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1/* SPDX-License-Identifier: GPL-2.0+ */
2/*
3 * RCU expedited grace periods
4 *
5 * Copyright IBM Corporation, 2016
6 *
7 * Authors: Paul E. McKenney <[email protected]>
8 */
9
10#include <linux/console.h>
11#include <linux/lockdep.h>
12
13static void rcu_exp_handler(void *unused);
14static int rcu_print_task_exp_stall(struct rcu_node *rnp);
15static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp);
16
17/*
18 * Record the start of an expedited grace period.
19 */
20static void rcu_exp_gp_seq_start(void)
21{
22 rcu_seq_start(&rcu_state.expedited_sequence);
23 rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_exp_snap);
24}
25
26/*
27 * Return the value that the expedited-grace-period counter will have
28 * at the end of the current grace period.
29 */
30static __maybe_unused unsigned long rcu_exp_gp_seq_endval(void)
31{
32 return rcu_seq_endval(&rcu_state.expedited_sequence);
33}
34
35/*
36 * Record the end of an expedited grace period.
37 */
38static void rcu_exp_gp_seq_end(void)
39{
40 rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_exp_snap);
41 rcu_seq_end(&rcu_state.expedited_sequence);
42 smp_mb(); /* Ensure that consecutive grace periods serialize. */
43}
44
45/*
46 * Take a snapshot of the expedited-grace-period counter, which is the
47 * earliest value that will indicate that a full grace period has
48 * elapsed since the current time.
49 */
50static unsigned long rcu_exp_gp_seq_snap(void)
51{
52 unsigned long s;
53
54 smp_mb(); /* Caller's modifications seen first by other CPUs. */
55 s = rcu_seq_snap(&rcu_state.expedited_sequence);
56 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("snap"));
57 return s;
58}
59
60/*
61 * Given a counter snapshot from rcu_exp_gp_seq_snap(), return true
62 * if a full expedited grace period has elapsed since that snapshot
63 * was taken.
64 */
65static bool rcu_exp_gp_seq_done(unsigned long s)
66{
67 return rcu_seq_done(&rcu_state.expedited_sequence, s);
68}
69
70/*
71 * Reset the ->expmaskinit values in the rcu_node tree to reflect any
72 * recent CPU-online activity. Note that these masks are not cleared
73 * when CPUs go offline, so they reflect the union of all CPUs that have
74 * ever been online. This means that this function normally takes its
75 * no-work-to-do fastpath.
76 */
77static void sync_exp_reset_tree_hotplug(void)
78{
79 bool done;
80 unsigned long flags;
81 unsigned long mask;
82 unsigned long oldmask;
83 int ncpus = smp_load_acquire(&rcu_state.ncpus); /* Order vs. locking. */
84 struct rcu_node *rnp;
85 struct rcu_node *rnp_up;
86
87 /* If no new CPUs onlined since last time, nothing to do. */
88 if (likely(ncpus == rcu_state.ncpus_snap))
89 return;
90 rcu_state.ncpus_snap = ncpus;
91
92 /*
93 * Each pass through the following loop propagates newly onlined
94 * CPUs for the current rcu_node structure up the rcu_node tree.
95 */
96 rcu_for_each_leaf_node(rnp) {
97 raw_spin_lock_irqsave_rcu_node(rnp, flags);
98 if (rnp->expmaskinit == rnp->expmaskinitnext) {
99 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
100 continue; /* No new CPUs, nothing to do. */
101 }
102
103 /* Update this node's mask, track old value for propagation. */
104 oldmask = rnp->expmaskinit;
105 rnp->expmaskinit = rnp->expmaskinitnext;
106 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
107
108 /* If was already nonzero, nothing to propagate. */
109 if (oldmask)
110 continue;
111
112 /* Propagate the new CPU up the tree. */
113 mask = rnp->grpmask;
114 rnp_up = rnp->parent;
115 done = false;
116 while (rnp_up) {
117 raw_spin_lock_irqsave_rcu_node(rnp_up, flags);
118 if (rnp_up->expmaskinit)
119 done = true;
120 rnp_up->expmaskinit |= mask;
121 raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags);
122 if (done)
123 break;
124 mask = rnp_up->grpmask;
125 rnp_up = rnp_up->parent;
126 }
127 }
128}
129
130/*
131 * Reset the ->expmask values in the rcu_node tree in preparation for
132 * a new expedited grace period.
133 */
134static void __maybe_unused sync_exp_reset_tree(void)
135{
136 unsigned long flags;
137 struct rcu_node *rnp;
138
139 sync_exp_reset_tree_hotplug();
140 rcu_for_each_node_breadth_first(rnp) {
141 raw_spin_lock_irqsave_rcu_node(rnp, flags);
142 WARN_ON_ONCE(rnp->expmask);
143 WRITE_ONCE(rnp->expmask, rnp->expmaskinit);
144 /*
145 * Need to wait for any blocked tasks as well. Note that
146 * additional blocking tasks will also block the expedited GP
147 * until such time as the ->expmask bits are cleared.
