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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_wait.h>
34 #include <linux/sched.h>
35 #include <linux/smp.h>
36 #include <linux/delay.h>
37 #include <linux/module.h>
38 #include <linux/srcu.h>
39
40 #include "rcu.h"
41 #include "rcu_segcblist.h"
42
43 /* Holdoff in nanoseconds for auto-expediting. */
44 #define DEFAULT_SRCU_EXP_HOLDOFF (25 * 1000)
45 static ulong exp_holdoff = DEFAULT_SRCU_EXP_HOLDOFF;
46 module_param(exp_holdoff, ulong, 0444);
47
48 /* Overflow-check frequency. N bits roughly says every 2**N grace periods. */
49 static ulong counter_wrap_check = (ULONG_MAX >> 2);
50 module_param(counter_wrap_check, ulong, 0444);
51
52 static void srcu_invoke_callbacks(struct work_struct *work);
53 static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay);
54 static void process_srcu(struct work_struct *work);
55
56 /*
57 * Initialize SRCU combining tree. Note that statically allocated
58 * srcu_struct structures might already have srcu_read_lock() and
59 * srcu_read_unlock() running against them. So if the is_static parameter
60 * is set, don't initialize ->srcu_lock_count[] and ->srcu_unlock_count[].
61 */
62 static void init_srcu_struct_nodes(struct srcu_struct *sp, bool is_static)
63 {
64 int cpu;
65 int i;
66 int level = 0;
67 int levelspread[RCU_NUM_LVLS];
68 struct srcu_data *sdp;
69 struct srcu_node *snp;
70 struct srcu_node *snp_first;
71
72 /* Work out the overall tree geometry. */
73 sp->level[0] = &sp->node[0];
74 for (i = 1; i < rcu_num_lvls; i++)
75 sp->level[i] = sp->level[i - 1] + num_rcu_lvl[i - 1];
76 rcu_init_levelspread(levelspread, num_rcu_lvl);
77
78 /* Each pass through this loop initializes one srcu_node structure. */
79 rcu_for_each_node_breadth_first(sp, snp) {
80 raw_spin_lock_init(&ACCESS_PRIVATE(snp, lock));
81 WARN_ON_ONCE(ARRAY_SIZE(snp->srcu_have_cbs) !=
82 ARRAY_SIZE(snp->srcu_data_have_cbs));
83 for (i = 0; i < ARRAY_SIZE(snp->srcu_have_cbs); i++) {
84 snp->srcu_have_cbs[i] = 0;
85 snp->srcu_data_have_cbs[i] = 0;
86 }
87 snp->srcu_gp_seq_needed_exp = 0;
88 snp->grplo = -1;
89 snp->grphi = -1;
90 if (snp == &sp->node[0]) {
91 /* Root node, special case. */
92 snp->srcu_parent = NULL;
93 continue;
94 }
95
96 /* Non-root node. */
97 if (snp == sp->level[level + 1])
98 level++;
99 snp->srcu_parent = sp->level[level - 1] +
100 (snp - sp->level[level]) /
101 levelspread[level - 1];
102 }
103
104 /*
105 * Initialize the per-CPU srcu_data array, which feeds into the
106 * leaves of the srcu_node tree.
107 */
108 WARN_ON_ONCE(ARRAY_SIZE(sdp->srcu_lock_count) !=
109 ARRAY_SIZE(sdp->srcu_unlock_count));
110 level = rcu_num_lvls - 1;
111 snp_first = sp->level[level];
112 for_each_possible_cpu(cpu) {
113 sdp = per_cpu_ptr(sp->sda, cpu);
114 raw_spin_lock_init(&ACCESS_PRIVATE(sdp, lock));
115 rcu_segcblist_init(&sdp->srcu_cblist);
116 sdp->srcu_cblist_invoking = false;
117 sdp->srcu_gp_seq_needed = sp->srcu_gp_seq;
118 sdp->srcu_gp_seq_needed_exp = sp->srcu_gp_seq;
119 sdp->mynode = &snp_first[cpu / levelspread[level]];
120 for (snp = sdp->mynode; snp != NULL; snp = snp->srcu_parent) {
121 if (snp->grplo < 0)
122 snp->grplo = cpu;
123 snp->grphi = cpu;
124 }
125 sdp->cpu = cpu;
126 INIT_DELAYED_WORK(&sdp->work, srcu_invoke_callbacks);
127 sdp->sp = sp;
128 sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
129 if (is_static)
130 continue;
131
132 /* Dynamically allocated, better be no srcu_read_locks()! */
133 for (i = 0; i < ARRAY_SIZE(sdp->srcu_lock_count); i++) {
134 sdp->srcu_lock_count[i] = 0;
135 sdp->srcu_unlock_count[i] = 0;
136 }
137 }
138 }
139
140 /*
141 * Initialize non-compile-time initialized fields, including the
142 * associated srcu_node and srcu_data structures. The is_static
143 * parameter is passed through to init_srcu_struct_nodes(), and
144 * also tells us that ->sda has already been wired up to srcu_data.
145 */
146 static int init_srcu_struct_fields(struct srcu_struct *sp, bool is_static)
147 {
148 mutex_init(&sp->srcu_cb_mutex);
149 mutex_init(&sp->srcu_gp_mutex);
150 sp->srcu_idx = 0;
151 sp->srcu_gp_seq = 0;
152 sp->srcu_barrier_seq = 0;
153 mutex_init(&sp->srcu_barrier_mutex);
154 atomic_set(&sp->srcu_barrier_cpu_cnt, 0);
155 INIT_DELAYED_WORK(&sp->work, process_srcu);
156 if (!is_static)
157 sp->sda = alloc_percpu(struct srcu_data);
158 init_srcu_struct_nodes(sp, is_static);
159 sp->srcu_gp_seq_needed_exp = 0;
160 sp->srcu_last_gp_end = ktime_get_mono_fast_ns();
161 smp_store_release(&sp->srcu_gp_seq_needed, 0); /* Init done. */
162 return sp->sda ? 0 : -ENOMEM;
163 }
164
165 #ifdef CONFIG_DEBUG_LOCK_ALLOC
166
167 int __init_srcu_struct(struct srcu_struct *sp, const char *name,
168 struct lock_class_key *key)
169 {
170 /* Don't re-initialize a lock while it is held. */
171 debug_check_no_locks_freed((void *)sp, sizeof(*sp));
172 lockdep_init_map(&sp->dep_map, name, key, 0);
173 raw_spin_lock_init(&ACCESS_PRIVATE(sp, lock));
174 return init_srcu_struct_fields(sp, false);
175 }
176 EXPORT_SYMBOL_GPL(__init_srcu_struct);
177
178 #else /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */
179
180 /**
181 * init_srcu_struct - initialize a sleep-RCU structure
182 * @sp: structure to initialize.
