1 // SPDX-License-Identifier: GPL-2.0+
2 /*
3 * RCU CPU stall warnings for normal RCU grace periods
4 *
5 * Copyright IBM Corporation, 2019
6 *
7 * Author: Paul E. McKenney <paulmck@linux.ibm.com>
8 */
9
10 #include <linux/kvm_para.h>
11 #include <linux/rcu_notifier.h>
12
13 //////////////////////////////////////////////////////////////////////////////
14 //
15 // Controlling CPU stall warnings, including delay calculation.
16
17 /* panic() on RCU Stall sysctl. */
18 int sysctl_panic_on_rcu_stall __read_mostly;
19 int sysctl_max_rcu_stall_to_panic __read_mostly;
20
21 #ifdef CONFIG_PROVE_RCU
22 #define RCU_STALL_DELAY_DELTA (5 * HZ)
23 #else
24 #define RCU_STALL_DELAY_DELTA 0
25 #endif
26 #define RCU_STALL_MIGHT_DIV 8
27 #define RCU_STALL_MIGHT_MIN (2 * HZ)
28
29 int rcu_exp_jiffies_till_stall_check(void)
30 {
31 int cpu_stall_timeout = READ_ONCE(rcu_exp_cpu_stall_timeout);
32 int exp_stall_delay_delta = 0;
33 int till_stall_check;
34
35 // Zero says to use rcu_cpu_stall_timeout, but in milliseconds.
36 if (!cpu_stall_timeout)
37 cpu_stall_timeout = jiffies_to_msecs(rcu_jiffies_till_stall_check());
38
39 // Limit check must be consistent with the Kconfig limits for
40 // CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, so check the allowed range.
41 // The minimum clamped value is "2UL", because at least one full
42 // tick has to be guaranteed.
43 till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 300UL * HZ);
44
45 if (cpu_stall_timeout && jiffies_to_msecs(till_stall_check) != cpu_stall_timeout)
46 WRITE_ONCE(rcu_exp_cpu_stall_timeout, jiffies_to_msecs(till_stall_check));
47
48 #ifdef CONFIG_PROVE_RCU
49 /* Add extra ~25% out of till_stall_check. */
50 exp_stall_delay_delta = ((till_stall_check * 25) / 100) + 1;
51 #endif
52
53 return till_stall_check + exp_stall_delay_delta;
54 }
55 EXPORT_SYMBOL_GPL(rcu_exp_jiffies_till_stall_check);
56
57 /* Limit-check stall timeouts specified at boottime and runtime. */
58 int rcu_jiffies_till_stall_check(void)
59 {
60 int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
61
62 /*
63 * Limit check must be consistent with the Kconfig limits
64 * for CONFIG_RCU_CPU_STALL_TIMEOUT.
65 */
66 if (till_stall_check < 3) {
67 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
68 till_stall_check = 3;
69 } else if (till_stall_check > 300) {
70 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
71 till_stall_check = 300;
72 }
73 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
74 }
75 EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
76
77 /**
78 * rcu_gp_might_be_stalled - Is it likely that the grace period is stalled?
79 *
80 * Returns @true if the current grace period is sufficiently old that
81 * it is reasonable to assume that it might be stalled. This can be
82 * useful when deciding whether to allocate memory to enable RCU-mediated
83 * freeing on the one hand or just invoking synchronize_rcu() on the other.
84 * The latter is preferable when the grace period is stalled.
85 *
86 * Note that sampling of the .gp_start and .gp_seq fields must be done
87 * carefully to avoid false positives at the beginnings and ends of
88 * grace periods.
89 */
90 bool rcu_gp_might_be_stalled(void)
91 {
92 unsigned long d = rcu_jiffies_till_stall_check() / RCU_STALL_MIGHT_DIV;
93 unsigned long j = jiffies;
94
95 if (d < RCU_STALL_MIGHT_MIN)
96 d = RCU_STALL_MIGHT_MIN;
97 smp_mb(); // jiffies before .gp_seq to avoid false positives.
98 if (!rcu_gp_in_progress())
99 return false;
100 // Long delays at this point avoids false positive, but a delay
101 // of ULONG_MAX/4 jiffies voids your no-false-positive warranty.
