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1/*
2 * RCU expedited grace periods
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 IBM Corporation, 2016
19 *
20 * Authors: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
21 */
22
23/* Wrapper functions for expedited grace periods. */
24static void rcu_exp_gp_seq_start(struct rcu_state *rsp)
25{
26 rcu_seq_start(&rsp->expedited_sequence);
27}
28static void rcu_exp_gp_seq_end(struct rcu_state *rsp)
29{
30 rcu_seq_end(&rsp->expedited_sequence);
31 smp_mb(); /* Ensure that consecutive grace periods serialize. */
32}
33static unsigned long rcu_exp_gp_seq_snap(struct rcu_state *rsp)
34{
35 unsigned long s;
36
37 smp_mb(); /* Caller's modifications seen first by other CPUs. */
38 s = rcu_seq_snap(&rsp->expedited_sequence);
39 trace_rcu_exp_grace_period(rsp->name, s, TPS("snap"));
40 return s;
41}
42static bool rcu_exp_gp_seq_done(struct rcu_state *rsp, unsigned long s)
43{
44 return rcu_seq_done(&rsp->expedited_sequence, s);
45}
46
47/*
48 * Reset the ->expmaskinit values in the rcu_node tree to reflect any
49 * recent CPU-online activity. Note that these masks are not cleared
50 * when CPUs go offline, so they reflect the union of all CPUs that have
51 * ever been online. This means that this function normally takes its
52 * no-work-to-do fastpath.
53 */
54static void sync_exp_reset_tree_hotplug(struct rcu_state *rsp)
55{
56 bool done;
57 unsigned long flags;
58 unsigned long mask;
59 unsigned long oldmask;
60 int ncpus = READ_ONCE(rsp->ncpus);
61 struct rcu_node *rnp;
62 struct rcu_node *rnp_up;
63
64 /* If no new CPUs onlined since last time, nothing to do. */
65 if (likely(ncpus == rsp->ncpus_snap))
66 return;
67 rsp->ncpus_snap = ncpus;
68
69 /*
70 * Each pass through the following loop propagates newly onlined
71 * CPUs for the current rcu_node structure up the rcu_node tree.
72 */
73 rcu_for_each_leaf_node(rsp, rnp) {
74 raw_spin_lock_irqsave_rcu_node(rnp, flags);
75 if (rnp->expmaskinit == rnp->expmaskinitnext) {
76 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
77 continue; /* No new CPUs, nothing to do. */
78 }
79
80 /* Update this node's mask, track old value for propagation. */
81 oldmask = rnp->expmaskinit;
82 rnp->expmaskinit = rnp->expmaskinitnext;
83 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
84
85 /* If was already nonzero, nothing to propagate. */
86 if (oldmask)
87 continue;
88
89 /* Propagate the new CPU up the tree. */
90 mask = rnp->grpmask;
91 rnp_up = rnp->parent;
92 done = false;
93 while (rnp_up) {
94 raw_spin_lock_irqsave_rcu_node(rnp_up, flags);
95 if (rnp_up->expmaskinit)
96 done = true;
97 rnp_up->expmaskinit |= mask;
98 raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags);
99 if (done)
100 break;
101 mask = rnp_up->grpmask;
102 rnp_up = rnp_up->parent;
103 }
104 }
105}
106
107/*
108 * Reset the ->expmask values in the rcu_node tree in preparation for
109 * a new expedited grace period.
110 */
111static void __maybe_unused sync_exp_reset_tree(struct rcu_state *rsp)
112{
113 unsigned long flags;
114 struct rcu_node *rnp;
115
116 sync_exp_reset_tree_hotplug(rsp);
117 rcu_for_each_node_breadth_first(rsp, rnp) {
118 raw_spin_lock_irqsave_rcu_node(rnp, flags);
119 WARN_ON_ONCE(rnp->expmask);
120 rnp->expmask = rnp->expmaskinit;
121 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
122 }
123}
124
125/*
126 * Return non-zero if there is no RCU expedited grace period in progress
127 * for the specified rcu_node structure, in other words, if all CPUs and
128 * tasks covered by the specified rcu_node structure have done their bit
129 * for the current expedited grace period. Works only for preemptible
130 * RCU -- other RCU implementation use other means.
