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TOMOYO Linux Cross Reference
Linux/kernel/rcu/srcu.c

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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, you can access it online at
 16  * http://www.gnu.org/licenses/gpl-2.0.html.
 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 "rcu.h"
 40 
 41 /*
 42  * Initialize an rcu_batch structure to empty.
 43  */
 44 static inline void rcu_batch_init(struct rcu_batch *b)
 45 {
 46         b->head = NULL;
 47         b->tail = &b->head;
 48 }
 49 
 50 /*
 51  * Enqueue a callback onto the tail of the specified rcu_batch structure.
 52  */
 53 static inline void rcu_batch_queue(struct rcu_batch *b, struct rcu_head *head)
 54 {
 55         *b->tail = head;
 56         b->tail = &head->next;
 57 }
 58 
 59 /*
 60  * Is the specified rcu_batch structure empty?
 61  */
 62 static inline bool rcu_batch_empty(struct rcu_batch *b)
 63 {
 64         return b->tail == &b->head;
 65 }
 66 
 67 /*
 68  * Remove the callback at the head of the specified rcu_batch structure
 69  * and return a pointer to it, or return NULL if the structure is empty.
 70  */
 71 static inline struct rcu_head *rcu_batch_dequeue(struct rcu_batch *b)
 72 {
 73         struct rcu_head *head;
 74 
 75         if (rcu_batch_empty(b))
 76                 return NULL;
 77 
 78         head = b->head;
 79         b->head = head->next;
 80         if (b->tail == &head->next)
 81                 rcu_batch_init(b);
 82 
 83         return head;
 84 }
 85 
 86 /*
 87  * Move all callbacks from the rcu_batch structure specified by "from" to
 88  * the structure specified by "to".
 89  */
 90 static inline void rcu_batch_move(struct rcu_batch *to, struct rcu_batch *from)
 91 {
 92         if (!rcu_batch_empty(from)) {
 93                 *to->tail = from->head;
 94                 to->tail = from->tail;
 95                 rcu_batch_init(from);
 96         }
 97 }
 98 
 99 static int init_srcu_struct_fields(struct srcu_struct *sp)
100 {
101         sp->completed = 0;
102         spin_lock_init(&sp->queue_lock);
103         sp->running = false;
104         rcu_batch_init(&sp->batch_queue);
105         rcu_batch_init(&sp->batch_check0);
106         rcu_batch_init(&sp->batch_check1);
107         rcu_batch_init(&sp->batch_done);
108         INIT_DELAYED_WORK(&sp->work, process_srcu);
109         sp->per_cpu_ref = alloc_percpu(struct srcu_struct_array);
110         return sp->per_cpu_ref ? 0 : -ENOMEM;
111 }
112 
113 #ifdef CONFIG_DEBUG_LOCK_ALLOC
114 
115 int __init_srcu_struct(struct srcu_struct *sp, const char *name,
116                        struct lock_class_key *key)
117 {
118         /* Don't re-initialize a lock while it is held. */
119         debug_check_no_locks_freed((void *)sp, sizeof(*sp));
120         lockdep_init_map(&sp->dep_map, name, key, 0);
121         return init_srcu_struct_fields(sp);
122 }
123 EXPORT_SYMBOL_GPL(__init_srcu_struct);
124 
125 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
126 
127 /**
128  * init_srcu_struct - initialize a sleep-RCU structure
129  * @sp: structure to initialize.
130  *
131  * Must invoke this on a given srcu_struct before passing that srcu_struct
132  * to any other function.  Each srcu_struct represents a separate domain
133  * of SRCU protection.
134  */
135 int init_srcu_struct(struct srcu_struct *sp)
136 {
137         return init_srcu_struct_fields(sp);
138 }
139 EXPORT_SYMBOL_GPL(init_srcu_struct);
140 
141 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
142 
143 /*
144  * Returns approximate total of the readers' ->seq[] values for the
145  * rank of per-CPU counters specified by idx.