183 *
184 * Must invoke this on a given srcu_struct before passing that srcu_struct
185 * to any other function. Each srcu_struct represents a separate domain
186 * of SRCU protection.
187 */
188 int init_srcu_struct(struct srcu_struct *sp)
189 {
190 raw_spin_lock_init(&ACCESS_PRIVATE(sp, lock));
191 return init_srcu_struct_fields(sp, false);
192 }
193 EXPORT_SYMBOL_GPL(init_srcu_struct);
194
195 #endif /* #else #ifdef CONFIG_DEBUG_LOCK_ALLOC */
196
197 /*
198 * First-use initialization of statically allocated srcu_struct
199 * structure. Wiring up the combining tree is more than can be
200 * done with compile-time initialization, so this check is added
201 * to each update-side SRCU primitive. Use sp->lock, which -is-
202 * compile-time initialized, to resolve races involving multiple
203 * CPUs trying to garner first-use privileges.
204 */
205 static void check_init_srcu_struct(struct srcu_struct *sp)
206 {
207 unsigned long flags;
208
209 WARN_ON_ONCE(rcu_scheduler_active == RCU_SCHEDULER_INIT);
210 /* The smp_load_acquire() pairs with the smp_store_release(). */
211 if (!rcu_seq_state(smp_load_acquire(&sp->srcu_gp_seq_needed))) /*^^^*/
212 return; /* Already initialized. */
213 raw_spin_lock_irqsave_rcu_node(sp, flags);
214 if (!rcu_seq_state(sp->srcu_gp_seq_needed)) {
215 raw_spin_unlock_irqrestore_rcu_node(sp, flags);
216 return;
217 }
218 init_srcu_struct_fields(sp, true);
219 raw_spin_unlock_irqrestore_rcu_node(sp, flags);
220 }
221
222 /*
223 * Returns approximate total of the readers' ->srcu_lock_count[] values
224 * for the rank of per-CPU counters specified by idx.
225 */
226 static unsigned long srcu_readers_lock_idx(struct srcu_struct *sp, int idx)
227 {
228 int cpu;
229 unsigned long sum = 0;
230
231 for_each_possible_cpu(cpu) {
232 struct srcu_data *cpuc = per_cpu_ptr(sp->sda, cpu);
233
234 sum += READ_ONCE(cpuc->srcu_lock_count[idx]);
235 }
236 return sum;
237 }
238
239 /*
240 * Returns approximate total of the readers' ->srcu_unlock_count[] values
241 * for the rank of per-CPU counters specified by idx.
242 */
243 static unsigned long srcu_readers_unlock_idx(struct srcu_struct *sp, int idx)
244 {
245 int cpu;
246 unsigned long sum = 0;
247
248 for_each_possible_cpu(cpu) {
249 struct srcu_data *cpuc = per_cpu_ptr(sp->sda, cpu);
250
251 sum += READ_ONCE(cpuc->srcu_unlock_count[idx]);
252 }
253 return sum;
254 }
255
256 /*
257 * Return true if the number of pre-existing readers is determined to
258 * be zero.
259 */
260 static bool srcu_readers_active_idx_check(struct srcu_struct *sp, int idx)
261 {
262 unsigned long unlocks;
263
264 unlocks = srcu_readers_unlock_idx(sp, idx);
265
266 /*
267 * Make sure that a lock is always counted if the corresponding
268 * unlock is counted. Needs to be a smp_mb() as the read side may
269 * contain a read from a variable that is written to before the
270 * synchronize_srcu() in the write side. In this case smp_mb()s
271 * A and B act like the store buffering pattern.
272 *
273 * This smp_mb() also pairs with smp_mb() C to prevent accesses
274 * after the synchronize_srcu() from being executed before the
275 * grace period ends.
276 */
277 smp_mb(); /* A */
278
279 /*
280 * If the locks are the same as the unlocks, then there must have
281 * been no readers on this index at some time in between. This does
282 * not mean that there are no more readers, as one could have read
283 * the current index but not have incremented the lock counter yet.
284 *
285 * So suppose that the updater is preempted here for so long
286 * that more than ULONG_MAX non-nested readers come and go in
287 * the meantime. It turns out that this cannot result in overflow
288 * because if a reader modifies its unlock count after we read it
289 * above, then that reader's next load of ->srcu_idx is guaranteed
290 * to get the new value, which will cause it to operate on the
291 * other bank of counters, where it cannot contribute to the
292 * overflow of these counters. This means that there is a maximum
293 * of 2*NR_CPUS increments, which cannot overflow given current
294 * systems, especially not on 64-bit systems.
295 *
296 * OK, how about nesting? This does impose a limit on nesting
297 * of floor(ULONG_MAX/NR_CPUS/2), which should be sufficient,
298 * especially on 64-bit systems.
299 */
300 return srcu_readers_lock_idx(sp, idx) == unlocks;
301 }
302
303 /**
304 * srcu_readers_active - returns true if there are readers. and false
305 * otherwise
306 * @sp: which srcu_struct to count active readers (holding srcu_read_lock).
307 *
308 * Note that this is not an atomic primitive, and can therefore suffer
309 * severe errors when invoked on an active srcu_struct. That said, it
310 * can be useful as an error check at cleanup time.
311 */
312 static bool srcu_readers_active(struct srcu_struct *sp)
313 {
314 int cpu;
315 unsigned long sum = 0;
316
317 for_each_possible_cpu(cpu) {
318 struct srcu_data *cpuc = per_cpu_ptr(sp->sda, cpu);
319
320 sum += READ_ONCE(cpuc->srcu_lock_count[0]);
321 sum += READ_ONCE(cpuc->srcu_lock_count[1]);
322 sum -= READ_ONCE(cpuc->srcu_unlock_count[0]);
323 sum -= READ_ONCE(cpuc->srcu_unlock_count[1]);
324 }
325 return sum;
326 }
327
328 #define SRCU_INTERVAL 1
329
330 /*
331 * Return grace-period delay, zero if there are expedited grace
332 * periods pending, SRCU_INTERVAL otherwise.
333 */
334 static unsigned long srcu_get_delay(struct srcu_struct *sp)
335 {
336 if (ULONG_CMP_LT(READ_ONCE(sp->srcu_gp_seq),
337 READ_ONCE(sp->srcu_gp_seq_needed_exp)))
338 return 0;
339 return SRCU_INTERVAL;
340 }
341
342 /**
343 * cleanup_srcu_struct - deconstruct a sleep-RCU structure
344 * @sp: structure to clean up.
345 *
346 * Must invoke this after you are finished using a given srcu_struct that
347 * was initialized via init_srcu_struct(), else you leak memory.