148 */
149 if (rcu_is_leaf_node(rnp) && rcu_preempt_has_tasks(rnp))
150 WRITE_ONCE(rnp->exp_tasks, rnp->blkd_tasks.next);
151 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
152 }
153}
154
155/*
156 * Return non-zero if there is no RCU expedited grace period in progress
157 * for the specified rcu_node structure, in other words, if all CPUs and
158 * tasks covered by the specified rcu_node structure have done their bit
159 * for the current expedited grace period.
160 */
161static bool sync_rcu_exp_done(struct rcu_node *rnp)
162{
163 raw_lockdep_assert_held_rcu_node(rnp);
164 return READ_ONCE(rnp->exp_tasks) == NULL &&
165 READ_ONCE(rnp->expmask) == 0;
166}
167
168/*
169 * Like sync_rcu_exp_done(), but where the caller does not hold the
170 * rcu_node's ->lock.
171 */
172static bool sync_rcu_exp_done_unlocked(struct rcu_node *rnp)
173{
174 unsigned long flags;
175 bool ret;
176
177 raw_spin_lock_irqsave_rcu_node(rnp, flags);
178 ret = sync_rcu_exp_done(rnp);
179 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
180
181 return ret;
182}
183
184/*
185 * Report the exit from RCU read-side critical section for the last task
186 * that queued itself during or before the current expedited preemptible-RCU
187 * grace period. This event is reported either to the rcu_node structure on
188 * which the task was queued or to one of that rcu_node structure's ancestors,
189 * recursively up the tree. (Calm down, calm down, we do the recursion
190 * iteratively!)
191 */
192static void __rcu_report_exp_rnp(struct rcu_node *rnp,
193 bool wake, unsigned long flags)
194 __releases(rnp->lock)
195{
196 unsigned long mask;
197
198 raw_lockdep_assert_held_rcu_node(rnp);
199 for (;;) {
200 if (!sync_rcu_exp_done(rnp)) {
201 if (!rnp->expmask)
202 rcu_initiate_boost(rnp, flags);
203 else
204 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
205 break;
206 }
207 if (rnp->parent == NULL) {
208 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
209 if (wake)
210 swake_up_one(&rcu_state.expedited_wq);
211
212 break;
213 }
214 mask = rnp->grpmask;
215 raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */
216 rnp = rnp->parent;
217 raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
218 WARN_ON_ONCE(!(rnp->expmask & mask));
219 WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
220 }
221}
222
223/*
224 * Report expedited quiescent state for specified node. This is a
225 * lock-acquisition wrapper function for __rcu_report_exp_rnp().
226 */
227static void __maybe_unused rcu_report_exp_rnp(struct rcu_node *rnp, bool wake)
228{
229 unsigned long flags;
230
231 raw_spin_lock_irqsave_rcu_node(rnp, flags);
232 __rcu_report_exp_rnp(rnp, wake, flags);
233}
234
235/*
236 * Report expedited quiescent state for multiple CPUs, all covered by the
237 * specified leaf rcu_node structure, which is acquired by the caller.
238 */
239static void rcu_report_exp_cpu_mult(struct rcu_node *rnp, unsigned long flags,
240 unsigned long mask_in, bool wake)
241 __releases(rnp->lock)
242{
243 int cpu;
244 unsigned long mask;
245 struct rcu_data *rdp;
246
247 raw_lockdep_assert_held_rcu_node(rnp);
248 if (!(rnp->expmask & mask_in)) {
249 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
250 return;
251 }
252 mask = mask_in & rnp->expmask;
253 WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
254 for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
255 rdp = per_cpu_ptr(&rcu_data, cpu);
256 if (!IS_ENABLED(CONFIG_NO_HZ_FULL) || !rdp->rcu_forced_tick_exp)
257 continue;
258 rdp->rcu_forced_tick_exp = false;
259 tick_dep_clear_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
260 }
261 __rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */
262}
263
264/*
265 * Report expedited quiescent state for specified rcu_data (CPU).
266 */
267static void rcu_report_exp_rdp(struct rcu_data *rdp)
268{
269 unsigned long flags;
270 struct rcu_node *rnp = rdp->mynode;
271
272 raw_spin_lock_irqsave_rcu_node(rnp, flags);
273 WRITE_ONCE(rdp->cpu_no_qs.b.exp, false);
274 ASSERT_EXCLUSIVE_WRITER(rdp->cpu_no_qs.b.exp);
275 rcu_report_exp_cpu_mult(rnp, flags, rdp->grpmask, true);
276}
277
278/* Common code for work-done checking. */
279static bool sync_exp_work_done(unsigned long s)
280{
281 if (rcu_exp_gp_seq_done(s)) {
282 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
283 /*
284 * Order GP completion with preceding accesses. Order also GP
285 * completion with post GP update side accesses. Pairs with
286 * rcu_seq_end().
287 */
288 smp_mb();
289 return true;
290 }
291 return false;
292}
293
294/*
295 * Funnel-lock acquisition for expedited grace periods. Returns true
296 * if some other task completed an expedited grace period that this task
297 * can piggy-back on, and with no mutex held. Otherwise, returns false
298 * with the mutex held, indicating that the caller must actually do the
299 * expedited grace period.