102 smp_mb(); // .gp_seq before second .gp_start
103 // And ditto here.
104 return !time_before(j, READ_ONCE(rcu_state.gp_start) + d);
105 }
106
107 /* Don't do RCU CPU stall warnings during long sysrq printouts. */
108 void rcu_sysrq_start(void)
109 {
110 if (!rcu_cpu_stall_suppress)
111 rcu_cpu_stall_suppress = 2;
112 }
113
114 void rcu_sysrq_end(void)
115 {
116 if (rcu_cpu_stall_suppress == 2)
117 rcu_cpu_stall_suppress = 0;
118 }
119
120 /* Don't print RCU CPU stall warnings during a kernel panic. */
121 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
122 {
123 rcu_cpu_stall_suppress = 1;
124 return NOTIFY_DONE;
125 }
126
127 static struct notifier_block rcu_panic_block = {
128 .notifier_call = rcu_panic,
129 };
130
131 static int __init check_cpu_stall_init(void)
132 {
133 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
134 return 0;
135 }
136 early_initcall(check_cpu_stall_init);
137
138 /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
139 static void panic_on_rcu_stall(void)
140 {
141 static int cpu_stall;
142
143 if (++cpu_stall < sysctl_max_rcu_stall_to_panic)
144 return;
145
146 if (sysctl_panic_on_rcu_stall)
147 panic("RCU Stall\n");
148 }
149
150 /**
151 * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
152 *
153 * To perform the reset request from the caller, disable stall detection until
154 * 3 fqs loops have passed. This is required to ensure a fresh jiffies is
155 * loaded. It should be safe to do from the fqs loop as enough timer
156 * interrupts and context switches should have passed.
157 *
158 * The caller must disable hard irqs.
159 */
160 void rcu_cpu_stall_reset(void)
161 {
162 WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 3);
163 WRITE_ONCE(rcu_state.jiffies_stall, ULONG_MAX);
164 }
165
166 //////////////////////////////////////////////////////////////////////////////
167 //
168 // Interaction with RCU grace periods
169
170 /* Start of new grace period, so record stall time (and forcing times). */
171 static void record_gp_stall_check_time(void)
172 {
173 unsigned long j = jiffies;
174 unsigned long j1;
175
176 WRITE_ONCE(rcu_state.gp_start, j);
177 j1 = rcu_jiffies_till_stall_check();
178 smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
179 WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 0);
180 WRITE_ONCE(rcu_state.jiffies_stall, j + j1);
181 rcu_state.jiffies_resched = j + j1 / 2;
182 rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
183 }
184
185 /* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
186 static void zero_cpu_stall_ticks(struct rcu_data *rdp)
187 {
188 rdp->ticks_this_gp = 0;
189 rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
190 WRITE_ONCE(rdp->last_fqs_resched, jiffies);
191 }
192
193 /*
194 * If too much time has passed in the current grace period, and if
195 * so configured, go kick the relevant kthreads.
196 */
197 static void rcu_stall_kick_kthreads(void)
198 {
199 unsigned long j;
200
201 if (!READ_ONCE(rcu_kick_kthreads))
202 return;
203 j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
204 if (time_after(jiffies, j) && rcu_state.gp_kthread &&
205 (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
206 WARN_ONCE(1, "Kicking %s grace-period kthread\n",
207 rcu_state.name);
208 rcu_ftrace_dump(DUMP_ALL);
209 wake_up_process(rcu_state.gp_kthread);
210 WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
211 }
212 }
213
214 /*
215 * Handler for the irq_work request posted about halfway into the RCU CPU
216 * stall timeout, and used to detect excessive irq disabling. Set state
217 * appropriately, but just complain if there is unexpected state on entry.
218 */
219 static void rcu_iw_handler(struct irq_work *iwp)
220 {
221 struct rcu_data *rdp;
222 struct rcu_node *rnp;
223
224 rdp = container_of(iwp, struct rcu_data, rcu_iw);
225 rnp = rdp->mynode;
226 raw_spin_lock_rcu_node(rnp);
227 if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
228 rdp->rcu_iw_gp_seq = rnp->gp_seq;
229 rdp->rcu_iw_pending = false;
230 }
231 raw_spin_unlock_rcu_node(rnp);
232 }
233
234 //////////////////////////////////////////////////////////////////////////////
235 //
236 // Printing RCU CPU stall warnings
237
238 #ifdef CONFIG_PREEMPT_RCU
239
240 /*
241 * Dump detailed information for all tasks blocking the current RCU
242 * grace period on the specified rcu_node structure.
