253 : 74 : return srcu_readers_seq_idx(sp, idx) == seq;
254 : : }
255 : :
256 : : /**
257 : : * srcu_readers_active - returns approximate number of readers.
258 : : * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
259 : : *
260 : : * Note that this is not an atomic primitive, and can therefore suffer
261 : : * severe errors when invoked on an active srcu_struct. That said, it
262 : : * can be useful as an error check at cleanup time.
263 : : */
264 : 0 : static int srcu_readers_active(struct srcu_struct *sp)
265 : : {
266 : : int cpu;
267 : : unsigned long sum = 0;
268 : :
269 [ # # ]: 0 : for_each_possible_cpu(cpu) {
270 : 0 : sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
271 : 0 : sum += ACCESS_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
272 : : }
273 : 0 : return sum;
274 : : }
275 : :
276 : : /**
277 : : * cleanup_srcu_struct - deconstruct a sleep-RCU structure
278 : : * @sp: structure to clean up.
279 : : *
280 : : * Must invoke this after you are finished using a given srcu_struct that
281 : : * was initialized via init_srcu_struct(), else you leak memory.
282 : : */
283 : 0 : void cleanup_srcu_struct(struct srcu_struct *sp)
284 : : {
285 [ # # ][ # # ]: 0 : if (WARN_ON(srcu_readers_active(sp)))
286 : 0 : return; /* Leakage unless caller handles error. */
287 : 0 : free_percpu(sp->per_cpu_ref);
288 : 0 : sp->per_cpu_ref = NULL;
289 : : }
290 : : EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
291 : :
292 : : /*
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 : : */
297 : 0 : int __srcu_read_lock(struct srcu_struct *sp)
298 : : {
299 : : int idx;
300 : :
301 : 5020573 : idx = ACCESS_ONCE(sp->completed) & 0x1;
302 : 5020573 : preempt_disable();
303 : 10074048 : ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->c[idx]) += 1;
304 : 5037024 : smp_mb(); /* B */ /* Avoid leaking the critical section. */
305 : 10027186 : ACCESS_ONCE(this_cpu_ptr(sp->per_cpu_ref)->seq[idx]) += 1;
306 : 5013593 : preempt_enable();
307 : 5027466 : return idx;
308 : : }
309 : : EXPORT_SYMBOL_GPL(__srcu_read_lock);
310 : :
311 : : /*
312 : : * Removes the count for the old reader from the appropriate per-CPU
313 : : * element of the srcu_struct. Note that this may well be a different
314 : : * CPU than that which was incremented by the corresponding srcu_read_lock().
315 : : * Must be called from process context.
316 : : */
317 : 0 : void __srcu_read_unlock(struct srcu_struct *sp, int idx)
318 : : {
319 : 4987523 : smp_mb(); /* C */ /* Avoid leaking the critical section. */
320 : 10045502 : this_cpu_dec(sp->per_cpu_ref->c[idx]);
321 : 5023452 : }
322 : : EXPORT_SYMBOL_GPL(__srcu_read_unlock);
323 : :
324 : : /*
325 : : * We use an adaptive strategy for synchronize_srcu() and especially for
326 : : * synchronize_srcu_expedited(). We spin for a fixed time period
327 : : * (defined below) to allow SRCU readers to exit their read-side critical
328 : : * sections. If there are still some readers after 10 microseconds,
329 : : * we repeatedly block for 1-millisecond time periods. This approach
330 : : * has done well in testing, so there is no need for a config parameter.
331 : : */
332 : : #define SRCU_RETRY_CHECK_DELAY 5
333 : : #define SYNCHRONIZE_SRCU_TRYCOUNT 2
334 : : #define SYNCHRONIZE_SRCU_EXP_TRYCOUNT 12
335 : :
336 : : /*
337 : : * @@@ Wait until all pre-existing readers complete. Such readers
338 : : * will have used the index specified by "idx".