300 */
301static bool exp_funnel_lock(unsigned long s)
302{
303 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
304 struct rcu_node *rnp = rdp->mynode;
305 struct rcu_node *rnp_root = rcu_get_root();
306
307 /* Low-contention fastpath. */
308 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
309 (rnp == rnp_root ||
310 ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
311 mutex_trylock(&rcu_state.exp_mutex))
312 goto fastpath;
313
314 /*
315 * Each pass through the following loop works its way up
316 * the rcu_node tree, returning if others have done the work or
317 * otherwise falls through to acquire ->exp_mutex. The mapping
318 * from CPU to rcu_node structure can be inexact, as it is just
319 * promoting locality and is not strictly needed for correctness.
320 */
321 for (; rnp != NULL; rnp = rnp->parent) {
322 if (sync_exp_work_done(s))
323 return true;
324
325 /* Work not done, either wait here or go up. */
326 spin_lock(&rnp->exp_lock);
327 if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {
328
329 /* Someone else doing GP, so wait for them. */
330 spin_unlock(&rnp->exp_lock);
331 trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
332 rnp->grplo, rnp->grphi,
333 TPS("wait"));
334 wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
335 sync_exp_work_done(s));
336 return true;
337 }
338 WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */
339 spin_unlock(&rnp->exp_lock);
340 trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
341 rnp->grplo, rnp->grphi, TPS("nxtlvl"));
342 }
343 mutex_lock(&rcu_state.exp_mutex);
344fastpath:
345 if (sync_exp_work_done(s)) {
346 mutex_unlock(&rcu_state.exp_mutex);
347 return true;
348 }
349 rcu_exp_gp_seq_start();
350 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("start"));
351 return false;
352}
353
354/*
355 * Select the CPUs within the specified rcu_node that the upcoming
356 * expedited grace period needs to wait for.
357 */
358static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
359{
360 int cpu;
361 unsigned long flags;
362 unsigned long mask_ofl_test;
363 unsigned long mask_ofl_ipi;
364 int ret;
365 struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew);
366
367 raw_spin_lock_irqsave_rcu_node(rnp, flags);
368
369 /* Each pass checks a CPU for identity, offline, and idle. */
370 mask_ofl_test = 0;
371 for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
372 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
373 unsigned long mask = rdp->grpmask;
374 int snap;
375
376 if (raw_smp_processor_id() == cpu ||
377 !(rnp->qsmaskinitnext & mask)) {
378 mask_ofl_test |= mask;
379 } else {
380 /*
381 * Full ordering between remote CPU's post idle accesses
382 * and updater's accesses prior to current GP (and also
383 * the started GP sequence number) is enforced by
384 * rcu_seq_start() implicit barrier, relayed by kworkers
385 * locking and even further by smp_mb__after_unlock_lock()
386 * barriers chained all the way throughout the rnp locking
387 * tree since sync_exp_reset_tree() and up to the current
388 * leaf rnp locking.
389 *
390 * Ordering between remote CPU's pre idle accesses and
391 * post grace period updater's accesses is enforced by the
392 * below acquire semantic.
393 */
394 snap = ct_rcu_watching_cpu_acquire(cpu);
395 if (rcu_watching_snap_in_eqs(snap))
396 mask_ofl_test |= mask;
397 else
398 rdp->exp_watching_snap = snap;
399 }
400 }
401 mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;
402
403 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
404
405 /* IPI the remaining CPUs for expedited quiescent state. */
406 for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) {
407 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
408 unsigned long mask = rdp->grpmask;
409
410retry_ipi:
411 if (rcu_watching_snap_stopped_since(rdp, rdp->exp_watching_snap)) {
412 mask_ofl_test |= mask;
413 continue;
414 }
415 if (get_cpu() == cpu) {
416 mask_ofl_test |= mask;
417 put_cpu();
418 continue;
419 }
420 ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
421 put_cpu();
422 /* The CPU will report the QS in response to the IPI. */
423 if (!ret)
424 continue;
425
426 /* Failed, raced with CPU hotplug operation. */
427 raw_spin_lock_irqsave_rcu_node(rnp, flags);
428 if ((rnp->qsmaskinitnext & mask) &&
429 (rnp->expmask & mask)) {
430 /* Online, so delay for a bit and try again. */
431 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
432 trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("selectofl"));
433 schedule_timeout_idle(1);
434 goto retry_ipi;
435 }
436 /* CPU really is offline, so we must report its QS. */
437 if (rnp->expmask & mask)
438 mask_ofl_test |= mask;
439 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
440 }
441 /* Report quiescent states for those that went offline. */
442 if (mask_ofl_test) {
443 raw_spin_lock_irqsave_rcu_node(rnp, flags);
444 rcu_report_exp_cpu_mult(rnp, flags, mask_ofl_test, false);
445 }
446}
447
448static void rcu_exp_sel_wait_wake(unsigned long s);
449
450static void sync_rcu_exp_select_node_cpus(struct kthread_work *wp)
451{
452 struct rcu_exp_work *rewp =
453 container_of(wp, struct rcu_exp_work, rew_work);
454
455 __sync_rcu_exp_select_node_cpus(rewp);
456}
457
458static inline bool rcu_exp_worker_started(void)
459{
460 return !!READ_ONCE(rcu_exp_gp_kworker);
461}
462
463static inline bool rcu_exp_par_worker_started(struct rcu_node *rnp)
464{
465 return !!READ_ONCE(rnp->exp_kworker);
466}
467
468static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
469{
470 kthread_init_work(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
471 /*
472 * Use rcu_exp_par_gp_kworker, because flushing a work item from
473 * another work item on the same kthread worker can result in
474 * deadlock.