146  */
147 static unsigned long srcu_readers_seq_idx(struct srcu_struct *sp, int idx)
148 {
149         int cpu;
150         unsigned long sum = 0;
151         unsigned long t;
152 
153         for_each_possible_cpu(cpu) {
154                 t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->seq[idx]);
155                 sum += t;
156         }
157         return sum;
158 }
159 
160 /*
161  * Returns approximate number of readers active on the specified rank
162  * of the per-CPU ->c[] counters.
163  */
164 static unsigned long srcu_readers_active_idx(struct srcu_struct *sp, int idx)
165 {
166         int cpu;
167         unsigned long sum = 0;
168         unsigned long t;
169 
170         for_each_possible_cpu(cpu) {
171                 t = READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[idx]);
172                 sum += t;
173         }
174         return sum;
175 }
176 
177 /*
178  * Return true if the number of pre-existing readers is determined to
179  * be stably zero.  An example unstable zero can occur if the call
180  * to srcu_readers_active_idx() misses an __srcu_read_lock() increment,
181  * but due to task migration, sees the corresponding __srcu_read_unlock()
182  * decrement.  This can happen because srcu_readers_active_idx() takes
183  * time to sum the array, and might in fact be interrupted or preempted
184  * partway through the summation.
185  */
186 static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
187 {
188         unsigned long seq;
189 
190         seq = srcu_readers_seq_idx(sp, idx);
191 
192         /*
193          * The following smp_mb() A pairs with the smp_mb() B located in
194          * __srcu_read_lock().  This pairing ensures that if an
195          * __srcu_read_lock() increments its counter after the summation
196          * in srcu_readers_active_idx(), then the corresponding SRCU read-side
197          * critical section will see any changes made prior to the start
198          * of the current SRCU grace period.
199          *
200          * Also, if the above call to srcu_readers_seq_idx() saw the
201          * increment of ->seq[], then the call to srcu_readers_active_idx()
202          * must see the increment of ->c[].
203          */
204         smp_mb(); /* A */
205 
206         /*
207          * Note that srcu_readers_active_idx() can incorrectly return
208          * zero even though there is a pre-existing reader throughout.
209          * To see this, suppose that task A is in a very long SRCU
210          * read-side critical section that started on CPU 0, and that
211          * no other reader exists, so that the sum of the counters
212          * is equal to one.  Then suppose that task B starts executing
213          * srcu_readers_active_idx(), summing up to CPU 1, and then that
214          * task C starts reading on CPU 0, so that its increment is not
215          * summed, but finishes reading on CPU 2, so that its decrement
216          * -is- summed.  Then when task B completes its sum, it will
217          * incorrectly get zero, despite the fact that task A has been
218          * in its SRCU read-side critical section the whole time.
219          *
220          * We therefore do a validation step should srcu_readers_active_idx()
221          * return zero.
222          */
223         if (srcu_readers_active_idx(sp, idx) != 0)
224                 return false;
225 
226         /*
227          * The remainder of this function is the validation step.
228          * The following smp_mb() D pairs with the smp_mb() C in
229          * __srcu_read_unlock().  If the __srcu_read_unlock() was seen
230          * by srcu_readers_active_idx() above, then any destructive
231          * operation performed after the grace period will happen after
232          * the corresponding SRCU read-side critical section.
233          *
234          * Note that there can be at most NR_CPUS worth of readers using
235          * the old index, which is not enough to overflow even a 32-bit
236          * integer.  (Yes, this does mean that systems having more than
237          * a billion or so CPUs need to be 64-bit systems.)  Therefore,
238          * the sum of the ->seq[] counters cannot possibly overflow.
239          * Therefore, the only way that the return values of the two
240          * calls to srcu_readers_seq_idx() can be equal is if there were
241          * no increments of the corresponding rank of ->seq[] counts
242          * in the interim.  But the missed-increment scenario laid out
243          * above includes an increment of the ->seq[] counter by
244          * the corresponding __srcu_read_lock().  Therefore, if this
245          * scenario occurs, the return values from the two calls to
246          * srcu_readers_seq_idx() will differ, and thus the validation
247          * step below suffices.
248          */
249         smp_mb(); /* D */
250 
251         return srcu_readers_seq_idx(sp, idx) == seq;
252 }
253 
254 /**
255  * srcu_readers_active - returns true if there are readers. and false
256  *                       otherwise
257  * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
258  *
259  * Note that this is not an atomic primitive, and can therefore suffer
260  * severe errors when invoked on an active srcu_struct.  That said, it
261  * can be useful as an error check at cleanup time.