348 */
349 void cleanup_srcu_struct(struct srcu_struct *sp)
350 {
351 int cpu;
352
353 if (WARN_ON(!srcu_get_delay(sp)))
354 return; /* Leakage unless caller handles error. */
355 if (WARN_ON(srcu_readers_active(sp)))
356 return; /* Leakage unless caller handles error. */
357 flush_delayed_work(&sp->work);
358 for_each_possible_cpu(cpu)
359 flush_delayed_work(&per_cpu_ptr(sp->sda, cpu)->work);
360 if (WARN_ON(rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) != SRCU_STATE_IDLE) ||
361 WARN_ON(srcu_readers_active(sp))) {
362 pr_info("cleanup_srcu_struct: Active srcu_struct %p state: %d\n", sp, rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)));
363 return; /* Caller forgot to stop doing call_srcu()? */
364 }
365 free_percpu(sp->sda);
366 sp->sda = NULL;
367 }
368 EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
369
370 /*
371 * Counts the new reader in the appropriate per-CPU element of the
372 * srcu_struct.
373 * Returns an index that must be passed to the matching srcu_read_unlock().
374 */
375 int __srcu_read_lock(struct srcu_struct *sp)
376 {
377 int idx;
378
379 idx = READ_ONCE(sp->srcu_idx) & 0x1;
380 this_cpu_inc(sp->sda->srcu_lock_count[idx]);
381 smp_mb(); /* B */ /* Avoid leaking the critical section. */
382 return idx;
383 }
384 EXPORT_SYMBOL_GPL(__srcu_read_lock);
385
386 /*
387 * Removes the count for the old reader from the appropriate per-CPU
388 * element of the srcu_struct. Note that this may well be a different
389 * CPU than that which was incremented by the corresponding srcu_read_lock().
390 */
391 void __srcu_read_unlock(struct srcu_struct *sp, int idx)
392 {
393 smp_mb(); /* C */ /* Avoid leaking the critical section. */
394 this_cpu_inc(sp->sda->srcu_unlock_count[idx]);
395 }
396 EXPORT_SYMBOL_GPL(__srcu_read_unlock);
397
398 /*
399 * We use an adaptive strategy for synchronize_srcu() and especially for
400 * synchronize_srcu_expedited(). We spin for a fixed time period
401 * (defined below) to allow SRCU readers to exit their read-side critical
402 * sections. If there are still some readers after a few microseconds,
403 * we repeatedly block for 1-millisecond time periods.
404 */
405 #define SRCU_RETRY_CHECK_DELAY 5
406
407 /*
408 * Start an SRCU grace period.
409 */
410 static void srcu_gp_start(struct srcu_struct *sp)
411 {
412 struct srcu_data *sdp = this_cpu_ptr(sp->sda);
413 int state;
414
415 lockdep_assert_held(&sp->lock);
416 WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
417 rcu_segcblist_advance(&sdp->srcu_cblist,
418 rcu_seq_current(&sp->srcu_gp_seq));
419 (void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
420 rcu_seq_snap(&sp->srcu_gp_seq));
421 smp_mb(); /* Order prior store to ->srcu_gp_seq_needed vs. GP start. */
422 rcu_seq_start(&sp->srcu_gp_seq);
423 state = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
424 WARN_ON_ONCE(state != SRCU_STATE_SCAN1);
425 }
426
427 /*
428 * Track online CPUs to guide callback workqueue placement.
429 */
430 DEFINE_PER_CPU(bool, srcu_online);
431
432 void srcu_online_cpu(unsigned int cpu)
433 {
434 WRITE_ONCE(per_cpu(srcu_online, cpu), true);
435 }
436
437 void srcu_offline_cpu(unsigned int cpu)
438 {
439 WRITE_ONCE(per_cpu(srcu_online, cpu), false);
440 }
441
442 /*
443 * Place the workqueue handler on the specified CPU if online, otherwise
444 * just run it whereever. This is useful for placing workqueue handlers
445 * that are to invoke the specified CPU's callbacks.
446 */
447 static bool srcu_queue_delayed_work_on(int cpu, struct workqueue_struct *wq,
448 struct delayed_work *dwork,
449 unsigned long delay)
450 {
451 bool ret;
452
453 preempt_disable();
454 if (READ_ONCE(per_cpu(srcu_online, cpu)))
455 ret = queue_delayed_work_on(cpu, wq, dwork, delay);
456 else
457 ret = queue_delayed_work(wq, dwork, delay);
458 preempt_enable();
459 return ret;
460 }
461
462 /*
463 * Schedule callback invocation for the specified srcu_data structure,
464 * if possible, on the corresponding CPU.
465 */
466 static void srcu_schedule_cbs_sdp(struct srcu_data *sdp, unsigned long delay)
467 {
468 srcu_queue_delayed_work_on(sdp->cpu, system_power_efficient_wq,
469 &sdp->work, delay);
470 }
471
472 /*
473 * Schedule callback invocation for all srcu_data structures associated
474 * with the specified srcu_node structure that have callbacks for the
475 * just-completed grace period, the one corresponding to idx. If possible,
476 * schedule this invocation on the corresponding CPUs.
477 */
478 static void srcu_schedule_cbs_snp(struct srcu_struct *sp, struct srcu_node *snp,
479 unsigned long mask, unsigned long delay)
480 {
481 int cpu;
482
483 for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
484 if (!(mask & (1 << (cpu - snp->grplo))))
485 continue;
486 srcu_schedule_cbs_sdp(per_cpu_ptr(sp->sda, cpu), delay);
487 }
488 }
489
490 /*
491 * Note the end of an SRCU grace period. Initiates callback invocation
492 * and starts a new grace period if needed.
493 *
494 * The ->srcu_cb_mutex acquisition does not protect any data, but
495 * instead prevents more than one grace period from starting while we
496 * are initiating callback invocation. This allows the ->srcu_have_cbs[]
497 * array to have a finite number of elements.