243 */
244 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
245 {
246 unsigned long flags;
247 struct task_struct *t;
248
249 raw_spin_lock_irqsave_rcu_node(rnp, flags);
250 if (!rcu_preempt_blocked_readers_cgp(rnp)) {
251 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
252 return;
253 }
254 t = list_entry(rnp->gp_tasks->prev,
255 struct task_struct, rcu_node_entry);
256 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
257 /*
258 * We could be printing a lot while holding a spinlock.
259 * Avoid triggering hard lockup.
260 */
261 touch_nmi_watchdog();
262 sched_show_task(t);
263 }
264 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
265 }
266
267 // Communicate task state back to the RCU CPU stall warning request.
268 struct rcu_stall_chk_rdr {
269 int nesting;
270 union rcu_special rs;
271 bool on_blkd_list;
272 };
273
274 /*
275 * Report out the state of a not-running task that is stalling the
276 * current RCU grace period.
277 */
278 static int check_slow_task(struct task_struct *t, void *arg)
279 {
280 struct rcu_stall_chk_rdr *rscrp = arg;
281
282 if (task_curr(t))
283 return -EBUSY; // It is running, so decline to inspect it.
284 rscrp->nesting = t->rcu_read_lock_nesting;
285 rscrp->rs = t->rcu_read_unlock_special;
286 rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
287 return 0;
288 }
289
290 /*
291 * Scan the current list of tasks blocked within RCU read-side critical
292 * sections, printing out the tid of each of the first few of them.
293 */
294 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
295 __releases(rnp->lock)
296 {
297 int i = 0;
298 int ndetected = 0;
299 struct rcu_stall_chk_rdr rscr;
300 struct task_struct *t;
301 struct task_struct *ts[8];
302
303 lockdep_assert_irqs_disabled();
304 if (!rcu_preempt_blocked_readers_cgp(rnp)) {
305 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
306 return 0;
307 }
308 pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
309 rnp->level, rnp->grplo, rnp->grphi);
310 t = list_entry(rnp->gp_tasks->prev,
311 struct task_struct, rcu_node_entry);
312 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
313 get_task_struct(t);
314 ts[i++] = t;
315 if (i >= ARRAY_SIZE(ts))
316 break;
317 }
318 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
319 while (i) {
320 t = ts[--i];
321 if (task_call_func(t, check_slow_task, &rscr))
322 pr_cont(" P%d", t->pid);
323 else
324 pr_cont(" P%d/%d:%c%c%c%c",
325 t->pid, rscr.nesting,
326 ".b"[rscr.rs.b.blocked],
327 ".q"[rscr.rs.b.need_qs],
328 ".e"[rscr.rs.b.exp_hint],
329 ".l"[rscr.on_blkd_list]);
330 lockdep_assert_irqs_disabled();
331 put_task_struct(t);
332 ndetected++;
333 }
334 pr_cont("\n");
335 return ndetected;
336 }
337
338 #else /* #ifdef CONFIG_PREEMPT_RCU */
339
340 /*
341 * Because preemptible RCU does not exist, we never have to check for
342 * tasks blocked within RCU read-side critical sections.
343 */
344 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
345 {
346 }
347
348 /*
349 * Because preemptible RCU does not exist, we never have to check for
350 * tasks blocked within RCU read-side critical sections.
351 */
352 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
353 __releases(rnp->lock)
354 {
355 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
356 return 0;
357 }
358 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
359
360 /*
361 * Dump stacks of all tasks running on stalled CPUs. First try using
362 * NMIs, but fall back to manual remote stack tracing on architectures
363 * that don't support NMI-based stack dumps. The NMI-triggered stack
364 * traces are more accurate because they are printed by the target CPU.