262  */
263 static bool srcu_readers_active(struct srcu_struct *sp)
264 {
265         int cpu;
266         unsigned long sum = 0;
267 
268         for_each_possible_cpu(cpu) {
269                 sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[0]);
270                 sum += READ_ONCE(per_cpu_ptr(sp->per_cpu_ref, cpu)->c[1]);
271         }
272         return sum;
273 }
274 
275 /**
276  * cleanup_srcu_struct - deconstruct a sleep-RCU structure
277  * @sp: structure to clean up.
278  *
279  * Must invoke this after you are finished using a given srcu_struct that
280  * was initialized via init_srcu_struct(), else you leak memory.
281  */
282 void cleanup_srcu_struct(struct srcu_struct *sp)
283 {
284         if (WARN_ON(srcu_readers_active(sp)))
285                 return; /* Leakage unless caller handles error. */
286         free_percpu(sp->per_cpu_ref);
287         sp->per_cpu_ref = NULL;
288 }
289 EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
290 
291 /*
292  * Counts the new reader in the appropriate per-CPU element of the
293  * srcu_struct.  Must be called from process context.
294  * Returns an index that must be passed to the matching srcu_read_unlock().
295  */
296 int __srcu_read_lock(struct srcu_struct *sp)
297 {
298         int idx;
299 
300         idx = READ_ONCE(sp->completed) & 0x1;
301         __this_cpu_inc(sp->per_cpu_ref->c[idx]);
302         smp_mb(); /* B */  /* Avoid leaking the critical section. */
303         __this_cpu_inc(sp->per_cpu_ref->seq[idx]);
304         return idx;
305 }
306 EXPORT_SYMBOL_GPL(__srcu_read_lock);
307 
308 /*
309  * Removes the count for the old reader from the appropriate per-CPU
310  * element of the srcu_struct.  Note that this may well be a different
311  * CPU than that which was incremented by the corresponding srcu_read_lock().
312  * Must be called from process context.
313  */
314 void __srcu_read_unlock(struct srcu_struct *sp, int idx)
315 {
316         smp_mb(); /* C */  /* Avoid leaking the critical section. */
317         this_cpu_dec(sp->per_cpu_ref->c[idx]);
318 }
319 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
320 
321 /*
322  * We use an adaptive strategy for synchronize_srcu() and especially for
323  * synchronize_srcu_expedited().  We spin for a fixed time period
324  * (defined below) to allow SRCU readers to exit their read-side critical
325  * sections.  If there are still some readers after 10 microseconds,
326  * we repeatedly block for 1-millisecond time periods.  This approach
327  * has done well in testing, so there is no need for a config parameter.
328  */
329 #define SRCU_RETRY_CHECK_DELAY          5
330 #define SYNCHRONIZE_SRCU_TRYCOUNT       2
331 #define SYNCHRONIZE_SRCU_EXP_TRYCOUNT   12
332 
333 /*
334  * @@@ Wait until all pre-existing readers complete.  Such readers
335  * will have used the index specified by "idx".
336  * the caller should ensures the ->completed is not changed while checking
337  * and idx = (->completed & 1) ^ 1
338  */
339 static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
340 {
341         for (;;) {
342                 if (srcu_readers_active_idx_check(sp, idx))
343                         return true;
344                 if (--trycount <= 0)
345                         return false;
346                 udelay(SRCU_RETRY_CHECK_DELAY);
347         }
348 }
349 
350 /*
351  * Increment the ->completed counter so that future SRCU readers will
352  * use the other rank of the ->c[] and ->seq[] arrays.  This allows
353  * us to wait for pre-existing readers in a starvation-free manner.
354  */
355 static void srcu_flip(struct srcu_struct *sp)
356 {
357         sp->completed++;
358 }
359 
360 /*
361  * Enqueue an SRCU callback on the specified srcu_struct structure,
362  * initiating grace-period processing if it is not already running.