339 : : * the caller should ensures the ->completed is not changed while checking
340 : : * and idx = (->completed & 1) ^ 1
341 : : */
342 : 74 : static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
343 : : {
344 : : for (;;) {
345 [ - + ]: 74 : if (srcu_readers_active_idx_check(sp, idx))
346 : : return true;
347 [ # # ]: 0 : if (--trycount <= 0)
348 : : return false;
349 : 0 : udelay(SRCU_RETRY_CHECK_DELAY);
350 : 0 : }
351 : : }
352 : :
353 : : /*
354 : : * Increment the ->completed counter so that future SRCU readers will
355 : : * use the other rank of the ->c[] and ->seq[] arrays. This allows
356 : : * us to wait for pre-existing readers in a starvation-free manner.
357 : : */
358 : : static void srcu_flip(struct srcu_struct *sp)
359 : : {
360 : 37 : sp->completed++;
361 : : }
362 : :
363 : : /*
364 : : * Enqueue an SRCU callback on the specified srcu_struct structure,
365 : : * initiating grace-period processing if it is not already running.
366 : : *
367 : : * Note that all CPUs must agree that the grace period extended beyond
368 : : * all pre-existing SRCU read-side critical section. On systems with
369 : : * more than one CPU, this means that when "func()" is invoked, each CPU
370 : : * is guaranteed to have executed a full memory barrier since the end of
371 : : * its last corresponding SRCU read-side critical section whose beginning
372 : : * preceded the call to call_rcu(). It also means that each CPU executing
373 : : * an SRCU read-side critical section that continues beyond the start of
374 : : * "func()" must have executed a memory barrier after the call_rcu()
375 : : * but before the beginning of that SRCU read-side critical section.
376 : : * Note that these guarantees include CPUs that are offline, idle, or
377 : : * executing in user mode, as well as CPUs that are executing in the kernel.
378 : : *
379 : : * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
380 : : * resulting SRCU callback function "func()", then both CPU A and CPU
381 : : * B are guaranteed to execute a full memory barrier during the time
382 : : * interval between the call to call_rcu() and the invocation of "func()".
383 : : * This guarantee applies even if CPU A and CPU B are the same CPU (but
384 : : * again only if the system has more than one CPU).
385 : : *
386 : : * Of course, these guarantees apply only for invocations of call_srcu(),
387 : : * srcu_read_lock(), and srcu_read_unlock() that are all passed the same
388 : : * srcu_struct structure.
389 : : */
390 : 0 : void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
391 : : void (*func)(struct rcu_head *head))
392 : : {
393 : : unsigned long flags;
394 : :
395 : 0 : head->next = NULL;
396 : 0 : head->func = func;
397 : 0 : spin_lock_irqsave(&sp->queue_lock, flags);
398 : : rcu_batch_queue(&sp->batch_queue, head);
399 [ # # ]: 0 : if (!sp->running) {
400 : 0 : sp->running = true;
401 : 0 : schedule_delayed_work(&sp->work, 0);
402 : : }
403 : : spin_unlock_irqrestore(&sp->queue_lock, flags);
404 : 0 : }
405 : : EXPORT_SYMBOL_GPL(call_srcu);
406 : :
407 : : struct rcu_synchronize {
408 : : struct rcu_head head;
409 : : struct completion completion;
410 : : };
411 : :
412 : : /*
413 : : * Awaken the corresponding synchronize_srcu() instance now that a
414 : : * grace period has elapsed.