365 */
366 static void rcu_dump_cpu_stacks(void)
367 {
368 int cpu;
369 unsigned long flags;
370 struct rcu_node *rnp;
371
372 rcu_for_each_leaf_node(rnp) {
373 raw_spin_lock_irqsave_rcu_node(rnp, flags);
374 for_each_leaf_node_possible_cpu(rnp, cpu)
375 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
376 if (cpu_is_offline(cpu))
377 pr_err("Offline CPU %d blocking current GP.\n", cpu);
378 else
379 dump_cpu_task(cpu);
380 }
381 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
382 }
383 }
384
385 static const char * const gp_state_names[] = {
386 [RCU_GP_IDLE] = "RCU_GP_IDLE",
387 [RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS",
388 [RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS",
389 [RCU_GP_ONOFF] = "RCU_GP_ONOFF",
390 [RCU_GP_INIT] = "RCU_GP_INIT",
391 [RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS",
392 [RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS",
393 [RCU_GP_CLEANUP] = "RCU_GP_CLEANUP",
394 [RCU_GP_CLEANED] = "RCU_GP_CLEANED",
395 };
396
397 /*
398 * Convert a ->gp_state value to a character string.
399 */
400 static const char *gp_state_getname(short gs)
401 {
402 if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
403 return "???";
404 return gp_state_names[gs];
405 }
406
407 /* Is the RCU grace-period kthread being starved of CPU time? */
408 static bool rcu_is_gp_kthread_starving(unsigned long *jp)
409 {
410 unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity);
411
412 if (jp)
413 *jp = j;
414 return j > 2 * HZ;
415 }
416
417 static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp)
418 {
419 int cpu;
420 struct task_struct *rcuc;
421 unsigned long j;
422
423 rcuc = rdp->rcu_cpu_kthread_task;
424 if (!rcuc)
425 return false;
426
427 cpu = task_cpu(rcuc);
428 if (cpu_is_offline(cpu) || idle_cpu(cpu))
429 return false;
430
431 j = jiffies - READ_ONCE(rdp->rcuc_activity);
432
433 if (jp)
434 *jp = j;
435 return j > 2 * HZ;
436 }
437
438 static void print_cpu_stat_info(int cpu)
439 {
440 struct rcu_snap_record rsr, *rsrp;
441 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
442 struct kernel_cpustat *kcsp = &kcpustat_cpu(cpu);
443
444 if (!rcu_cpu_stall_cputime)
445 return;
446
447 rsrp = &rdp->snap_record;
448 if (rsrp->gp_seq != rdp->gp_seq)
449 return;
450
451 rsr.cputime_irq = kcpustat_field(kcsp, CPUTIME_IRQ, cpu);
452 rsr.cputime_softirq = kcpustat_field(kcsp, CPUTIME_SOFTIRQ, cpu);
453 rsr.cputime_system = kcpustat_field(kcsp, CPUTIME_SYSTEM, cpu);
454
455 pr_err("\t hardirqs softirqs csw/system\n");
456 pr_err("\t number: %8ld %10d %12lld\n",
457 kstat_cpu_irqs_sum(cpu) - rsrp->nr_hardirqs,
458 kstat_cpu_softirqs_sum(cpu) - rsrp->nr_softirqs,
459 nr_context_switches_cpu(cpu) - rsrp->nr_csw);
460 pr_err("\tcputime: %8lld %10lld %12lld ==> %d(ms)\n",
461 div_u64(rsr.cputime_irq - rsrp->cputime_irq, NSEC_PER_MSEC),
462 div_u64(rsr.cputime_softirq - rsrp->cputime_softirq, NSEC_PER_MSEC),
463 div_u64(rsr.cputime_system - rsrp->cputime_system, NSEC_PER_MSEC),
464 jiffies_to_msecs(jiffies - rsrp->jiffies));
465 }
466
467 /*
468 * Print out diagnostic information for the specified stalled CPU.
469 *
470 * If the specified CPU is aware of the current RCU grace period, then
471 * print the number of scheduling clock interrupts the CPU has taken
472 * during the time that it has been aware. Otherwise, print the number
473 * of RCU grace periods that this CPU is ignorant of, for example, "1"
474 * if the CPU was aware of the previous grace period.
475 *
476 * Also print out idle info.
477 */
478 static void print_cpu_stall_info(int cpu)
479 {
480 unsigned long delta;
481 bool falsepositive;
482 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
483 char *ticks_title;
484 unsigned long ticks_value;
485 bool rcuc_starved;
486 unsigned long j;
487 char buf[32];
488
489 /*
490 * We could be printing a lot while holding a spinlock. Avoid
491 * triggering hard lockup.