363  *
364  * Note that all CPUs must agree that the grace period extended beyond
365  * all pre-existing SRCU read-side critical section.  On systems with
366  * more than one CPU, this means that when "func()" is invoked, each CPU
367  * is guaranteed to have executed a full memory barrier since the end of
368  * its last corresponding SRCU read-side critical section whose beginning
369  * preceded the call to call_rcu().  It also means that each CPU executing
370  * an SRCU read-side critical section that continues beyond the start of
371  * "func()" must have executed a memory barrier after the call_rcu()
372  * but before the beginning of that SRCU read-side critical section.
373  * Note that these guarantees include CPUs that are offline, idle, or
374  * executing in user mode, as well as CPUs that are executing in the kernel.
375  *
376  * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
377  * resulting SRCU callback function "func()", then both CPU A and CPU
378  * B are guaranteed to execute a full memory barrier during the time
379  * interval between the call to call_rcu() and the invocation of "func()".
380  * This guarantee applies even if CPU A and CPU B are the same CPU (but
381  * again only if the system has more than one CPU).
382  *
383  * Of course, these guarantees apply only for invocations of call_srcu(),
384  * srcu_read_lock(), and srcu_read_unlock() that are all passed the same
385  * srcu_struct structure.
386  */
387 void call_srcu(struct srcu_struct *sp, struct rcu_head *head,
388                rcu_callback_t func)
389 {
390         unsigned long flags;
391 
392         head->next = NULL;
393         head->func = func;
394         spin_lock_irqsave(&sp->queue_lock, flags);
395         rcu_batch_queue(&sp->batch_queue, head);
396         if (!sp->running) {
397                 sp->running = true;
398                 queue_delayed_work(system_power_efficient_wq, &sp->work, 0);
399         }
400         spin_unlock_irqrestore(&sp->queue_lock, flags);
401 }
402 EXPORT_SYMBOL_GPL(call_srcu);
403 
404 static void srcu_advance_batches(struct srcu_struct *sp, int trycount);
405 static void srcu_reschedule(struct srcu_struct *sp);
406 
407 /*
408  * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
409  */
410 static void __synchronize_srcu(struct srcu_struct *sp, int trycount)
411 {
412         struct rcu_synchronize rcu;
413         struct rcu_head *head = &rcu.head;
414         bool done = false;
415 
416         RCU_LOCKDEP_WARN(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 in RCU) read-side critical section");
421 
422         might_sleep();
423         init_completion(&rcu.completion);
424 
425         head->next = NULL;
426         head->func = wakeme_after_rcu;
427         spin_lock_irq(&sp->queue_lock);
428         if (!sp->running) {
429                 /* steal the processing owner */
430                 sp->running = true;
431                 rcu_batch_queue(&sp->batch_check0, head);
432                 spin_unlock_irq(&sp->queue_lock);
433 
434                 srcu_advance_batches(sp, trycount);
435                 if (!rcu_batch_empty(&sp->batch_done)) {
436                         BUG_ON(sp->batch_done.head != head);
437                         rcu_batch_dequeue(&sp->batch_done);
438                         done = true;
439                 }
440                 /* give the processing owner to work_struct */
441                 srcu_reschedule(sp);
442         } else {
443                 rcu_batch_queue(&sp->batch_queue, head);
444                 spin_unlock_irq(&sp->queue_lock);
445         }
446 
447         if (!done)
448                 wait_for_completion(&rcu.completion);
449 }
450 
451 /**
452  * synchronize_srcu - wait for prior SRCU read-side critical-section completion
453  * @sp: srcu_struct with which to synchronize.
454  *
455  * Wait for the count to drain to zero of both indexes. To avoid the
456  * possible starvation of synchronize_srcu(), it waits for the count of
457  * the index=((->completed & 1) ^ 1) to drain to zero at first,
458  * and then flip the completed and wait for the count of the other index.
459  *
460  * Can block; must be called from process context.
461  *
462  * Note that it is illegal to call synchronize_srcu() from the corresponding
463  * SRCU read-side critical section; doing so will result in deadlock.
464  * However, it is perfectly legal to call synchronize_srcu() on one
465  * srcu_struct from some other srcu_struct's read-side critical section,
466  * as long as the resulting graph of srcu_structs is acyclic.