415 : : */
416 : 0 : static void wakeme_after_rcu(struct rcu_head *head)
417 : : {
418 : : struct rcu_synchronize *rcu;
419 : :
420 : : rcu = container_of(head, struct rcu_synchronize, head);
421 : 0 : complete(&rcu->completion);
422 : 0 : }
423 : :
424 : : static void srcu_advance_batches(struct srcu_struct *sp, int trycount);
425 : : static void srcu_reschedule(struct srcu_struct *sp);
426 : :
427 : : /*
428 : : * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
429 : : */
430 : 0 : static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
431 : : {
432 : : struct rcu_synchronize rcu;
433 : : struct rcu_head *head = &rcu.head;
434 : : bool done = false;
435 : :
436 : : rcu_lockdep_assert(!lock_is_held(&sp->dep_map) &&
437 : : !lock_is_held(&rcu_bh_lock_map) &&
438 : : !lock_is_held(&rcu_lock_map) &&
439 : : !lock_is_held(&rcu_sched_lock_map),
440 : : "Illegal synchronize_srcu() in same-type SRCU (or RCU) read-side critical section");
441 : :
442 : : might_sleep();
443 : : init_completion(&rcu.completion);
444 : :
445 : 37 : head->next = NULL;
446 : 37 : head->func = wakeme_after_rcu;
447 : : spin_lock_irq(&sp->queue_lock);
448 [ + - ]: 37 : if (!sp->running) {
449 : : /* steal the processing owner */
450 : 37 : sp->running = true;
451 : : rcu_batch_queue(&sp->batch_check0, head);
452 : : spin_unlock_irq(&sp->queue_lock);
453 : :
454 : 37 : srcu_advance_batches(sp, trycount);
455 [ + - ]: 37 : if (!rcu_batch_empty(&sp->batch_done)) {
456 [ - + ]: 37 : BUG_ON(sp->batch_done.head != head);
457 : : rcu_batch_dequeue(&sp->batch_done);
458 : : done = true;
459 : : }
460 : : /* give the processing owner to work_struct */
461 : 37 : srcu_reschedule(sp);
462 : : } else {
463 : : rcu_batch_queue(&sp->batch_queue, head);
464 : : spin_unlock_irq(&sp->queue_lock);
465 : : }
466 : :
467 [ - + ]: 37 : if (!done)
468 : 0 : wait_for_completion(&rcu.completion);
469 : 37 : }
470 : :
471 : : /**
472 : : * synchronize_srcu - wait for prior SRCU read-side critical-section completion
473 : : * @sp: srcu_struct with which to synchronize.
474 : : *
475 : : * Wait for the count to drain to zero of both indexes. To avoid the
476 : : * possible starvation of synchronize_srcu(), it waits for the count of
477 : : * the index=((->completed & 1) ^ 1) to drain to zero at first,
478 : : * and then flip the completed and wait for the count of the other index.
479 : : *
480 : : * Can block; must be called from process context.
481 : : *
482 : : * Note that it is illegal to call synchronize_srcu() from the corresponding
483 : : * SRCU read-side critical section; doing so will result in deadlock.
484 : : * However, it is perfectly legal to call synchronize_srcu() on one
485 : : * srcu_struct from some other srcu_struct's read-side critical section,
486 : : * as long as the resulting graph of srcu_structs is acyclic.
487 : : *
488 : : * There are memory-ordering constraints implied by synchronize_srcu().
489 : : * On systems with more than one CPU, when synchronize_srcu() returns,
490 : : * each CPU is guaranteed to have executed a full memory barrier since
491 : : * the end of its last corresponding SRCU-sched read-side critical section
492 : : * whose beginning preceded the call to synchronize_srcu(). In addition,
493 : : * each CPU having an SRCU read-side critical section that extends beyond
494 : : * the return from synchronize_srcu() is guaranteed to have executed a
495 : : * full memory barrier after the beginning of synchronize_srcu() and before
496 : : * the beginning of that SRCU read-side critical section. Note that these
497 : : * guarantees include CPUs that are offline, idle, or executing in user mode,
498 : : * as well as CPUs that are executing in the kernel.
499 : : *
500 : : * Furthermore, if CPU A invoked synchronize_srcu(), which returned
501 : : * to its caller on CPU B, then both CPU A and CPU B are guaranteed
502 : : * to have executed a full memory barrier during the execution of
503 : : * synchronize_srcu(). This guarantee applies even if CPU A and CPU B
504 : : * are the same CPU, but again only if the system has more than one CPU.
505 : : *
506 : : * Of course, these memory-ordering guarantees apply only when
507 : : * synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
508 : : * passed the same srcu_struct structure.