131 *
132 * Caller must hold the rcu_state's exp_mutex.
133 */
134static int sync_rcu_preempt_exp_done(struct rcu_node *rnp)
135{
136 return rnp->exp_tasks == NULL &&
137 READ_ONCE(rnp->expmask) == 0;
138}
139
140/*
141 * Report the exit from RCU read-side critical section for the last task
142 * that queued itself during or before the current expedited preemptible-RCU
143 * grace period. This event is reported either to the rcu_node structure on
144 * which the task was queued or to one of that rcu_node structure's ancestors,
145 * recursively up the tree. (Calm down, calm down, we do the recursion
146 * iteratively!)
147 *
148 * Caller must hold the rcu_state's exp_mutex and the specified rcu_node
149 * structure's ->lock.
150 */
151static void __rcu_report_exp_rnp(struct rcu_state *rsp, struct rcu_node *rnp,
152 bool wake, unsigned long flags)
153 __releases(rnp->lock)
154{
155 unsigned long mask;
156
157 for (;;) {
158 if (!sync_rcu_preempt_exp_done(rnp)) {
159 if (!rnp->expmask)
160 rcu_initiate_boost(rnp, flags);
161 else
162 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
163 break;
164 }
165 if (rnp->parent == NULL) {
166 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
167 if (wake) {
168 smp_mb(); /* EGP done before wake_up(). */
169 swake_up(&rsp->expedited_wq);
170 }
171 break;
172 }
173 mask = rnp->grpmask;
174 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */
175 rnp = rnp->parent;
176 raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
177 WARN_ON_ONCE(!(rnp->expmask & mask));
178 rnp->expmask &= ~mask;
179 }
180}
181
182/*
183 * Report expedited quiescent state for specified node. This is a
184 * lock-acquisition wrapper function for __rcu_report_exp_rnp().
185 *
186 * Caller must hold the rcu_state's exp_mutex.
187 */
188static void __maybe_unused rcu_report_exp_rnp(struct rcu_state *rsp,
189 struct rcu_node *rnp, bool wake)
190{
191 unsigned long flags;
192
193 raw_spin_lock_irqsave_rcu_node(rnp, flags);
194 __rcu_report_exp_rnp(rsp, rnp, wake, flags);
195}
196
197/*
198 * Report expedited quiescent state for multiple CPUs, all covered by the
199 * specified leaf rcu_node structure. Caller must hold the rcu_state's
200 * exp_mutex.
201 */
202static void rcu_report_exp_cpu_mult(struct rcu_state *rsp, struct rcu_node *rnp,
203 unsigned long mask, bool wake)
204{
205 unsigned long flags;
206
207 raw_spin_lock_irqsave_rcu_node(rnp, flags);
208 if (!(rnp->expmask & mask)) {
209 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
210 return;
211 }
212 rnp->expmask &= ~mask;
213 __rcu_report_exp_rnp(rsp, rnp, wake, flags); /* Releases rnp->lock. */
214}
215
216/*
217 * Report expedited quiescent state for specified rcu_data (CPU).