475 */
476 kthread_queue_work(READ_ONCE(rnp->exp_kworker), &rnp->rew.rew_work);
477}
478
479static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
480{
481 kthread_flush_work(&rnp->rew.rew_work);
482}
483
484/*
485 * Work-queue handler to drive an expedited grace period forward.
486 */
487static void wait_rcu_exp_gp(struct kthread_work *wp)
488{
489 struct rcu_exp_work *rewp;
490
491 rewp = container_of(wp, struct rcu_exp_work, rew_work);
492 rcu_exp_sel_wait_wake(rewp->rew_s);
493}
494
495static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
496{
497 kthread_init_work(&rew->rew_work, wait_rcu_exp_gp);
498 kthread_queue_work(rcu_exp_gp_kworker, &rew->rew_work);
499}
500
501/*
502 * Select the nodes that the upcoming expedited grace period needs
503 * to wait for.
504 */
505static void sync_rcu_exp_select_cpus(void)
506{
507 struct rcu_node *rnp;
508
509 trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset"));
510 sync_exp_reset_tree();
511 trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("select"));
512
513 /* Schedule work for each leaf rcu_node structure. */
514 rcu_for_each_leaf_node(rnp) {
515 rnp->exp_need_flush = false;
516 if (!READ_ONCE(rnp->expmask))
517 continue; /* Avoid early boot non-existent wq. */
518 if (!rcu_exp_par_worker_started(rnp) ||
519 rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
520 rcu_is_last_leaf_node(rnp)) {
521 /* No worker started yet or last leaf, do direct call. */
522 sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
523 continue;
524 }
525 sync_rcu_exp_select_cpus_queue_work(rnp);
526 rnp->exp_need_flush = true;
527 }
528
529 /* Wait for jobs (if any) to complete. */
530 rcu_for_each_leaf_node(rnp)
531 if (rnp->exp_need_flush)
532 sync_rcu_exp_select_cpus_flush_work(rnp);
533}
534
535/*
536 * Wait for the expedited grace period to elapse, within time limit.
537 * If the time limit is exceeded without the grace period elapsing,
538 * return false, otherwise return true.
539 */
540static bool synchronize_rcu_expedited_wait_once(long tlimit)
541{
542 int t;
543 struct rcu_node *rnp_root = rcu_get_root();
544
545 t = swait_event_timeout_exclusive(rcu_state.expedited_wq,
546 sync_rcu_exp_done_unlocked(rnp_root),
547 tlimit);
548 // Workqueues should not be signaled.
549 if (t > 0 || sync_rcu_exp_done_unlocked(rnp_root))
550 return true;
551 WARN_ON(t < 0); /* workqueues should not be signaled. */
552 return false;
553}
554
555/*
556 * Print out an expedited RCU CPU stall warning message.
557 */
558static void synchronize_rcu_expedited_stall(unsigned long jiffies_start, unsigned long j)
559{
560 int cpu;
561 unsigned long mask;
562 int ndetected;
563 struct rcu_node *rnp;
564 struct rcu_node *rnp_root = rcu_get_root();
565
566 if (READ_ONCE(csd_lock_suppress_rcu_stall) && csd_lock_is_stuck()) {
567 pr_err("INFO: %s detected expedited stalls, but suppressed full report due to a stuck CSD-lock.\n", rcu_state.name);
568 return;
569 }
570 pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {", rcu_state.name);
571 ndetected = 0;
572 rcu_for_each_leaf_node(rnp) {
573 ndetected += rcu_print_task_exp_stall(rnp);
574 for_each_leaf_node_possible_cpu(rnp, cpu) {
575 struct rcu_data *rdp;
576
577 mask = leaf_node_cpu_bit(rnp, cpu);
578 if (!(READ_ONCE(rnp->expmask) & mask))
579 continue;
580 ndetected++;
581 cpumask_set_cpu(cpu, &rcu_exp_stall_cpumask);
582 rdp = per_cpu_ptr(&rcu_data, cpu);
583 pr_cont(" %d-%c%c%c%c", cpu,
584 "O."[!!cpu_online(cpu)],
585 "o."[!!(rdp->grpmask & rnp->expmaskinit)],
586 "N."[!!(rdp->grpmask & rnp->expmaskinitnext)],
587 "D."[!!data_race(rdp->cpu_no_qs.b.exp)]);
588 }
589 }
590 pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
591 j - jiffies_start, rcu_state.expedited_sequence, data_race(rnp_root->expmask),
592 ".T"[!!data_race(rnp_root->exp_tasks)]);
593 if (!ndetected) {
594 // This is invoked from the grace-period worker, so
595 // a new grace period cannot have started. And if this
596 // worker were stalled, we would not get here. ;-)
597 pr_err("INFO: Expedited stall ended before state dump start\n");