509 : : */
510 : 0 : void synchronize_srcu(struct srcu_struct *sp)
511 : : {
512 [ # # ][ + - ]: 37 : __synchronize_srcu(sp, rcu_expedited
513 : 37 : ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
514 : : : SYNCHRONIZE_SRCU_TRYCOUNT);
515 : 37 : }
516 : : EXPORT_SYMBOL_GPL(synchronize_srcu);
517 : :
518 : : /**
519 : : * synchronize_srcu_expedited - Brute-force SRCU grace period
520 : : * @sp: srcu_struct with which to synchronize.
521 : : *
522 : : * Wait for an SRCU grace period to elapse, but be more aggressive about
523 : : * spinning rather than blocking when waiting.
524 : : *
525 : : * Note that synchronize_srcu_expedited() has the same deadlock and
526 : : * memory-ordering properties as does synchronize_srcu().
527 : : */
528 : 0 : void synchronize_srcu_expedited(struct srcu_struct *sp)
529 : : {
530 : 0 : __synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
531 : 0 : }
532 : : EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
533 : :
534 : : /**
535 : : * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
536 : : * @sp: srcu_struct on which to wait for in-flight callbacks.
537 : : */
538 : 0 : void srcu_barrier(struct srcu_struct *sp)
539 : : {
540 : : synchronize_srcu(sp);
541 : 0 : }
542 : : EXPORT_SYMBOL_GPL(srcu_barrier);
543 : :
544 : : /**
545 : : * srcu_batches_completed - return batches completed.
546 : : * @sp: srcu_struct on which to report batch completion.
547 : : *
548 : : * Report the number of batches, correlated with, but not necessarily
549 : : * precisely the same as, the number of grace periods that have elapsed.
550 : : */
551 : 0 : long srcu_batches_completed(struct srcu_struct *sp)
552 : : {
553 : 0 : return sp->completed;
554 : : }
555 : : EXPORT_SYMBOL_GPL(srcu_batches_completed);
556 : :
557 : : #define SRCU_CALLBACK_BATCH 10
558 : : #define SRCU_INTERVAL 1
559 : :
560 : : /*
561 : : * Move any new SRCU callbacks to the first stage of the SRCU grace
562 : : * period pipeline.
563 : : */
564 : 0 : static void srcu_collect_new(struct srcu_struct *sp)
565 : : {
566 [ # # ]: 0 : if (!rcu_batch_empty(&sp->batch_queue)) {
567 : : spin_lock_irq(&sp->queue_lock);
568 : : rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
569 : : spin_unlock_irq(&sp->queue_lock);
570 : : }
571 : 0 : }
572 : :
573 : : /*
574 : : * Core SRCU state machine. Advance callbacks from ->batch_check0 to
575 : : * ->batch_check1 and then to ->batch_done as readers drain.
576 : : */
577 : 0 : static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
578 : : {
579 : 37 : int idx = 1 ^ (sp->completed & 1);
580 : :
581 : : /*
582 : : * Because readers might be delayed for an extended period after
583 : : * fetching ->completed for their index, at any point in time there
584 : : * might well be readers using both idx=0 and idx=1. We therefore
585 : : * need to wait for readers to clear from both index values before
586 : : * invoking a callback.
587 : : */
588 : :
589 [ - + ][ # # ]: 37 : if (rcu_batch_empty(&sp->batch_check0) &&
590 : : rcu_batch_empty(&sp->batch_check1))
591 : : return; /* no callbacks need to be advanced */
592 : :
593 [ + - ]: 37 : if (!try_check_zero(sp, idx, trycount))
594 : : return; /* failed to advance, will try after SRCU_INTERVAL */
595 : :
596 : : /*
597 : : * The callbacks in ->batch_check1 have already done with their
598 : : * first zero check and flip back when they were enqueued on
599 : : * ->batch_check0 in a previous invocation of srcu_advance_batches().
600 : : * (Presumably try_check_zero() returned false during that
601 : : * invocation, leaving the callbacks stranded on ->batch_check1.)
602 : : * They are therefore ready to invoke, so move them to ->batch_done.