498 */
499 static void srcu_gp_end(struct srcu_struct *sp)
500 {
501 unsigned long cbdelay;
502 bool cbs;
503 int cpu;
504 unsigned long flags;
505 unsigned long gpseq;
506 int idx;
507 int idxnext;
508 unsigned long mask;
509 struct srcu_data *sdp;
510 struct srcu_node *snp;
511
512 /* Prevent more than one additional grace period. */
513 mutex_lock(&sp->srcu_cb_mutex);
514
515 /* End the current grace period. */
516 raw_spin_lock_irq_rcu_node(sp);
517 idx = rcu_seq_state(sp->srcu_gp_seq);
518 WARN_ON_ONCE(idx != SRCU_STATE_SCAN2);
519 cbdelay = srcu_get_delay(sp);
520 sp->srcu_last_gp_end = ktime_get_mono_fast_ns();
521 rcu_seq_end(&sp->srcu_gp_seq);
522 gpseq = rcu_seq_current(&sp->srcu_gp_seq);
523 if (ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, gpseq))
524 sp->srcu_gp_seq_needed_exp = gpseq;
525 raw_spin_unlock_irq_rcu_node(sp);
526 mutex_unlock(&sp->srcu_gp_mutex);
527 /* A new grace period can start at this point. But only one. */
528
529 /* Initiate callback invocation as needed. */
530 idx = rcu_seq_ctr(gpseq) % ARRAY_SIZE(snp->srcu_have_cbs);
531 idxnext = (idx + 1) % ARRAY_SIZE(snp->srcu_have_cbs);
532 rcu_for_each_node_breadth_first(sp, snp) {
533 raw_spin_lock_irq_rcu_node(snp);
534 cbs = false;
535 if (snp >= sp->level[rcu_num_lvls - 1])
536 cbs = snp->srcu_have_cbs[idx] == gpseq;
537 snp->srcu_have_cbs[idx] = gpseq;
538 rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
539 if (ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, gpseq))
540 snp->srcu_gp_seq_needed_exp = gpseq;
541 mask = snp->srcu_data_have_cbs[idx];
542 snp->srcu_data_have_cbs[idx] = 0;
543 raw_spin_unlock_irq_rcu_node(snp);
544 if (cbs)
545 srcu_schedule_cbs_snp(sp, snp, mask, cbdelay);
546
547 /* Occasionally prevent srcu_data counter wrap. */
548 if (!(gpseq & counter_wrap_check))
549 for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
550 sdp = per_cpu_ptr(sp->sda, cpu);
551 raw_spin_lock_irqsave_rcu_node(sdp, flags);
552 if (ULONG_CMP_GE(gpseq,
553 sdp->srcu_gp_seq_needed + 100))
554 sdp->srcu_gp_seq_needed = gpseq;
555 raw_spin_unlock_irqrestore_rcu_node(sdp, flags);
556 }
557 }
558
559 /* Callback initiation done, allow grace periods after next. */
560 mutex_unlock(&sp->srcu_cb_mutex);
561
562 /* Start a new grace period if needed. */
563 raw_spin_lock_irq_rcu_node(sp);
564 gpseq = rcu_seq_current(&sp->srcu_gp_seq);
565 if (!rcu_seq_state(gpseq) &&
566 ULONG_CMP_LT(gpseq, sp->srcu_gp_seq_needed)) {
567 srcu_gp_start(sp);
568 raw_spin_unlock_irq_rcu_node(sp);
569 /* Throttle expedited grace periods: Should be rare! */
570 srcu_reschedule(sp, rcu_seq_ctr(gpseq) & 0x3ff
571 ? 0 : SRCU_INTERVAL);
572 } else {
573 raw_spin_unlock_irq_rcu_node(sp);
574 }
575 }
576
577 /*
578 * Funnel-locking scheme to scalably mediate many concurrent expedited
579 * grace-period requests. This function is invoked for the first known
580 * expedited request for a grace period that has already been requested,
581 * but without expediting. To start a completely new grace period,
582 * whether expedited or not, use srcu_funnel_gp_start() instead.
583 */
584 static void srcu_funnel_exp_start(struct srcu_struct *sp, struct srcu_node *snp,
585 unsigned long s)
586 {
587 unsigned long flags;
588
589 for (; snp != NULL; snp = snp->srcu_parent) {
590 if (rcu_seq_done(&sp->srcu_gp_seq, s) ||
591 ULONG_CMP_GE(READ_ONCE(snp->srcu_gp_seq_needed_exp), s))
592 return;
593 raw_spin_lock_irqsave_rcu_node(snp, flags);
594 if (ULONG_CMP_GE(snp->srcu_gp_seq_needed_exp, s)) {
595 raw_spin_unlock_irqrestore_rcu_node(snp, flags);
596 return;
597 }
598 WRITE_ONCE(snp->srcu_gp_seq_needed_exp, s);
599 raw_spin_unlock_irqrestore_rcu_node(snp, flags);
600 }
601 raw_spin_lock_irqsave_rcu_node(sp, flags);
602 if (!ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
603 sp->srcu_gp_seq_needed_exp = s;
604 raw_spin_unlock_irqrestore_rcu_node(sp, flags);
605 }
606
607 /*
608 * Funnel-locking scheme to scalably mediate many concurrent grace-period
609 * requests. The winner has to do the work of actually starting grace
610 * period s. Losers must either ensure that their desired grace-period
611 * number is recorded on at least their leaf srcu_node structure, or they
612 * must take steps to invoke their own callbacks.
613 */
614 static void srcu_funnel_gp_start(struct srcu_struct *sp, struct srcu_data *sdp,
615 unsigned long s, bool do_norm)
616 {
617 unsigned long flags;
618 int idx = rcu_seq_ctr(s) % ARRAY_SIZE(sdp->mynode->srcu_have_cbs);
619 struct srcu_node *snp = sdp->mynode;
620 unsigned long snp_seq;
621
622 /* Each pass through the loop does one level of the srcu_node tree. */
623 for (; snp != NULL; snp = snp->srcu_parent) {
624 if (rcu_seq_done(&sp->srcu_gp_seq, s) && snp != sdp->mynode)
625 return; /* GP already done and CBs recorded. */
626 raw_spin_lock_irqsave_rcu_node(snp, flags);
627 if (ULONG_CMP_GE(snp->srcu_have_cbs[idx], s)) {
628 snp_seq = snp->srcu_have_cbs[idx];
629 if (snp == sdp->mynode && snp_seq == s)
630 snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
631 raw_spin_unlock_irqrestore_rcu_node(snp, flags);
632 if (snp == sdp->mynode && snp_seq != s) {
633 srcu_schedule_cbs_sdp(sdp, do_norm
634 ? SRCU_INTERVAL
635 : 0);
636 return;
637 }
638 if (!do_norm)
639 srcu_funnel_exp_start(sp, snp, s);
640 return;
641 }
642 snp->srcu_have_cbs[idx] = s;
643 if (snp == sdp->mynode)
644 snp->srcu_data_have_cbs[idx] |= sdp->grpmask;
645 if (!do_norm && ULONG_CMP_LT(snp->srcu_gp_seq_needed_exp, s))
646 snp->srcu_gp_seq_needed_exp = s;
647 raw_spin_unlock_irqrestore_rcu_node(snp, flags);
648 }
649
650 /* Top of tree, must ensure the grace period will be started. */
651 raw_spin_lock_irqsave_rcu_node(sp, flags);
652 if (ULONG_CMP_LT(sp->srcu_gp_seq_needed, s)) {
653 /*
654 * Record need for grace period s. Pair with load
655 * acquire setting up for initialization.