598 } else {
599 pr_err("blocking rcu_node structures (internal RCU debug):");
600 rcu_for_each_node_breadth_first(rnp) {
601 if (rnp == rnp_root)
602 continue; /* printed unconditionally */
603 if (sync_rcu_exp_done_unlocked(rnp))
604 continue;
605 pr_cont(" l=%u:%d-%d:%#lx/%c",
606 rnp->level, rnp->grplo, rnp->grphi, data_race(rnp->expmask),
607 ".T"[!!data_race(rnp->exp_tasks)]);
608 }
609 pr_cont("\n");
610 }
611 rcu_for_each_leaf_node(rnp) {
612 for_each_leaf_node_possible_cpu(rnp, cpu) {
613 mask = leaf_node_cpu_bit(rnp, cpu);
614 if (!(READ_ONCE(rnp->expmask) & mask))
615 continue;
616 dump_cpu_task(cpu);
617 }
618 rcu_exp_print_detail_task_stall_rnp(rnp);
619 }
620}
621
622/*
623 * Wait for the expedited grace period to elapse, issuing any needed
624 * RCU CPU stall warnings along the way.
625 */
626static void synchronize_rcu_expedited_wait(void)
627{
628 int cpu;
629 unsigned long j;
630 unsigned long jiffies_stall;
631 unsigned long jiffies_start;
632 unsigned long mask;
633 struct rcu_data *rdp;
634 struct rcu_node *rnp;
635 unsigned long flags;
636
637 trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
638 jiffies_stall = rcu_exp_jiffies_till_stall_check();
639 jiffies_start = jiffies;
640 if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) {
641 if (synchronize_rcu_expedited_wait_once(1))
642 return;
643 rcu_for_each_leaf_node(rnp) {
644 raw_spin_lock_irqsave_rcu_node(rnp, flags);
645 mask = READ_ONCE(rnp->expmask);
646 for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
647 rdp = per_cpu_ptr(&rcu_data, cpu);
648 if (rdp->rcu_forced_tick_exp)
649 continue;
650 rdp->rcu_forced_tick_exp = true;
651 if (cpu_online(cpu))
652 tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
653 }
654 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
655 }
656 j = READ_ONCE(jiffies_till_first_fqs);
657 if (synchronize_rcu_expedited_wait_once(j + HZ))
658 return;
659 }
660
661 for (;;) {
662 unsigned long j;
663
664 if (synchronize_rcu_expedited_wait_once(jiffies_stall))
665 return;
666 if (rcu_stall_is_suppressed())
667 continue;
668
669 cpumask_clear(&rcu_exp_stall_cpumask);
670
671 nbcon_cpu_emergency_enter();
672
673 j = jiffies;
674 rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_EXP, (void *)(j - jiffies_start));
675 trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
676 synchronize_rcu_expedited_stall(jiffies_start, j);
677 jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
678
679 nbcon_cpu_emergency_exit();
680
681 panic_on_rcu_stall(&rcu_exp_stall_cpumask);
682 }
683}
684
685/*
686 * Wait for the current expedited grace period to complete, and then
687 * wake up everyone who piggybacked on the just-completed expedited
688 * grace period. Also update all the ->exp_seq_rq counters as needed
689 * in order to avoid counter-wrap problems.
690 */
691static void rcu_exp_wait_wake(unsigned long s)
692{
693 struct rcu_node *rnp;
694
695 synchronize_rcu_expedited_wait();
696
697 // Switch over to wakeup mode, allowing the next GP to proceed.
698 // End the previous grace period only after acquiring the mutex
699 // to ensure that only one GP runs concurrently with wakeups.
700 mutex_lock(&rcu_state.exp_wake_mutex);
701 rcu_exp_gp_seq_end();
702 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));
703
704 rcu_for_each_node_breadth_first(rnp) {
705 if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
706 spin_lock(&rnp->exp_lock);
707 /* Recheck, avoid hang in case someone just arrived. */
708 if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
709 WRITE_ONCE(rnp->exp_seq_rq, s);
710 spin_unlock(&rnp->exp_lock);
711 }
712 smp_mb(); /* All above changes before wakeup. */
713 wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);
714 if (rcu_is_leaf_node(rnp))
715 rcu_nocb_exp_cleanup(rnp);
716 }
717 trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
718 mutex_unlock(&rcu_state.exp_wake_mutex);
719}
720
721/*
722 * Common code to drive an expedited grace period forward, used by
723 * workqueues and mid-boot-time tasks.