603 : : */
604 : : rcu_batch_move(&sp->batch_done, &sp->batch_check1);
605 : :
606 [ + - ]: 37 : if (rcu_batch_empty(&sp->batch_check0))
607 : : return; /* no callbacks need to be advanced */
608 : : srcu_flip(sp);
609 : :
610 : : /*
611 : : * The callbacks in ->batch_check0 just finished their
612 : : * first check zero and flip, so move them to ->batch_check1
613 : : * for future checking on the other idx.
614 : : */
615 : : rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
616 : :
617 : : /*
618 : : * SRCU read-side critical sections are normally short, so check
619 : : * at least twice in quick succession after a flip.
620 : : */
621 : 37 : trycount = trycount < 2 ? 2 : trycount;
622 [ + - ]: 37 : if (!try_check_zero(sp, idx^1, trycount))
623 : : return; /* failed to advance, will try after SRCU_INTERVAL */
624 : :
625 : : /*
626 : : * The callbacks in ->batch_check1 have now waited for all
627 : : * pre-existing readers using both idx values. They are therefore
628 : : * ready to invoke, so move them to ->batch_done.
629 : : */
630 : : rcu_batch_move(&sp->batch_done, &sp->batch_check1);
631 : : }
632 : :
633 : : /*
634 : : * Invoke a limited number of SRCU callbacks that have passed through
635 : : * their grace period. If there are more to do, SRCU will reschedule
636 : : * the workqueue.
637 : : */
638 : 0 : static void srcu_invoke_callbacks(struct srcu_struct *sp)
639 : : {
640 : : int i;
641 : : struct rcu_head *head;
642 : :
643 [ # # ]: 0 : for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
644 : : head = rcu_batch_dequeue(&sp->batch_done);
645 [ # # ]: 0 : if (!head)
646 : : break;
647 : : local_bh_disable();
648 : 0 : head->func(head);
649 : : local_bh_enable();
650 : : }
651 : 0 : }
652 : :
653 : : /*
654 : : * Finished one round of SRCU grace period. Start another if there are
655 : : * more SRCU callbacks queued, otherwise put SRCU into not-running state.
656 : : */
657 : 0 : static void srcu_reschedule(struct srcu_struct *sp)
658 : : {
659 : : bool pending = true;
660 : :
661 [ + - ][ + - ]: 37 : if (rcu_batch_empty(&sp->batch_done) &&
662 [ + - ]: 37 : rcu_batch_empty(&sp->batch_check1) &&
663 [ + - ]: 37 : rcu_batch_empty(&sp->batch_check0) &&
664 : : rcu_batch_empty(&sp->batch_queue)) {
665 : : spin_lock_irq(&sp->queue_lock);
666 [ + - ][ + - ]: 37 : if (rcu_batch_empty(&sp->batch_done) &&
667 [ + - ]: 37 : rcu_batch_empty(&sp->batch_check1) &&
668 [ + - ]: 37 : rcu_batch_empty(&sp->batch_check0) &&
669 : : rcu_batch_empty(&sp->batch_queue)) {
670 : 37 : sp->running = false;
671 : : pending = false;
672 : : }
673 : : spin_unlock_irq(&sp->queue_lock);
674 : : }
675 : :
676 [ - + ]: 37 : if (pending)
677 : 0 : schedule_delayed_work(&sp->work, SRCU_INTERVAL);
678 : 37 : }
679 : :
680 : : /*
681 : : * This is the work-queue function that handles SRCU grace periods.
682 : : */
683 : 0 : void process_srcu(struct work_struct *work)
684 : : {
685 : : struct srcu_struct *sp;
686 : :
687 : 0 : sp = container_of(work, struct srcu_struct, work.work);
688 : :
689 : 0 : srcu_collect_new(sp);
690 : 0 : srcu_advance_batches(sp, 1);
691 : 0 : srcu_invoke_callbacks(sp);
692 : 0 : srcu_reschedule(sp);
693 : 0 : }
694 : : EXPORT_SYMBOL_GPL(process_srcu);
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