656 */
657 smp_store_release(&sp->srcu_gp_seq_needed, s); /*^^^*/
658 }
659 if (!do_norm && ULONG_CMP_LT(sp->srcu_gp_seq_needed_exp, s))
660 sp->srcu_gp_seq_needed_exp = s;
661
662 /* If grace period not already done and none in progress, start it. */
663 if (!rcu_seq_done(&sp->srcu_gp_seq, s) &&
664 rcu_seq_state(sp->srcu_gp_seq) == SRCU_STATE_IDLE) {
665 WARN_ON_ONCE(ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed));
666 srcu_gp_start(sp);
667 queue_delayed_work(system_power_efficient_wq, &sp->work,
668 srcu_get_delay(sp));
669 }
670 raw_spin_unlock_irqrestore_rcu_node(sp, flags);
671 }
672
673 /*
674 * Wait until all readers counted by array index idx complete, but
675 * loop an additional time if there is an expedited grace period pending.
676 * The caller must ensure that ->srcu_idx is not changed while checking.
677 */
678 static bool try_check_zero(struct srcu_struct *sp, int idx, int trycount)
679 {
680 for (;;) {
681 if (srcu_readers_active_idx_check(sp, idx))
682 return true;
683 if (--trycount + !srcu_get_delay(sp) <= 0)
684 return false;
685 udelay(SRCU_RETRY_CHECK_DELAY);
686 }
687 }
688
689 /*
690 * Increment the ->srcu_idx counter so that future SRCU readers will
691 * use the other rank of the ->srcu_(un)lock_count[] arrays. This allows
692 * us to wait for pre-existing readers in a starvation-free manner.
693 */
694 static void srcu_flip(struct srcu_struct *sp)
695 {
696 /*
697 * Ensure that if this updater saw a given reader's increment
698 * from __srcu_read_lock(), that reader was using an old value
699 * of ->srcu_idx. Also ensure that if a given reader sees the
700 * new value of ->srcu_idx, this updater's earlier scans cannot
701 * have seen that reader's increments (which is OK, because this
702 * grace period need not wait on that reader).
703 */
704 smp_mb(); /* E */ /* Pairs with B and C. */
705
706 WRITE_ONCE(sp->srcu_idx, sp->srcu_idx + 1);
707
708 /*
709 * Ensure that if the updater misses an __srcu_read_unlock()
710 * increment, that task's next __srcu_read_lock() will see the
711 * above counter update. Note that both this memory barrier
712 * and the one in srcu_readers_active_idx_check() provide the
713 * guarantee for __srcu_read_lock().
714 */
715 smp_mb(); /* D */ /* Pairs with C. */
716 }
717
718 /*
719 * If SRCU is likely idle, return true, otherwise return false.
720 *
721 * Note that it is OK for several current from-idle requests for a new
722 * grace period from idle to specify expediting because they will all end
723 * up requesting the same grace period anyhow. So no loss.
724 *
725 * Note also that if any CPU (including the current one) is still invoking
726 * callbacks, this function will nevertheless say "idle". This is not
727 * ideal, but the overhead of checking all CPUs' callback lists is even
728 * less ideal, especially on large systems. Furthermore, the wakeup
729 * can happen before the callback is fully removed, so we have no choice
730 * but to accept this type of error.
731 *
732 * This function is also subject to counter-wrap errors, but let's face
733 * it, if this function was preempted for enough time for the counters
734 * to wrap, it really doesn't matter whether or not we expedite the grace
735 * period. The extra overhead of a needlessly expedited grace period is
736 * negligible when amoritized over that time period, and the extra latency
737 * of a needlessly non-expedited grace period is similarly negligible.
738 */
739 static bool srcu_might_be_idle(struct srcu_struct *sp)
740 {
741 unsigned long curseq;
742 unsigned long flags;
743 struct srcu_data *sdp;
744 unsigned long t;
745
746 /* If the local srcu_data structure has callbacks, not idle. */
747 local_irq_save(flags);
748 sdp = this_cpu_ptr(sp->sda);
749 if (rcu_segcblist_pend_cbs(&sdp->srcu_cblist)) {
750 local_irq_restore(flags);
751 return false; /* Callbacks already present, so not idle. */
752 }
753 local_irq_restore(flags);
754
755 /*
756 * No local callbacks, so probabalistically probe global state.
757 * Exact information would require acquiring locks, which would
758 * kill scalability, hence the probabalistic nature of the probe.
759 */
760
761 /* First, see if enough time has passed since the last GP. */
762 t = ktime_get_mono_fast_ns();
763 if (exp_holdoff == 0 ||
764 time_in_range_open(t, sp->srcu_last_gp_end,
765 sp->srcu_last_gp_end + exp_holdoff))
766 return false; /* Too soon after last GP. */
767
768 /* Next, check for probable idleness. */
769 curseq = rcu_seq_current(&sp->srcu_gp_seq);
770 smp_mb(); /* Order ->srcu_gp_seq with ->srcu_gp_seq_needed. */
771 if (ULONG_CMP_LT(curseq, READ_ONCE(sp->srcu_gp_seq_needed)))
772 return false; /* Grace period in progress, so not idle. */
773 smp_mb(); /* Order ->srcu_gp_seq with prior access. */
774 if (curseq != rcu_seq_current(&sp->srcu_gp_seq))
775 return false; /* GP # changed, so not idle. */
776 return true; /* With reasonable probability, idle! */
777 }
778
779 /*
780 * SRCU callback function to leak a callback.
781 */
782 static void srcu_leak_callback(struct rcu_head *rhp)
783 {
784 }
785
786 /*
787 * Enqueue an SRCU callback on the srcu_data structure associated with
788 * the current CPU and the specified srcu_struct structure, initiating
789 * grace-period processing if it is not already running.
790 *
791 * Note that all CPUs must agree that the grace period extended beyond
792 * all pre-existing SRCU read-side critical section. On systems with
793 * more than one CPU, this means that when "func()" is invoked, each CPU
794 * is guaranteed to have executed a full memory barrier since the end of
795 * its last corresponding SRCU read-side critical section whose beginning
796 * preceded the call to call_rcu(). It also means that each CPU executing
797 * an SRCU read-side critical section that continues beyond the start of
798 * "func()" must have executed a memory barrier after the call_rcu()
799 * but before the beginning of that SRCU read-side critical section.
800 * Note that these guarantees include CPUs that are offline, idle, or
801 * executing in user mode, as well as CPUs that are executing in the kernel.
802 *
803 * Furthermore, if CPU A invoked call_rcu() and CPU B invoked the
804 * resulting SRCU callback function "func()", then both CPU A and CPU
805 * B are guaranteed to execute a full memory barrier during the time
806 * interval between the call to call_rcu() and the invocation of "func()".
807 * This guarantee applies even if CPU A and CPU B are the same CPU (but
808 * again only if the system has more than one CPU).
809 *
810 * Of course, these guarantees apply only for invocations of call_srcu(),
811 * srcu_read_lock(), and srcu_read_unlock() that are all passed the same
812 * srcu_struct structure.