218 */
219static void rcu_report_exp_rdp(struct rcu_state *rsp, struct rcu_data *rdp,
220 bool wake)
221{
222 rcu_report_exp_cpu_mult(rsp, rdp->mynode, rdp->grpmask, wake);
223}
224
225/* Common code for synchronize_{rcu,sched}_expedited() work-done checking. */
226static bool sync_exp_work_done(struct rcu_state *rsp, atomic_long_t *stat,
227 unsigned long s)
228{
229 if (rcu_exp_gp_seq_done(rsp, s)) {
230 trace_rcu_exp_grace_period(rsp->name, s, TPS("done"));
231 /* Ensure test happens before caller kfree(). */
232 smp_mb__before_atomic(); /* ^^^ */
233 atomic_long_inc(stat);
234 return true;
235 }
236 return false;
237}
238
239/*
240 * Funnel-lock acquisition for expedited grace periods. Returns true
241 * if some other task completed an expedited grace period that this task
242 * can piggy-back on, and with no mutex held. Otherwise, returns false
243 * with the mutex held, indicating that the caller must actually do the
244 * expedited grace period.
245 */
246static bool exp_funnel_lock(struct rcu_state *rsp, unsigned long s)
247{
248 struct rcu_data *rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
249 struct rcu_node *rnp = rdp->mynode;
250 struct rcu_node *rnp_root = rcu_get_root(rsp);
251
252 /* Low-contention fastpath. */
253 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
254 (rnp == rnp_root ||
255 ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
256 mutex_trylock(&rsp->exp_mutex))
257 goto fastpath;
258
259 /*
260 * Each pass through the following loop works its way up
261 * the rcu_node tree, returning if others have done the work or
262 * otherwise falls through to acquire rsp->exp_mutex. The mapping
263 * from CPU to rcu_node structure can be inexact, as it is just
264 * promoting locality and is not strictly needed for correctness.
265 */
266 for (; rnp != NULL; rnp = rnp->parent) {
267 if (sync_exp_work_done(rsp, &rdp->exp_workdone1, s))
268 return true;
269
270 /* Work not done, either wait here or go up. */
271 spin_lock(&rnp->exp_lock);
272 if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {
273
274 /* Someone else doing GP, so wait for them. */
275 spin_unlock(&rnp->exp_lock);
276 trace_rcu_exp_funnel_lock(rsp->name, rnp->level,
277 rnp->grplo, rnp->grphi,
278 TPS("wait"));
279 wait_event(rnp->exp_wq[(s >> 1) & 0x3],
280 sync_exp_work_done(rsp,
281 &rdp->exp_workdone2, s));
282 return true;
283 }
284 rnp->exp_seq_rq = s; /* Followers can wait on us. */
285 spin_unlock(&rnp->exp_lock);
286 trace_rcu_exp_funnel_lock(rsp->name, rnp->level, rnp->grplo,
287 rnp->grphi, TPS("nxtlvl"));
288 }
289 mutex_lock(&rsp->exp_mutex);
290fastpath:
291 if (sync_exp_work_done(rsp, &rdp->exp_workdone3, s)) {
292 mutex_unlock(&rsp->exp_mutex);
293 return true;
294 }
295 rcu_exp_gp_seq_start(rsp);
296 trace_rcu_exp_grace_period(rsp->name, s, TPS("start"));
297 return false;
298}
299
300/* Invoked on each online non-idle CPU for expedited quiescent state. */
301static void sync_sched_exp_handler(void *data)
302{
303 struct rcu_data *rdp;
304 struct rcu_node *rnp;
305 struct rcu_state *rsp = data;
306
307 rdp = this_cpu_ptr(rsp->rda);
308 rnp = rdp->mynode;
309 if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
310 __this_cpu_read(rcu_sched_data.cpu_no_qs.b.exp))
311 return;
312 if (rcu_is_cpu_rrupt_from_idle()) {
313 rcu_report_exp_rdp(&rcu_sched_state,
314 this_cpu_ptr(&rcu_sched_data), true);
315 return;
316 }
317 __this_cpu_write(rcu_sched_data.cpu_no_qs.b.exp, true);
318 resched_cpu(smp_processor_id());
319}
320
321/* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
322static void sync_sched_exp_online_cleanup(int cpu)
323{
324 struct rcu_data *rdp;
325 int ret;
326 struct rcu_node *rnp;
327 struct rcu_state *rsp = &rcu_sched_state;
328
329 rdp = per_cpu_ptr(rsp->rda, cpu);
330 rnp = rdp->mynode;
331 if (!(READ_ONCE(rnp->expmask) & rdp->grpmask))
332 return;
333 ret = smp_call_function_single(cpu, sync_sched_exp_handler, rsp, 0);
334 WARN_ON_ONCE(ret);
335}
336
337/*
338 * Select the nodes that the upcoming expedited grace period needs
339 * to wait for.