492 */
493 touch_nmi_watchdog();
494
495 ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
496 if (ticks_value) {
497 ticks_title = "GPs behind";
498 } else {
499 ticks_title = "ticks this GP";
500 ticks_value = rdp->ticks_this_gp;
501 }
502 delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
503 falsepositive = rcu_is_gp_kthread_starving(NULL) &&
504 rcu_dynticks_in_eqs(ct_dynticks_cpu(cpu));
505 rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);
506 if (rcuc_starved)
507 // Print signed value, as negative values indicate a probable bug.
508 snprintf(buf, sizeof(buf), " rcuc=%ld jiffies(starved)", j);
509 pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%04x/%ld/%#lx softirq=%u/%u fqs=%ld%s%s\n",
510 cpu,
511 "O."[!!cpu_online(cpu)],
512 "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
513 "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
514 !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
515 rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '' :
516 "!."[!delta],
517 ticks_value, ticks_title,
518 ct_dynticks_cpu(cpu) & 0xffff,
519 ct_dynticks_nesting_cpu(cpu), ct_dynticks_nmi_nesting_cpu(cpu),
520 rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
521 data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
522 rcuc_starved ? buf : "",
523 falsepositive ? " (false positive?)" : "");
524
525 print_cpu_stat_info(cpu);
526 }
527
528 /* Complain about starvation of grace-period kthread. */
529 static void rcu_check_gp_kthread_starvation(void)
530 {
531 int cpu;
532 struct task_struct *gpk = rcu_state.gp_kthread;
533 unsigned long j;
534
535 if (rcu_is_gp_kthread_starving(&j)) {
536 cpu = gpk ? task_cpu(gpk) : -1;
537 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n",
538 rcu_state.name, j,
539 (long)rcu_seq_current(&rcu_state.gp_seq),
540 data_race(READ_ONCE(rcu_state.gp_flags)),
541 gp_state_getname(rcu_state.gp_state),
542 data_race(READ_ONCE(rcu_state.gp_state)),
543 gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu);
544 if (gpk) {
545 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
546
547 pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
548 pr_err("RCU grace-period kthread stack dump:\n");
549 sched_show_task(gpk);
550 if (cpu_is_offline(cpu)) {
551 pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
552 } else if (!(data_race(READ_ONCE(rdp->mynode->qsmask)) & rdp->grpmask)) {
553 pr_err("Stack dump where RCU GP kthread last ran:\n");
554 dump_cpu_task(cpu);
555 }
556 wake_up_process(gpk);
557 }
558 }
559 }
560
561 /* Complain about missing wakeups from expired fqs wait timer */
562 static void rcu_check_gp_kthread_expired_fqs_timer(void)
563 {
564 struct task_struct *gpk = rcu_state.gp_kthread;
565 short gp_state;
566 unsigned long jiffies_fqs;
567 int cpu;
568
569 /*
570 * Order reads of .gp_state and .jiffies_force_qs.
571 * Matching smp_wmb() is present in rcu_gp_fqs_loop().
572 */
573 gp_state = smp_load_acquire(&rcu_state.gp_state);
574 jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs);
575
576 if (gp_state == RCU_GP_WAIT_FQS &&
577 time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) &&
578 gpk && !READ_ONCE(gpk->on_rq)) {
579 cpu = task_cpu(gpk);
580 pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n",
581 rcu_state.name, (jiffies - jiffies_fqs),
582 (long)rcu_seq_current(&rcu_state.gp_seq),
583 data_race(READ_ONCE(rcu_state.gp_flags)), // Diagnostic read
584 gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
585 data_race(READ_ONCE(gpk->__state)));
586 pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",
587 cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu));
588 }
589 }
590
591 static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
592 {
593 int cpu;
594 unsigned long flags;
595 unsigned long gpa;
596 unsigned long j;
597 int ndetected = 0;
598 struct rcu_node *rnp;
599 long totqlen = 0;
600
601 lockdep_assert_irqs_disabled();
602
603 /* Kick and suppress, if so configured. */
604 rcu_stall_kick_kthreads();
605 if (rcu_stall_is_suppressed())
606 return;
607
608 /*
609 * OK, time to rat on our buddy...