813 */
814 void __call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
815 rcu_callback_t func, bool do_norm)
816 {
817 unsigned long flags;
818 bool needexp = false;
819 bool needgp = false;
820 unsigned long s;
821 struct srcu_data *sdp;
822
823 check_init_srcu_struct(sp);
824 if (debug_rcu_head_queue(rhp)) {
825 /* Probable double call_srcu(), so leak the callback. */
826 WRITE_ONCE(rhp->func, srcu_leak_callback);
827 WARN_ONCE(1, "call_srcu(): Leaked duplicate callback\n");
828 return;
829 }
830 rhp->func = func;
831 local_irq_save(flags);
832 sdp = this_cpu_ptr(sp->sda);
833 raw_spin_lock_rcu_node(sdp);
834 rcu_segcblist_enqueue(&sdp->srcu_cblist, rhp, false);
835 rcu_segcblist_advance(&sdp->srcu_cblist,
836 rcu_seq_current(&sp->srcu_gp_seq));
837 s = rcu_seq_snap(&sp->srcu_gp_seq);
838 (void)rcu_segcblist_accelerate(&sdp->srcu_cblist, s);
839 if (ULONG_CMP_LT(sdp->srcu_gp_seq_needed, s)) {
840 sdp->srcu_gp_seq_needed = s;
841 needgp = true;
842 }
843 if (!do_norm && ULONG_CMP_LT(sdp->srcu_gp_seq_needed_exp, s)) {
844 sdp->srcu_gp_seq_needed_exp = s;
845 needexp = true;
846 }
847 raw_spin_unlock_irqrestore_rcu_node(sdp, flags);
848 if (needgp)
849 srcu_funnel_gp_start(sp, sdp, s, do_norm);
850 else if (needexp)
851 srcu_funnel_exp_start(sp, sdp->mynode, s);
852 }
853
854 /**
855 * call_srcu() - Queue a callback for invocation after an SRCU grace period
856 * @sp: srcu_struct in queue the callback
857 * @head: structure to be used for queueing the SRCU callback.
858 * @func: function to be invoked after the SRCU grace period
859 *
860 * The callback function will be invoked some time after a full SRCU
861 * grace period elapses, in other words after all pre-existing SRCU
862 * read-side critical sections have completed. However, the callback
863 * function might well execute concurrently with other SRCU read-side
864 * critical sections that started after call_srcu() was invoked. SRCU
865 * read-side critical sections are delimited by srcu_read_lock() and
866 * srcu_read_unlock(), and may be nested.
867 *
868 * The callback will be invoked from process context, but must nevertheless
869 * be fast and must not block.
870 */
871 void call_srcu(struct srcu_struct *sp, struct rcu_head *rhp,
872 rcu_callback_t func)
873 {
874 __call_srcu(sp, rhp, func, true);
875 }
876 EXPORT_SYMBOL_GPL(call_srcu);
877
878 /*
879 * Helper function for synchronize_srcu() and synchronize_srcu_expedited().
880 */
881 static void __synchronize_srcu(struct srcu_struct *sp, bool do_norm)
882 {
883 struct rcu_synchronize rcu;
884
885 RCU_LOCKDEP_WARN(lock_is_held(&sp->dep_map) ||
886 lock_is_held(&rcu_bh_lock_map) ||
887 lock_is_held(&rcu_lock_map) ||
888 lock_is_held(&rcu_sched_lock_map),
889 "Illegal synchronize_srcu() in same-type SRCU (or in RCU) read-side critical section");
890
891 if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
892 return;
893 might_sleep();
894 check_init_srcu_struct(sp);
895 init_completion(&rcu.completion);
896 init_rcu_head_on_stack(&rcu.head);
897 __call_srcu(sp, &rcu.head, wakeme_after_rcu, do_norm);
898 wait_for_completion(&rcu.completion);
899 destroy_rcu_head_on_stack(&rcu.head);
900
901 /*
902 * Make sure that later code is ordered after the SRCU grace
903 * period. This pairs with the raw_spin_lock_irq_rcu_node()
904 * in srcu_invoke_callbacks(). Unlike Tree RCU, this is needed
905 * because the current CPU might have been totally uninvolved with
906 * (and thus unordered against) that grace period.
907 */
908 smp_mb();
909 }
910
911 /**
912 * synchronize_srcu_expedited - Brute-force SRCU grace period
913 * @sp: srcu_struct with which to synchronize.
914 *
915 * Wait for an SRCU grace period to elapse, but be more aggressive about
916 * spinning rather than blocking when waiting.
917 *
918 * Note that synchronize_srcu_expedited() has the same deadlock and
919 * memory-ordering properties as does synchronize_srcu().
920 */
921 void synchronize_srcu_expedited(struct srcu_struct *sp)
922 {
923 __synchronize_srcu(sp, rcu_gp_is_normal());
924 }
925 EXPORT_SYMBOL_GPL(synchronize_srcu_expedited);
926
927 /**
928 * synchronize_srcu - wait for prior SRCU read-side critical-section completion
929 * @sp: srcu_struct with which to synchronize.
930 *
931 * Wait for the count to drain to zero of both indexes. To avoid the
932 * possible starvation of synchronize_srcu(), it waits for the count of
933 * the index=((->srcu_idx & 1) ^ 1) to drain to zero at first,
934 * and then flip the srcu_idx and wait for the count of the other index.
935 *
936 * Can block; must be called from process context.
937 *
938 * Note that it is illegal to call synchronize_srcu() from the corresponding
939 * SRCU read-side critical section; doing so will result in deadlock.
940 * However, it is perfectly legal to call synchronize_srcu() on one
941 * srcu_struct from some other srcu_struct's read-side critical section,
942 * as long as the resulting graph of srcu_structs is acyclic.
943 *
944 * There are memory-ordering constraints implied by synchronize_srcu().
945 * On systems with more than one CPU, when synchronize_srcu() returns,
946 * each CPU is guaranteed to have executed a full memory barrier since
947 * the end of its last corresponding SRCU-sched read-side critical section
948 * whose beginning preceded the call to synchronize_srcu(). In addition,
949 * each CPU having an SRCU read-side critical section that extends beyond
950 * the return from synchronize_srcu() is guaranteed to have executed a
951 * full memory barrier after the beginning of synchronize_srcu() and before
952 * the beginning of that SRCU read-side critical section. Note that these
953 * guarantees include CPUs that are offline, idle, or executing in user mode,
954 * as well as CPUs that are executing in the kernel.
955 *
956 * Furthermore, if CPU A invoked synchronize_srcu(), which returned
957 * to its caller on CPU B, then both CPU A and CPU B are guaranteed
958 * to have executed a full memory barrier during the execution of
959 * synchronize_srcu(). This guarantee applies even if CPU A and CPU B
960 * are the same CPU, but again only if the system has more than one CPU.
961 *
962 * Of course, these memory-ordering guarantees apply only when
963 * synchronize_srcu(), srcu_read_lock(), and srcu_read_unlock() are
964 * passed the same srcu_struct structure.