340 */
341static void sync_rcu_exp_select_cpus(struct rcu_state *rsp,
342 smp_call_func_t func)
343{
344 int cpu;
345 unsigned long flags;
346 unsigned long mask_ofl_test;
347 unsigned long mask_ofl_ipi;
348 int ret;
349 struct rcu_node *rnp;
350
351 sync_exp_reset_tree(rsp);
352 rcu_for_each_leaf_node(rsp, rnp) {
353 raw_spin_lock_irqsave_rcu_node(rnp, flags);
354
355 /* Each pass checks a CPU for identity, offline, and idle. */
356 mask_ofl_test = 0;
357 for_each_leaf_node_possible_cpu(rnp, cpu) {
358 struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
359 struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
360
361 rdp->exp_dynticks_snap =
362 atomic_add_return(0, &rdtp->dynticks);
363 if (raw_smp_processor_id() == cpu ||
364 !(rdp->exp_dynticks_snap & 0x1) ||
365 !(rnp->qsmaskinitnext & rdp->grpmask))
366 mask_ofl_test |= rdp->grpmask;
367 }
368 mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;
369
370 /*
371 * Need to wait for any blocked tasks as well. Note that
372 * additional blocking tasks will also block the expedited
373 * GP until such time as the ->expmask bits are cleared.
374 */
375 if (rcu_preempt_has_tasks(rnp))
376 rnp->exp_tasks = rnp->blkd_tasks.next;
377 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
378
379 /* IPI the remaining CPUs for expedited quiescent state. */
380 for_each_leaf_node_possible_cpu(rnp, cpu) {
381 unsigned long mask = leaf_node_cpu_bit(rnp, cpu);
382 struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
383 struct rcu_dynticks *rdtp = &per_cpu(rcu_dynticks, cpu);
384
385 if (!(mask_ofl_ipi & mask))
386 continue;
387retry_ipi:
388 if (atomic_add_return(0, &rdtp->dynticks) !=
389 rdp->exp_dynticks_snap) {
390 mask_ofl_test |= mask;
391 continue;
392 }
393 ret = smp_call_function_single(cpu, func, rsp, 0);
394 if (!ret) {
395 mask_ofl_ipi &= ~mask;
396 continue;
397 }
398 /* Failed, raced with CPU hotplug operation. */
399 raw_spin_lock_irqsave_rcu_node(rnp, flags);
400 if ((rnp->qsmaskinitnext & mask) &&
401 (rnp->expmask & mask)) {
402 /* Online, so delay for a bit and try again. */
403 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
404 schedule_timeout_uninterruptible(1);
405 goto retry_ipi;
406 }
407 /* CPU really is offline, so we can ignore it. */
408 if (!(rnp->expmask & mask))
409 mask_ofl_ipi &= ~mask;
410 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
411 }
412 /* Report quiescent states for those that went offline. */
413 mask_ofl_test |= mask_ofl_ipi;
414 if (mask_ofl_test)
415 rcu_report_exp_cpu_mult(rsp, rnp, mask_ofl_test, false);
416 }
417}
418
419static void synchronize_sched_expedited_wait(struct rcu_state *rsp)
420{
421 int cpu;
422 unsigned long jiffies_stall;
423 unsigned long jiffies_start;
424 unsigned long mask;
425 int ndetected;
426 struct rcu_node *rnp;
427 struct rcu_node *rnp_root = rcu_get_root(rsp);
428 int ret;
429
430 jiffies_stall = rcu_jiffies_till_stall_check();
431 jiffies_start = jiffies;
432
433 for (;;) {
434 ret = swait_event_timeout(
435 rsp->expedited_wq,
436 sync_rcu_preempt_exp_done(rnp_root),
437 jiffies_stall);