467  *
468  * There are memory-ordering constraints implied by synchronize_srcu().
469  * On systems with more than one CPU, when synchronize_srcu() returns,
470  * each CPU is guaranteed to have executed a full memory barrier since
471  * the end of its last corresponding SRCU-sched read-side critical section
472  * whose beginning preceded the call to synchronize_srcu().  In addition,
473  * each CPU having an SRCU read-side critical section that extends beyond
474  * the return from synchronize_srcu() is guaranteed to have executed a
475  * full memory barrier after the beginning of synchronize_srcu() and before
476  * the beginning of that SRCU read-side critical section.  Note that these
477  * guarantees include CPUs that are offline, idle, or executing in user mode,
478  * as well as CPUs that are executing in the kernel.
479  *
480  * Furthermore, if CPU A invoked synchronize_srcu(), which returned
481  * to its caller on CPU B, then both CPU A and CPU B are guaranteed
482  * to have executed a full memory barrier during the execution of
483  * synchronize_srcu().  This guarantee applies even if CPU A and CPU B
484  * are the same CPU, but again only if the system has more than one CPU.
485  *
486  * Of course, these memory-ordering guarantees apply only when
487  * synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
488  * passed the same srcu_struct structure.
489  */
490 void synchronize_srcu(struct srcu_struct *sp)
491 {
492         __synchronize_srcu(sp, (rcu_gp_is_expedited() && !rcu_gp_is_normal())
493                            ? SYNCHRONIZE_SRCU_EXP_TRYCOUNT
494                            : SYNCHRONIZE_SRCU_TRYCOUNT);
495 }
496 EXPORT_SYMBOL_GPL(synchronize_srcu);
497 
498 /**
499  * synchronize_srcu_expedited - Brute-force SRCU grace period
500  * @sp: srcu_struct with which to synchronize.
501  *
502  * Wait for an SRCU grace period to elapse, but be more aggressive about
503  * spinning rather than blocking when waiting.
504  *
505  * Note that synchronize_srcu_expedited() has the same deadlock and
506  * memory-ordering properties as does synchronize_srcu().
507  */
508 void synchronize_srcu_expedited(struct srcu_struct *sp)
509 {
510         __synchronize_srcu(sp, SYNCHRONIZE_SRCU_EXP_TRYCOUNT);
511 }
512 EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
513 
514 /**
515  * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
516  * @sp: srcu_struct on which to wait for in-flight callbacks.
517  */
518 void srcu_barrier(struct srcu_struct *sp)
519 {
520         synchronize_srcu(sp);
521 }
522 EXPORT_SYMBOL_GPL(srcu_barrier);
523 
524 /**
525  * srcu_batches_completed - return batches completed.
526  * @sp: srcu_struct on which to report batch completion.
527  *
528  * Report the number of batches, correlated with, but not necessarily
529  * precisely the same as, the number of grace periods that have elapsed.
530  */
531 unsigned long srcu_batches_completed(struct srcu_struct *sp)
532 {
533         return sp->completed;
534 }
535 EXPORT_SYMBOL_GPL(srcu_batches_completed);
536 
537 #define SRCU_CALLBACK_BATCH     10
538 #define SRCU_INTERVAL           1
539 
540 /*
541  * Move any new SRCU callbacks to the first stage of the SRCU grace
542  * period pipeline.
543  */
544 static void srcu_collect_new(struct srcu_struct *sp)
545 {
546         if (!rcu_batch_empty(&sp->batch_queue)) {
547                 spin_lock_irq(&sp->queue_lock);
548                 rcu_batch_move(&sp->batch_check0, &sp->batch_queue);
549                 spin_unlock_irq(&sp->queue_lock);
550         }
551 }
552 
553 /*
554  * Core SRCU state machine.  Advance callbacks from ->batch_check0 to
555  * ->batch_check1 and then to ->batch_done as readers drain.
556  */
557 static void srcu_advance_batches(struct srcu_struct *sp, int trycount)
558 {
559         int idx = 1 ^ (sp->completed & 1);
560 
561         /*
562          * Because readers might be delayed for an extended period after
563          * fetching ->completed for their index, at any point in time there
564          * might well be readers using both idx=0 and idx=1.  We therefore
565          * need to wait for readers to clear from both index values before
566          * invoking a callback.