610 * See Documentation/RCU/stallwarn.rst for info on how to debug
611 * RCU CPU stall warnings.
612 */
613 trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
614 pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
615 rcu_for_each_leaf_node(rnp) {
616 raw_spin_lock_irqsave_rcu_node(rnp, flags);
617 if (rnp->qsmask != 0) {
618 for_each_leaf_node_possible_cpu(rnp, cpu)
619 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
620 print_cpu_stall_info(cpu);
621 ndetected++;
622 }
623 }
624 ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
625 lockdep_assert_irqs_disabled();
626 }
627
628 for_each_possible_cpu(cpu)
629 totqlen += rcu_get_n_cbs_cpu(cpu);
630 pr_err("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n",
631 smp_processor_id(), (long)(jiffies - gps),
632 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen,
633 data_race(rcu_state.n_online_cpus)); // Diagnostic read
634 if (ndetected) {
635 rcu_dump_cpu_stacks();
636
637 /* Complain about tasks blocking the grace period. */
638 rcu_for_each_leaf_node(rnp)
639 rcu_print_detail_task_stall_rnp(rnp);
640 } else {
641 if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
642 pr_err("INFO: Stall ended before state dump start\n");
643 } else {
644 j = jiffies;
645 gpa = data_race(READ_ONCE(rcu_state.gp_activity));
646 pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
647 rcu_state.name, j - gpa, j, gpa,
648 data_race(READ_ONCE(jiffies_till_next_fqs)),
649 data_race(READ_ONCE(rcu_get_root()->qsmask)));
650 }
651 }
652 /* Rewrite if needed in case of slow consoles. */
653 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
654 WRITE_ONCE(rcu_state.jiffies_stall,
655 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
656
657 rcu_check_gp_kthread_expired_fqs_timer();
658 rcu_check_gp_kthread_starvation();
659
660 panic_on_rcu_stall();
661
662 rcu_force_quiescent_state(); /* Kick them all. */
663 }
664
665 static void print_cpu_stall(unsigned long gps)
666 {
667 int cpu;
668 unsigned long flags;
669 struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
670 struct rcu_node *rnp = rcu_get_root();
671 long totqlen = 0;
672
673 lockdep_assert_irqs_disabled();
674
675 /* Kick and suppress, if so configured. */
676 rcu_stall_kick_kthreads();
677 if (rcu_stall_is_suppressed())
678 return;
679
680 /*
681 * OK, time to rat on ourselves...
682 * See Documentation/RCU/stallwarn.rst for info on how to debug
683 * RCU CPU stall warnings.
684 */
685 trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
686 pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
687 raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
688 print_cpu_stall_info(smp_processor_id());
689 raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
690 for_each_possible_cpu(cpu)
691 totqlen += rcu_get_n_cbs_cpu(cpu);
692 pr_err("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n",
693 jiffies - gps,
694 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen,
695 data_race(rcu_state.n_online_cpus)); // Diagnostic read
696
697 rcu_check_gp_kthread_expired_fqs_timer();
698 rcu_check_gp_kthread_starvation();
699
700 rcu_dump_cpu_stacks();
701
702 raw_spin_lock_irqsave_rcu_node(rnp, flags);
703 /* Rewrite if needed in case of slow consoles. */
704 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
705 WRITE_ONCE(rcu_state.jiffies_stall,
706 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
707 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
708
709 panic_on_rcu_stall();
710
711 /*
712 * Attempt to revive the RCU machinery by forcing a context switch.
713 *
714 * A context switch would normally allow the RCU state machine to make
715 * progress and it could be we're stuck in kernel space without context
716 * switches for an entirely unreasonable amount of time.
717 */
718 set_tsk_need_resched(current);
719 set_preempt_need_resched();
720 }
721
722 static void check_cpu_stall(struct rcu_data *rdp)
723 {
724 bool self_detected;
725 unsigned long gs1;
726 unsigned long gs2;
727 unsigned long gps;
728 unsigned long j;
729 unsigned long jn;
730 unsigned long js;
731 struct rcu_node *rnp;
732
733 lockdep_assert_irqs_disabled();
734 if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
735 !rcu_gp_in_progress())
736 return;
737 rcu_stall_kick_kthreads();
738
739 /*
740 * Check if it was requested (via rcu_cpu_stall_reset()) that the FQS
741 * loop has to set jiffies to ensure a non-stale jiffies value. This
742 * is required to have good jiffies value after coming out of long
743 * breaks of jiffies updates. Not doing so can cause false positives.