438 if (ret > 0 || sync_rcu_preempt_exp_done(rnp_root))
439 return;
440 WARN_ON(ret < 0); /* workqueues should not be signaled. */
441 if (rcu_cpu_stall_suppress)
442 continue;
443 panic_on_rcu_stall();
444 pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
445 rsp->name);
446 ndetected = 0;
447 rcu_for_each_leaf_node(rsp, rnp) {
448 ndetected += rcu_print_task_exp_stall(rnp);
449 for_each_leaf_node_possible_cpu(rnp, cpu) {
450 struct rcu_data *rdp;
451
452 mask = leaf_node_cpu_bit(rnp, cpu);
453 if (!(rnp->expmask & mask))
454 continue;
455 ndetected++;
456 rdp = per_cpu_ptr(rsp->rda, cpu);
457 pr_cont(" %d-%c%c%c", cpu,
458 "O."[!!cpu_online(cpu)],
459 "o."[!!(rdp->grpmask & rnp->expmaskinit)],
460 "N."[!!(rdp->grpmask & rnp->expmaskinitnext)]);
461 }
462 }
463 pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
464 jiffies - jiffies_start, rsp->expedited_sequence,
465 rnp_root->expmask, ".T"[!!rnp_root->exp_tasks]);
466 if (ndetected) {
467 pr_err("blocking rcu_node structures:");
468 rcu_for_each_node_breadth_first(rsp, rnp) {
469 if (rnp == rnp_root)
470 continue; /* printed unconditionally */
471 if (sync_rcu_preempt_exp_done(rnp))
472 continue;
473 pr_cont(" l=%u:%d-%d:%#lx/%c",
474 rnp->level, rnp->grplo, rnp->grphi,
475 rnp->expmask,
476 ".T"[!!rnp->exp_tasks]);
477 }
478 pr_cont("\n");
479 }
480 rcu_for_each_leaf_node(rsp, rnp) {
481 for_each_leaf_node_possible_cpu(rnp, cpu) {
482 mask = leaf_node_cpu_bit(rnp, cpu);
483 if (!(rnp->expmask & mask))
484 continue;
485 dump_cpu_task(cpu);
486 }
487 }
488 jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3;
489 }
490}
491
492/*
493 * Wait for the current expedited grace period to complete, and then
494 * wake up everyone who piggybacked on the just-completed expedited
495 * grace period. Also update all the ->exp_seq_rq counters as needed
496 * in order to avoid counter-wrap problems.
497 */
498static void rcu_exp_wait_wake(struct rcu_state *rsp, unsigned long s)
499{
500 struct rcu_node *rnp;
501
502 synchronize_sched_expedited_wait(rsp);
503 rcu_exp_gp_seq_end(rsp);
504 trace_rcu_exp_grace_period(rsp->name, s, TPS("end"));
505
506 /*
507 * Switch over to wakeup mode, allowing the next GP, but -only- the
508 * next GP, to proceed.
509 */
510 mutex_lock(&rsp->exp_wake_mutex);
511
512 rcu_for_each_node_breadth_first(rsp, rnp) {
513 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
514 spin_lock(&rnp->exp_lock);
515 /* Recheck, avoid hang in case someone just arrived. */
516 if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
517 rnp->exp_seq_rq = s;
518 spin_unlock(&rnp->exp_lock);
519 }
520 wake_up_all(&rnp->exp_wq[(rsp->expedited_sequence >> 1) & 0x3]);
521 }
522 trace_rcu_exp_grace_period(rsp->name, s, TPS("endwake"));
523 mutex_unlock(&rsp->exp_wake_mutex);
524}
525
526/* Let the workqueue handler know what it is supposed to do. */
527struct rcu_exp_work {
528 smp_call_func_t rew_func;
529 struct rcu_state *rew_rsp;
530 unsigned long rew_s;
531 struct work_struct rew_work;
532};
533
534/*
535 * Common code to drive an expedited grace period forward, used by
536 * workqueues and mid-boot-time tasks.