965 *
966 * If SRCU is likely idle, expedite the first request. This semantic
967 * was provided by Classic SRCU, and is relied upon by its users, so TREE
968 * SRCU must also provide it. Note that detecting idleness is heuristic
969 * and subject to both false positives and negatives.
970 */
971 void synchronize_srcu(struct srcu_struct *sp)
972 {
973 if (srcu_might_be_idle(sp) || rcu_gp_is_expedited())
974 synchronize_srcu_expedited(sp);
975 else
976 __synchronize_srcu(sp, true);
977 }
978 EXPORT_SYMBOL_GPL(synchronize_srcu);
979
980 /*
981 * Callback function for srcu_barrier() use.
982 */
983 static void srcu_barrier_cb(struct rcu_head *rhp)
984 {
985 struct srcu_data *sdp;
986 struct srcu_struct *sp;
987
988 sdp = container_of(rhp, struct srcu_data, srcu_barrier_head);
989 sp = sdp->sp;
990 if (atomic_dec_and_test(&sp->srcu_barrier_cpu_cnt))
991 complete(&sp->srcu_barrier_completion);
992 }
993
994 /**
995 * srcu_barrier - Wait until all in-flight call_srcu() callbacks complete.
996 * @sp: srcu_struct on which to wait for in-flight callbacks.
997 */
998 void srcu_barrier(struct srcu_struct *sp)
999 {
1000 int cpu;
1001 struct srcu_data *sdp;
1002 unsigned long s = rcu_seq_snap(&sp->srcu_barrier_seq);
1003
1004 check_init_srcu_struct(sp);
1005 mutex_lock(&sp->srcu_barrier_mutex);
1006 if (rcu_seq_done(&sp->srcu_barrier_seq, s)) {
1007 smp_mb(); /* Force ordering following return. */
1008 mutex_unlock(&sp->srcu_barrier_mutex);
1009 return; /* Someone else did our work for us. */
1010 }
1011 rcu_seq_start(&sp->srcu_barrier_seq);
1012 init_completion(&sp->srcu_barrier_completion);
1013
1014 /* Initial count prevents reaching zero until all CBs are posted. */
1015 atomic_set(&sp->srcu_barrier_cpu_cnt, 1);
1016
1017 /*
1018 * Each pass through this loop enqueues a callback, but only
1019 * on CPUs already having callbacks enqueued. Note that if
1020 * a CPU already has callbacks enqueue, it must have already
1021 * registered the need for a future grace period, so all we
1022 * need do is enqueue a callback that will use the same
1023 * grace period as the last callback already in the queue.
1024 */
1025 for_each_possible_cpu(cpu) {
1026 sdp = per_cpu_ptr(sp->sda, cpu);
1027 raw_spin_lock_irq_rcu_node(sdp);
1028 atomic_inc(&sp->srcu_barrier_cpu_cnt);
1029 sdp->srcu_barrier_head.func = srcu_barrier_cb;
1030 debug_rcu_head_queue(&sdp->srcu_barrier_head);
1031 if (!rcu_segcblist_entrain(&sdp->srcu_cblist,
1032 &sdp->srcu_barrier_head, 0)) {
1033 debug_rcu_head_unqueue(&sdp->srcu_barrier_head);
1034 atomic_dec(&sp->srcu_barrier_cpu_cnt);
1035 }
1036 raw_spin_unlock_irq_rcu_node(sdp);
1037 }
1038
1039 /* Remove the initial count, at which point reaching zero can happen. */
1040 if (atomic_dec_and_test(&sp->srcu_barrier_cpu_cnt))
1041 complete(&sp->srcu_barrier_completion);
1042 wait_for_completion(&sp->srcu_barrier_completion);
1043
1044 rcu_seq_end(&sp->srcu_barrier_seq);
1045 mutex_unlock(&sp->srcu_barrier_mutex);
1046 }
1047 EXPORT_SYMBOL_GPL(srcu_barrier);
1048
1049 /**
1050 * srcu_batches_completed - return batches completed.
1051 * @sp: srcu_struct on which to report batch completion.
1052 *
1053 * Report the number of batches, correlated with, but not necessarily
1054 * precisely the same as, the number of grace periods that have elapsed.
1055 */
1056 unsigned long srcu_batches_completed(struct srcu_struct *sp)
1057 {
1058 return sp->srcu_idx;
1059 }
1060 EXPORT_SYMBOL_GPL(srcu_batches_completed);
1061
1062 /*
1063 * Core SRCU state machine. Push state bits of ->srcu_gp_seq
1064 * to SRCU_STATE_SCAN2, and invoke srcu_gp_end() when scan has
1065 * completed in that state.
1066 */
1067 static void srcu_advance_state(struct srcu_struct *sp)
1068 {
1069 int idx;
1070
1071 mutex_lock(&sp->srcu_gp_mutex);
1072
1073 /*
1074 * Because readers might be delayed for an extended period after
1075 * fetching ->srcu_idx for their index, at any point in time there
1076 * might well be readers using both idx=0 and idx=1. We therefore
1077 * need to wait for readers to clear from both index values before
1078 * invoking a callback.
1079 *
1080 * The load-acquire ensures that we see the accesses performed
1081 * by the prior grace period.
1082 */
1083 idx = rcu_seq_state(smp_load_acquire(&sp->srcu_gp_seq)); /* ^^^ */
1084 if (idx == SRCU_STATE_IDLE) {
1085 raw_spin_lock_irq_rcu_node(sp);
1086 if (ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed)) {
1087 WARN_ON_ONCE(rcu_seq_state(sp->srcu_gp_seq));
1088 raw_spin_unlock_irq_rcu_node(sp);
1089 mutex_unlock(&sp->srcu_gp_mutex);
1090 return;
1091 }
1092 idx = rcu_seq_state(READ_ONCE(sp->srcu_gp_seq));
1093 if (idx == SRCU_STATE_IDLE)
1094 srcu_gp_start(sp);
1095 raw_spin_unlock_irq_rcu_node(sp);
1096 if (idx != SRCU_STATE_IDLE) {
1097 mutex_unlock(&sp->srcu_gp_mutex);
1098 return; /* Someone else started the grace period. */
1099 }
1100 }
1101
1102 if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN1) {
1103 idx = 1 ^ (sp->srcu_idx & 1);
1104 if (!try_check_zero(sp, idx, 1)) {
1105 mutex_unlock(&sp->srcu_gp_mutex);
1106 return; /* readers present, retry later. */
1107 }
1108 srcu_flip(sp);
1109 rcu_seq_set_state(&sp->srcu_gp_seq, SRCU_STATE_SCAN2);
1110 }
1111
1112 if (rcu_seq_state(READ_ONCE(sp->srcu_gp_seq)) == SRCU_STATE_SCAN2) {
1113
1114 /*
1115 * SRCU read-side critical sections are normally short,
1116 * so check at least twice in quick succession after a flip.