744 */
745 if (READ_ONCE(rcu_state.nr_fqs_jiffies_stall) > 0)
746 return;
747
748 j = jiffies;
749
750 /*
751 * Lots of memory barriers to reject false positives.
752 *
753 * The idea is to pick up rcu_state.gp_seq, then
754 * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
755 * another copy of rcu_state.gp_seq. These values are updated in
756 * the opposite order with memory barriers (or equivalent) during
757 * grace-period initialization and cleanup. Now, a false positive
758 * can occur if we get an new value of rcu_state.gp_start and a old
759 * value of rcu_state.jiffies_stall. But given the memory barriers,
760 * the only way that this can happen is if one grace period ends
761 * and another starts between these two fetches. This is detected
762 * by comparing the second fetch of rcu_state.gp_seq with the
763 * previous fetch from rcu_state.gp_seq.
764 *
765 * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
766 * and rcu_state.gp_start suffice to forestall false positives.
767 */
768 gs1 = READ_ONCE(rcu_state.gp_seq);
769 smp_rmb(); /* Pick up ->gp_seq first... */
770 js = READ_ONCE(rcu_state.jiffies_stall);
771 smp_rmb(); /* ...then ->jiffies_stall before the rest... */
772 gps = READ_ONCE(rcu_state.gp_start);
773 smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
774 gs2 = READ_ONCE(rcu_state.gp_seq);
775 if (gs1 != gs2 ||
776 ULONG_CMP_LT(j, js) ||
777 ULONG_CMP_GE(gps, js))
778 return; /* No stall or GP completed since entering function. */
779 rnp = rdp->mynode;
780 jn = jiffies + ULONG_MAX / 2;
781 self_detected = READ_ONCE(rnp->qsmask) & rdp->grpmask;
782 if (rcu_gp_in_progress() &&
783 (self_detected || ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) &&
784 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
785 /*
786 * If a virtual machine is stopped by the host it can look to
787 * the watchdog like an RCU stall. Check to see if the host
788 * stopped the vm.
789 */
790 if (kvm_check_and_clear_guest_paused())
791 return;
792
793 rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_NORM, (void *)j - gps);
794 if (self_detected) {
795 /* We haven't checked in, so go dump stack. */
796 print_cpu_stall(gps);
797 } else {
798 /* They had a few time units to dump stack, so complain. */
799 print_other_cpu_stall(gs2, gps);
800 }
801
802 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
803 rcu_ftrace_dump(DUMP_ALL);
804
805 if (READ_ONCE(rcu_state.jiffies_stall) == jn) {
806 jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
807 WRITE_ONCE(rcu_state.jiffies_stall, jn);
808 }
809 }
810 }
811
812 //////////////////////////////////////////////////////////////////////////////
813 //
814 // RCU forward-progress mechanisms, including for callback invocation.
815
816
817 /*
818 * Check to see if a failure to end RCU priority inversion was due to
819 * a CPU not passing through a quiescent state. When this happens, there
820 * is nothing that RCU priority boosting can do to help, so we shouldn't
821 * count this as an RCU priority boosting failure. A return of true says
822 * RCU priority boosting is to blame, and false says otherwise. If false
823 * is returned, the first of the CPUs to blame is stored through cpup.
824 * If there was no CPU blocking the current grace period, but also nothing
825 * in need of being boosted, *cpup is set to -1. This can happen in case
826 * of vCPU preemption while the last CPU is reporting its quiscent state,
827 * for example.