724 */
725static void rcu_exp_sel_wait_wake(unsigned long s)
726{
727 /* Initialize the rcu_node tree in preparation for the wait. */
728 sync_rcu_exp_select_cpus();
729
730 /* Wait and clean up, including waking everyone. */
731 rcu_exp_wait_wake(s);
732}
733
734/* Request an expedited quiescent state. */
735static void rcu_exp_need_qs(void)
736{
737 lockdep_assert_irqs_disabled();
738 ASSERT_EXCLUSIVE_WRITER_SCOPED(*this_cpu_ptr(&rcu_data.cpu_no_qs.b.exp));
739 this_cpu_write(rcu_data.cpu_no_qs.b.exp, true);
740 /* Store .exp before .rcu_urgent_qs. */
741 smp_store_release(this_cpu_ptr(&rcu_data.rcu_urgent_qs), true);
742 set_need_resched_current();
743}
744
745#ifdef CONFIG_PREEMPT_RCU
746
747/*
748 * Remote handler for smp_call_function_single(). If there is an
749 * RCU read-side critical section in effect, request that the
750 * next rcu_read_unlock() record the quiescent state up the
751 * ->expmask fields in the rcu_node tree. Otherwise, immediately
752 * report the quiescent state.
753 */
754static void rcu_exp_handler(void *unused)
755{
756 int depth = rcu_preempt_depth();
757 unsigned long flags;
758 struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
759 struct rcu_node *rnp = rdp->mynode;
760 struct task_struct *t = current;
761
762 /*
763 * WARN if the CPU is unexpectedly already looking for a
764 * QS or has already reported one.
765 */
766 ASSERT_EXCLUSIVE_WRITER_SCOPED(rdp->cpu_no_qs.b.exp);
767 if (WARN_ON_ONCE(!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
768 READ_ONCE(rdp->cpu_no_qs.b.exp)))
769 return;
770
771 /*
772 * Second, the common case of not being in an RCU read-side
773 * critical section. If also enabled or idle, immediately
774 * report the quiescent state, otherwise defer.
775 */
776 if (!depth) {
777 if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
778 rcu_is_cpu_rrupt_from_idle())
779 rcu_report_exp_rdp(rdp);
780 else
781 rcu_exp_need_qs();
782 return;
783 }
784
785 /*
786 * Third, the less-common case of being in an RCU read-side
787 * critical section. In this case we can count on a future
788 * rcu_read_unlock(). However, this rcu_read_unlock() might
789 * execute on some other CPU, but in that case there will be
790 * a future context switch. Either way, if the expedited
791 * grace period is still waiting on this CPU, set ->deferred_qs
792 * so that the eventual quiescent state will be reported.
793 * Note that there is a large group of race conditions that
794 * can have caused this quiescent state to already have been
795 * reported, so we really do need to check ->expmask.
796 */
797 if (depth > 0) {
798 raw_spin_lock_irqsave_rcu_node(rnp, flags);
799 if (rnp->expmask & rdp->grpmask) {
800 WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
801 t->rcu_read_unlock_special.b.exp_hint = true;
802 }
803 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
804 return;
805 }
806
807 // Fourth and finally, negative nesting depth should not happen.
808 WARN_ON_ONCE(1);
809}
810
811/*
812 * Scan the current list of tasks blocked within RCU read-side critical
813 * sections, printing out the tid of each that is blocking the current
814 * expedited grace period.
815 */
816static int rcu_print_task_exp_stall(struct rcu_node *rnp)
817{
818 unsigned long flags;
819 int ndetected = 0;
820 struct task_struct *t;
821
822 raw_spin_lock_irqsave_rcu_node(rnp, flags);
823 if (!rnp->exp_tasks) {
824 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
825 return 0;
826 }
827 t = list_entry(rnp->exp_tasks->prev,
828 struct task_struct, rcu_node_entry);
829 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
830 pr_cont(" P%d", t->pid);
831 ndetected++;
832 }
833 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
834 return ndetected;
835}
836
837/*
838 * Scan the current list of tasks blocked within RCU read-side critical
839 * sections, dumping the stack of each that is blocking the current
840 * expedited grace period.
841 */
842static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
843{
844 unsigned long flags;
845 struct task_struct *t;
846
847 if (!rcu_exp_stall_task_details)
848 return;
849 raw_spin_lock_irqsave_rcu_node(rnp, flags);
850 if (!READ_ONCE(rnp->exp_tasks)) {
851 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
852 return;
853 }
854 t = list_entry(rnp->exp_tasks->prev,
855 struct task_struct, rcu_node_entry);
856 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
857 /*
858 * We could be printing a lot while holding a spinlock.
859 * Avoid triggering hard lockup.
860 */
861 touch_nmi_watchdog();
862 sched_show_task(t);
863 }
864 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
865}
866
867#else /* #ifdef CONFIG_PREEMPT_RCU */
868
869/* Invoked on each online non-idle CPU for expedited quiescent state. */
870static void rcu_exp_handler(void *unused)
871{
872 struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
873 struct rcu_node *rnp = rdp->mynode;
874 bool preempt_bh_enabled = !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
875
876 ASSERT_EXCLUSIVE_WRITER_SCOPED(rdp->cpu_no_qs.b.exp);
877 if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
878 this_cpu_read(rcu_data.cpu_no_qs.b.exp))
879 return;
880 if (rcu_is_cpu_rrupt_from_idle() ||
881 (IS_ENABLED(CONFIG_PREEMPT_COUNT) && preempt_bh_enabled)) {
882 rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
883 return;
884 }
885 rcu_exp_need_qs();
886}
887
888/*
889 * Because preemptible RCU does not exist, we never have to check for
890 * tasks blocked within RCU read-side critical sections that are
891 * blocking the current expedited grace period.