567          */
568 
569         if (rcu_batch_empty(&sp->batch_check0) &&
570             rcu_batch_empty(&sp->batch_check1))
571                 return; /* no callbacks need to be advanced */
572 
573         if (!try_check_zero(sp, idx, trycount))
574                 return; /* failed to advance, will try after SRCU_INTERVAL */
575 
576         /*
577          * The callbacks in ->batch_check1 have already done with their
578          * first zero check and flip back when they were enqueued on
579          * ->batch_check0 in a previous invocation of srcu_advance_batches().
580          * (Presumably try_check_zero() returned false during that
581          * invocation, leaving the callbacks stranded on ->batch_check1.)
582          * They are therefore ready to invoke, so move them to ->batch_done.
583          */
584         rcu_batch_move(&sp->batch_done, &sp->batch_check1);
585 
586         if (rcu_batch_empty(&sp->batch_check0))
587                 return; /* no callbacks need to be advanced */
588         srcu_flip(sp);
589 
590         /*
591          * The callbacks in ->batch_check0 just finished their
592          * first check zero and flip, so move them to ->batch_check1
593          * for future checking on the other idx.
594          */
595         rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
596 
597         /*
598          * SRCU read-side critical sections are normally short, so check
599          * at least twice in quick succession after a flip.
600          */
601         trycount = trycount < 2 ? 2 : trycount;
602         if (!try_check_zero(sp, idx^1, trycount))
603                 return; /* failed to advance, will try after SRCU_INTERVAL */
604 
605         /*
606          * The callbacks in ->batch_check1 have now waited for all
607          * pre-existing readers using both idx values.  They are therefore
608          * ready to invoke, so move them to ->batch_done.
609          */
610         rcu_batch_move(&sp->batch_done, &sp->batch_check1);
611 }
612 
613 /*
614  * Invoke a limited number of SRCU callbacks that have passed through
615  * their grace period.  If there are more to do, SRCU will reschedule
616  * the workqueue.
617  */
618 static void srcu_invoke_callbacks(struct srcu_struct *sp)
619 {
620         int i;
621         struct rcu_head *head;
622 
623         for (i = 0; i < SRCU_CALLBACK_BATCH; i++) {
624                 head = rcu_batch_dequeue(&sp->batch_done);
625                 if (!head)
626                         break;
627                 local_bh_disable();
628                 head->func(head);
629                 local_bh_enable();
630         }
631 }
632 
633 /*
634  * Finished one round of SRCU grace period.  Start another if there are
635  * more SRCU callbacks queued, otherwise put SRCU into not-running state.
636  */
637 static void srcu_reschedule(struct srcu_struct *sp)
638 {
639         bool pending = true;
640 
641         if (rcu_batch_empty(&sp->batch_done) &&
642             rcu_batch_empty(&sp->batch_check1) &&
643             rcu_batch_empty(&sp->batch_check0) &&
644             rcu_batch_empty(&sp->batch_queue)) {
645                 spin_lock_irq(&sp->queue_lock);
646                 if (rcu_batch_empty(&sp->batch_done) &&
647                     rcu_batch_empty(&sp->batch_check1) &&
648                     rcu_batch_empty(&sp->batch_check0) &&
649                     rcu_batch_empty(&sp->batch_queue)) {
650                         sp->running = false;
651                         pending = false;
652                 }
653                 spin_unlock_irq(&sp->queue_lock);
654         }
655 
656         if (pending)
657                 queue_delayed_work(system_power_efficient_wq,
658                                    &sp->work, SRCU_INTERVAL);
659 }
660 
661 /*
662  * This is the work-queue function that handles SRCU grace periods.
663  */
664 void process_srcu(struct work_struct *work)
665 {
666         struct srcu_struct *sp;
667 
668         sp = container_of(work, struct srcu_struct, work.work);
669 
670         srcu_collect_new(sp);
671         srcu_advance_batches(sp, 1);
672         srcu_invoke_callbacks(sp);
673         srcu_reschedule(sp);
674 }
675 EXPORT_SYMBOL_GPL(process_srcu);
676 

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