537 */
538static void rcu_exp_sel_wait_wake(struct rcu_state *rsp,
539 smp_call_func_t func, unsigned long s)
540{
541 /* Initialize the rcu_node tree in preparation for the wait. */
542 sync_rcu_exp_select_cpus(rsp, func);
543
544 /* Wait and clean up, including waking everyone. */
545 rcu_exp_wait_wake(rsp, s);
546}
547
548/*
549 * Work-queue handler to drive an expedited grace period forward.
550 */
551static void wait_rcu_exp_gp(struct work_struct *wp)
552{
553 struct rcu_exp_work *rewp;
554
555 rewp = container_of(wp, struct rcu_exp_work, rew_work);
556 rcu_exp_sel_wait_wake(rewp->rew_rsp, rewp->rew_func, rewp->rew_s);
557}
558
559/*
560 * Given an rcu_state pointer and a smp_call_function() handler, kick
561 * off the specified flavor of expedited grace period.
562 */
563static void _synchronize_rcu_expedited(struct rcu_state *rsp,
564 smp_call_func_t func)
565{
566 struct rcu_data *rdp;
567 struct rcu_exp_work rew;
568 struct rcu_node *rnp;
569 unsigned long s;
570
571 /* If expedited grace periods are prohibited, fall back to normal. */
572 if (rcu_gp_is_normal()) {
573 wait_rcu_gp(rsp->call);
574 return;
575 }
576
577 /* Take a snapshot of the sequence number. */
578 s = rcu_exp_gp_seq_snap(rsp);
579 if (exp_funnel_lock(rsp, s))
580 return; /* Someone else did our work for us. */
581
582 /* Ensure that load happens before action based on it. */
583 if (unlikely(rcu_scheduler_active == RCU_SCHEDULER_INIT)) {
584 /* Direct call during scheduler init and early_initcalls(). */
585 rcu_exp_sel_wait_wake(rsp, func, s);
586 } else {
587 /* Marshall arguments & schedule the expedited grace period. */
588 rew.rew_func = func;
589 rew.rew_rsp = rsp;
590 rew.rew_s = s;
591 INIT_WORK_ONSTACK(&rew.rew_work, wait_rcu_exp_gp);
592 schedule_work(&rew.rew_work);
593 }
594
595 /* Wait for expedited grace period to complete. */
596 rdp = per_cpu_ptr(rsp->rda, raw_smp_processor_id());
597 rnp = rcu_get_root(rsp);
598 wait_event(rnp->exp_wq[(s >> 1) & 0x3],
599 sync_exp_work_done(rsp,
600 &rdp->exp_workdone0, s));
601
602 /* Let the next expedited grace period start. */
603 mutex_unlock(&rsp->exp_mutex);
604}
605
606/**
607 * synchronize_sched_expedited - Brute-force RCU-sched grace period
608 *
609 * Wait for an RCU-sched grace period to elapse, but use a "big hammer"
610 * approach to force the grace period to end quickly. This consumes
611 * significant time on all CPUs and is unfriendly to real-time workloads,
612 * so is thus not recommended for any sort of common-case code. In fact,
613 * if you are using synchronize_sched_expedited() in a loop, please
614 * restructure your code to batch your updates, and then use a single
615 * synchronize_sched() instead.