892 */
893static int rcu_print_task_exp_stall(struct rcu_node *rnp)
894{
895 return 0;
896}
897
898/*
899 * Because preemptible RCU does not exist, we never have to print out
900 * tasks blocked within RCU read-side critical sections that are blocking
901 * the current expedited grace period.
902 */
903static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
904{
905}
906
907#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
908
909/**
910 * synchronize_rcu_expedited - Brute-force RCU grace period
911 *
912 * Wait for an RCU grace period, but expedite it. The basic idea is to
913 * IPI all non-idle non-nohz online CPUs. The IPI handler checks whether
914 * the CPU is in an RCU critical section, and if so, it sets a flag that
915 * causes the outermost rcu_read_unlock() to report the quiescent state
916 * for RCU-preempt or asks the scheduler for help for RCU-sched. On the
917 * other hand, if the CPU is not in an RCU read-side critical section,
918 * the IPI handler reports the quiescent state immediately.
919 *
920 * Although this is a great improvement over previous expedited
921 * implementations, it is still unfriendly to real-time workloads, so is
922 * thus not recommended for any sort of common-case code. In fact, if
923 * you are using synchronize_rcu_expedited() in a loop, please restructure
924 * your code to batch your updates, and then use a single synchronize_rcu()
925 * instead.
926 *
927 * This has the same semantics as (but is more brutal than) synchronize_rcu().
928 */
929void synchronize_rcu_expedited(void)
930{
931 unsigned long flags;
932 struct rcu_exp_work rew;
933 struct rcu_node *rnp;
934 unsigned long s;
935
936 RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
937 lock_is_held(&rcu_lock_map) ||
938 lock_is_held(&rcu_sched_lock_map),
939 "Illegal synchronize_rcu_expedited() in RCU read-side critical section");
940
941 /* Is the state is such that the call is a grace period? */
942 if (rcu_blocking_is_gp()) {
943 // Note well that this code runs with !PREEMPT && !SMP.
944 // In addition, all code that advances grace periods runs
945 // at process level. Therefore, this expedited GP overlaps
946 // with other expedited GPs only by being fully nested within
947 // them, which allows reuse of ->gp_seq_polled_exp_snap.
948 rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_exp_snap);
949 rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_exp_snap);
950
951 local_irq_save(flags);
952 WARN_ON_ONCE(num_online_cpus() > 1);
953 rcu_state.expedited_sequence += (1 << RCU_SEQ_CTR_SHIFT);
954 local_irq_restore(flags);
955 return; // Context allows vacuous grace periods.
956 }
957
958 /* If expedited grace periods are prohibited, fall back to normal. */
959 if (rcu_gp_is_normal()) {
960 synchronize_rcu_normal();
961 return;
962 }
963
964 /* Take a snapshot of the sequence number. */
965 s = rcu_exp_gp_seq_snap();
966 if (exp_funnel_lock(s))
967 return; /* Someone else did our work for us. */
968
969 /* Ensure that load happens before action based on it. */
970 if (unlikely((rcu_scheduler_active == RCU_SCHEDULER_INIT) || !rcu_exp_worker_started())) {
971 /* Direct call during scheduler init and early_initcalls(). */
972 rcu_exp_sel_wait_wake(s);
973 } else {
974 /* Marshall arguments & schedule the expedited grace period. */
975 rew.rew_s = s;
976 synchronize_rcu_expedited_queue_work(&rew);
977 }
978
979 /* Wait for expedited grace period to complete. */
980 rnp = rcu_get_root();
981 wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
982 sync_exp_work_done(s));
983
984 /* Let the next expedited grace period start. */
985 mutex_unlock(&rcu_state.exp_mutex);
986}
987EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);
988
989/*
990 * Ensure that start_poll_synchronize_rcu_expedited() has the expedited
991 * RCU grace periods that it needs.
992 */
993static void sync_rcu_do_polled_gp(struct work_struct *wp)
994{
995 unsigned long flags;
996 int i = 0;
997 struct rcu_node *rnp = container_of(wp, struct rcu_node, exp_poll_wq);
998 unsigned long s;
999
1000 raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
1001 s = rnp->exp_seq_poll_rq;
1002 rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
1003 raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
1004 if (s == RCU_GET_STATE_COMPLETED)
1005 return;
1006 while (!poll_state_synchronize_rcu(s)) {
1007 synchronize_rcu_expedited();
1008 if (i == 10 || i == 20)
1009 pr_info("%s: i = %d s = %lx gp_seq_polled = %lx\n", __func__, i, s, READ_ONCE(rcu_state.gp_seq_polled));
1010 i++;
1011 }
1012 raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
1013 s = rnp->exp_seq_poll_rq;
1014 if (poll_state_synchronize_rcu(s))
1015 rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
1016 raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
1017}
1018
1019/**
1020 * start_poll_synchronize_rcu_expedited - Snapshot current RCU state and start expedited grace period
1021 *
1022 * Returns a cookie to pass to a call to cond_synchronize_rcu(),
1023 * cond_synchronize_rcu_expedited(), or poll_state_synchronize_rcu(),
1024 * allowing them to determine whether or not any sort of grace period has
1025 * elapsed in the meantime. If the needed expedited grace period is not
1026 * already slated to start, initiates that grace period.