1117 */
1118 idx = 1 ^ (sp->srcu_idx & 1);
1119 if (!try_check_zero(sp, idx, 2)) {
1120 mutex_unlock(&sp->srcu_gp_mutex);
1121 return; /* readers present, retry later. */
1122 }
1123 srcu_gp_end(sp); /* Releases ->srcu_gp_mutex. */
1124 }
1125 }
1126
1127 /*
1128 * Invoke a limited number of SRCU callbacks that have passed through
1129 * their grace period. If there are more to do, SRCU will reschedule
1130 * the workqueue. Note that needed memory barriers have been executed
1131 * in this task's context by srcu_readers_active_idx_check().
1132 */
1133 static void srcu_invoke_callbacks(struct work_struct *work)
1134 {
1135 bool more;
1136 struct rcu_cblist ready_cbs;
1137 struct rcu_head *rhp;
1138 struct srcu_data *sdp;
1139 struct srcu_struct *sp;
1140
1141 sdp = container_of(work, struct srcu_data, work.work);
1142 sp = sdp->sp;
1143 rcu_cblist_init(&ready_cbs);
1144 raw_spin_lock_irq_rcu_node(sdp);
1145 rcu_segcblist_advance(&sdp->srcu_cblist,
1146 rcu_seq_current(&sp->srcu_gp_seq));
1147 if (sdp->srcu_cblist_invoking ||
1148 !rcu_segcblist_ready_cbs(&sdp->srcu_cblist)) {
1149 raw_spin_unlock_irq_rcu_node(sdp);
1150 return; /* Someone else on the job or nothing to do. */
1151 }
1152
1153 /* We are on the job! Extract and invoke ready callbacks. */
1154 sdp->srcu_cblist_invoking = true;
1155 rcu_segcblist_extract_done_cbs(&sdp->srcu_cblist, &ready_cbs);
1156 raw_spin_unlock_irq_rcu_node(sdp);
1157 rhp = rcu_cblist_dequeue(&ready_cbs);
1158 for (; rhp != NULL; rhp = rcu_cblist_dequeue(&ready_cbs)) {
1159 debug_rcu_head_unqueue(rhp);
1160 local_bh_disable();
1161 rhp->func(rhp);
1162 local_bh_enable();
1163 }
1164
1165 /*
1166 * Update counts, accelerate new callbacks, and if needed,
1167 * schedule another round of callback invocation.
1168 */
1169 raw_spin_lock_irq_rcu_node(sdp);
1170 rcu_segcblist_insert_count(&sdp->srcu_cblist, &ready_cbs);
1171 (void)rcu_segcblist_accelerate(&sdp->srcu_cblist,
1172 rcu_seq_snap(&sp->srcu_gp_seq));
1173 sdp->srcu_cblist_invoking = false;
1174 more = rcu_segcblist_ready_cbs(&sdp->srcu_cblist);
1175 raw_spin_unlock_irq_rcu_node(sdp);
1176 if (more)
1177 srcu_schedule_cbs_sdp(sdp, 0);
1178 }
1179
1180 /*
1181 * Finished one round of SRCU grace period. Start another if there are
1182 * more SRCU callbacks queued, otherwise put SRCU into not-running state.
1183 */
1184 static void srcu_reschedule(struct srcu_struct *sp, unsigned long delay)
1185 {
1186 bool pushgp = true;
1187
1188 raw_spin_lock_irq_rcu_node(sp);
1189 if (ULONG_CMP_GE(sp->srcu_gp_seq, sp->srcu_gp_seq_needed)) {
1190 if (!WARN_ON_ONCE(rcu_seq_state(sp->srcu_gp_seq))) {
1191 /* All requests fulfilled, time to go idle. */
1192 pushgp = false;
1193 }
1194 } else if (!rcu_seq_state(sp->srcu_gp_seq)) {
1195 /* Outstanding request and no GP. Start one. */
1196 srcu_gp_start(sp);
1197 }
1198 raw_spin_unlock_irq_rcu_node(sp);
1199
1200 if (pushgp)
1201 queue_delayed_work(system_power_efficient_wq, &sp->work, delay);
1202 }
1203
1204 /*
1205 * This is the work-queue function that handles SRCU grace periods.
1206 */
1207 static void process_srcu(struct work_struct *work)
1208 {
1209 struct srcu_struct *sp;
1210
1211 sp = container_of(work, struct srcu_struct, work.work);
1212
1213 srcu_advance_state(sp);
1214 srcu_reschedule(sp, srcu_get_delay(sp));
1215 }
1216
1217 void srcutorture_get_gp_data(enum rcutorture_type test_type,
1218 struct srcu_struct *sp, int *flags,
1219 unsigned long *gpnum, unsigned long *completed)
1220 {
1221 if (test_type != SRCU_FLAVOR)
1222 return;
1223 *flags = 0;
1224 *completed = rcu_seq_ctr(sp->srcu_gp_seq);
1225 *gpnum = rcu_seq_ctr(sp->srcu_gp_seq_needed);
1226 }
1227 EXPORT_SYMBOL_GPL(srcutorture_get_gp_data);
1228
1229 void srcu_torture_stats_print(struct srcu_struct *sp, char *tt, char *tf)
1230 {
1231 int cpu;
1232 int idx;
1233 unsigned long s0 = 0, s1 = 0;
1234
1235 idx = sp->srcu_idx & 0x1;
1236 pr_alert("%s%s Tree SRCU per-CPU(idx=%d):", tt, tf, idx);
1237 for_each_possible_cpu(cpu) {
1238 unsigned long l0, l1;
1239 unsigned long u0, u1;
1240 long c0, c1;
1241 struct srcu_data *counts;
1242
1243 counts = per_cpu_ptr(sp->sda, cpu);
1244 u0 = counts->srcu_unlock_count[!idx];
1245 u1 = counts->srcu_unlock_count[idx];
1246
1247 /*
1248 * Make sure that a lock is always counted if the corresponding
1249 * unlock is counted.
1250 */
1251 smp_rmb();
1252
1253 l0 = counts->srcu_lock_count[!idx];
1254 l1 = counts->srcu_lock_count[idx];
1255
1256 c0 = l0 - u0;
1257 c1 = l1 - u1;
1258 pr_cont(" %d(%ld,%ld)", cpu, c0, c1);
1259 s0 += c0;
1260 s1 += c1;
1261 }
1262 pr_cont(" T(%ld,%ld)\n", s0, s1);
1263 }
1264 EXPORT_SYMBOL_GPL(srcu_torture_stats_print);
1265
1266 static int __init srcu_bootup_announce(void)
1267 {
1268 pr_info("Hierarchical SRCU implementation.\n");
1269 if (exp_holdoff != DEFAULT_SRCU_EXP_HOLDOFF)
1270 pr_info("\tNon-default auto-expedite holdoff of %lu ns.\n", exp_holdoff);
1271 return 0;
1272 }
1273 early_initcall(srcu_bootup_announce);