616 *
617 * This implementation can be thought of as an application of sequence
618 * locking to expedited grace periods, but using the sequence counter to
619 * determine when someone else has already done the work instead of for
620 * retrying readers.
621 */
622void synchronize_sched_expedited(void)
623{
624 struct rcu_state *rsp = &rcu_sched_state;
625
626 /* If only one CPU, this is automatically a grace period. */
627 if (rcu_blocking_is_gp())
628 return;
629
630 _synchronize_rcu_expedited(rsp, sync_sched_exp_handler);
631}
632EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
633
634#ifdef CONFIG_PREEMPT_RCU
635
636/*
637 * Remote handler for smp_call_function_single(). If there is an
638 * RCU read-side critical section in effect, request that the
639 * next rcu_read_unlock() record the quiescent state up the
640 * ->expmask fields in the rcu_node tree. Otherwise, immediately
641 * report the quiescent state.
642 */
643static void sync_rcu_exp_handler(void *info)
644{
645 struct rcu_data *rdp;
646 struct rcu_state *rsp = info;
647 struct task_struct *t = current;
648
649 /*
650 * Within an RCU read-side critical section, request that the next
651 * rcu_read_unlock() report. Unless this RCU read-side critical
652 * section has already blocked, in which case it is already set
653 * up for the expedited grace period to wait on it.
654 */
655 if (t->rcu_read_lock_nesting > 0 &&
656 !t->rcu_read_unlock_special.b.blocked) {
657 t->rcu_read_unlock_special.b.exp_need_qs = true;
658 return;
659 }
660
661 /*
662 * We are either exiting an RCU read-side critical section (negative
663 * values of t->rcu_read_lock_nesting) or are not in one at all
664 * (zero value of t->rcu_read_lock_nesting). Or we are in an RCU
665 * read-side critical section that blocked before this expedited
666 * grace period started. Either way, we can immediately report
667 * the quiescent state.
668 */
669 rdp = this_cpu_ptr(rsp->rda);
670 rcu_report_exp_rdp(rsp, rdp, true);
671}
672
673/**
674 * synchronize_rcu_expedited - Brute-force RCU grace period
675 *
676 * Wait for an RCU-preempt grace period, but expedite it. The basic
677 * idea is to IPI all non-idle non-nohz online CPUs. The IPI handler
678 * checks whether the CPU is in an RCU-preempt critical section, and
679 * if so, it sets a flag that causes the outermost rcu_read_unlock()
680 * to report the quiescent state. On the other hand, if the CPU is
681 * not in an RCU read-side critical section, the IPI handler reports
682 * the quiescent state immediately.
683 *
684 * Although this is a greate improvement over previous expedited
685 * implementations, it is still unfriendly to real-time workloads, so is
686 * thus not recommended for any sort of common-case code. In fact, if
687 * you are using synchronize_rcu_expedited() in a loop, please restructure
688 * your code to batch your updates, and then Use a single synchronize_rcu()
689 * instead.
690 */
691void synchronize_rcu_expedited(void)
692{
693 struct rcu_state *rsp = rcu_state_p;
694
695 if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
696 return;
697 _synchronize_rcu_expedited(rsp, sync_rcu_exp_handler);
698}
699EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
700
701#else /* #ifdef CONFIG_PREEMPT_RCU */
702
703/*
704 * Wait for an rcu-preempt grace period, but make it happen quickly.
705 * But because preemptible RCU does not exist, map to rcu-sched.
706 */
707void synchronize_rcu_expedited(void)
708{
709 synchronize_sched_expedited();
710}
711EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
712
713#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
714
715/*
716 * Switch to run-time mode once Tree RCU has fully initialized.
717 */
718static int __init rcu_exp_runtime_mode(void)
719{
720 rcu_test_sync_prims();
721 rcu_scheduler_active = RCU_SCHEDULER_RUNNING;
722 rcu_test_sync_prims();
723 return 0;
724}
725core_initcall(rcu_exp_runtime_mode);