1027 */
1028unsigned long start_poll_synchronize_rcu_expedited(void)
1029{
1030 unsigned long flags;
1031 struct rcu_data *rdp;
1032 struct rcu_node *rnp;
1033 unsigned long s;
1034
1035 s = get_state_synchronize_rcu();
1036 rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
1037 rnp = rdp->mynode;
1038 if (rcu_init_invoked())
1039 raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
1040 if (!poll_state_synchronize_rcu(s)) {
1041 if (rcu_init_invoked()) {
1042 rnp->exp_seq_poll_rq = s;
1043 queue_work(rcu_gp_wq, &rnp->exp_poll_wq);
1044 }
1045 }
1046 if (rcu_init_invoked())
1047 raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
1048
1049 return s;
1050}
1051EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited);
1052
1053/**
1054 * start_poll_synchronize_rcu_expedited_full - Take a full snapshot and start expedited grace period
1055 * @gsp: Place to put snapshot of grace-period state
1056 *
1057 * Places the normal and expedited grace-period states in gsp. This
1058 * state value can be passed to a later call to cond_synchronize_rcu_full()
1059 * or poll_state_synchronize_rcu_full() to determine whether or not a
1060 * grace period (whether normal or expedited) has elapsed in the meantime.
1061 * If the needed expedited grace period is not already slated to start,
1062 * initiates that grace period.
1063 */
1064void start_poll_synchronize_rcu_expedited_full(struct rcu_gp_seq *gsp)
1065{
1066 get_state_synchronize_rcu_full(gsp);
1067 (void)start_poll_synchronize_rcu_expedited();
1068}
1069EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited_full);
1070
1071/**
1072 * cond_synchronize_rcu_expedited - Conditionally wait for an expedited RCU grace period
1073 *
1074 * @oldstate: value from get_state_synchronize_rcu(), start_poll_synchronize_rcu(), or start_poll_synchronize_rcu_expedited()
1075 *
1076 * If any type of full RCU grace period has elapsed since the earlier
1077 * call to get_state_synchronize_rcu(), start_poll_synchronize_rcu(),
1078 * or start_poll_synchronize_rcu_expedited(), just return. Otherwise,
1079 * invoke synchronize_rcu_expedited() to wait for a full grace period.
1080 *
1081 * Yes, this function does not take counter wrap into account.
1082 * But counter wrap is harmless. If the counter wraps, we have waited for
1083 * more than 2 billion grace periods (and way more on a 64-bit system!),
1084 * so waiting for a couple of additional grace periods should be just fine.
1085 *
1086 * This function provides the same memory-ordering guarantees that
1087 * would be provided by a synchronize_rcu() that was invoked at the call
1088 * to the function that provided @oldstate and that returned at the end
1089 * of this function.
1090 */
1091void cond_synchronize_rcu_expedited(unsigned long oldstate)
1092{
1093 if (!poll_state_synchronize_rcu(oldstate))
1094 synchronize_rcu_expedited();
1095}
1096EXPORT_SYMBOL_GPL(cond_synchronize_rcu_expedited);
1097
1098/**
1099 * cond_synchronize_rcu_expedited_full - Conditionally wait for an expedited RCU grace period
1100 * @gsp: value from get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(), or start_poll_synchronize_rcu_expedited_full()
1101 *
1102 * If a full RCU grace period has elapsed since the call to
1103 * get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(),
1104 * or start_poll_synchronize_rcu_expedited_full() from which @gsp was
1105 * obtained, just return. Otherwise, invoke synchronize_rcu_expedited()
1106 * to wait for a full grace period.
1107 *
1108 * Yes, this function does not take counter wrap into account.
1109 * But counter wrap is harmless. If the counter wraps, we have waited for
1110 * more than 2 billion grace periods (and way more on a 64-bit system!),
1111 * so waiting for a couple of additional grace periods should be just fine.
1112 *
1113 * This function provides the same memory-ordering guarantees that
1114 * would be provided by a synchronize_rcu() that was invoked at the call
1115 * to the function that provided @gsp and that returned at the end of
1116 * this function.
1117 */
1118void cond_synchronize_rcu_expedited_full(struct rcu_gp_seq *gsp)
1119{
1120 if (!poll_state_synchronize_rcu_full(gsp))
1121 synchronize_rcu_expedited();
1122}
1123EXPORT_SYMBOL_GPL(cond_synchronize_rcu_expedited_full);