828 *
829 * If cpup is NULL, then a lockless quick check is carried out, suitable
830 * for high-rate usage. On the other hand, if cpup is non-NULL, each
831 * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
832 */
833 bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
834 {
835 bool atb = false;
836 int cpu;
837 unsigned long flags;
838 struct rcu_node *rnp;
839
840 rcu_for_each_leaf_node(rnp) {
841 if (!cpup) {
842 if (data_race(READ_ONCE(rnp->qsmask))) {
843 return false;
844 } else {
845 if (READ_ONCE(rnp->gp_tasks))
846 atb = true;
847 continue;
848 }
849 }
850 *cpup = -1;
851 raw_spin_lock_irqsave_rcu_node(rnp, flags);
852 if (rnp->gp_tasks)
853 atb = true;
854 if (!rnp->qsmask) {
855 // No CPUs without quiescent states for this rnp.
856 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
857 continue;
858 }
859 // Find the first holdout CPU.
860 for_each_leaf_node_possible_cpu(rnp, cpu) {
861 if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) {
862 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
863 *cpup = cpu;
864 return false;
865 }
866 }
867 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
868 }
869 // Can't blame CPUs, so must blame RCU priority boosting.
870 return atb;
871 }
872 EXPORT_SYMBOL_GPL(rcu_check_boost_fail);
873
874 /*
875 * Show the state of the grace-period kthreads.
876 */
877 void show_rcu_gp_kthreads(void)
878 {
879 unsigned long cbs = 0;
880 int cpu;
881 unsigned long j;
882 unsigned long ja;
883 unsigned long jr;
884 unsigned long js;
885 unsigned long jw;
886 struct rcu_data *rdp;
887 struct rcu_node *rnp;
888 struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
889
890 j = jiffies;
891 ja = j - data_race(READ_ONCE(rcu_state.gp_activity));
892 jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity));
893 js = j - data_race(READ_ONCE(rcu_state.gp_start));
894 jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time));
895 pr_info("%s: wait state: %s(%d) ->state: %#x ->rt_priority %u delta ->gp_start %lu ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_max %lu ->gp_flags %#x\n",
896 rcu_state.name, gp_state_getname(rcu_state.gp_state),
897 data_race(READ_ONCE(rcu_state.gp_state)),
898 t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU,
899 js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)),
900 (long)data_race(READ_ONCE(rcu_state.gp_seq)),
901 (long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)),
902 data_race(READ_ONCE(rcu_state.gp_max)),
903 data_race(READ_ONCE(rcu_state.gp_flags)));
904 rcu_for_each_node_breadth_first(rnp) {
905 if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
906 !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
907 !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
908 continue;
909 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
910 rnp->grplo, rnp->grphi,
911 (long)data_race(READ_ONCE(rnp->gp_seq)),
912 (long)data_race(READ_ONCE(rnp->gp_seq_needed)),
913 data_race(READ_ONCE(rnp->qsmask)),
914 ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
915 ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
916 ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],
917 ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))],
918 data_race(READ_ONCE(rnp->n_boosts)));
919 if (!rcu_is_leaf_node(rnp))
920 continue;
921 for_each_leaf_node_possible_cpu(rnp, cpu) {
922 rdp = per_cpu_ptr(&rcu_data, cpu);
923 if (READ_ONCE(rdp->gpwrap) ||
924 ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
925 READ_ONCE(rdp->gp_seq_needed)))
926 continue;
927 pr_info("\tcpu %d ->gp_seq_needed %ld\n",
928 cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed)));
929 }
930 }
931 for_each_possible_cpu(cpu) {
932 rdp = per_cpu_ptr(&rcu_data, cpu);
933 cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
934 if (rcu_segcblist_is_offloaded(&rdp->cblist))
935 show_rcu_nocb_state(rdp);
936 }
937 pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
938 show_rcu_tasks_gp_kthreads();
939 }
940 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
941
942 /*
943 * This function checks for grace-period requests that fail to motivate
944 * RCU to come out of its idle mode.
945 */
946 static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
947 const unsigned long gpssdelay)
948 {
949 unsigned long flags;
950 unsigned long j;
951 struct rcu_node *rnp_root = rcu_get_root();
952 static atomic_t warned = ATOMIC_INIT(0);
953
954 if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
955 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
956 READ_ONCE(rnp_root->gp_seq_needed)) ||
957 !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
958 return;
959 j = jiffies; /* Expensive access, and in common case don't get here. */
960 if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
961 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
962 atomic_read(&warned))
963 return;
964
965 raw_spin_lock_irqsave_rcu_node(rnp, flags);
966 j = jiffies;
967 if (rcu_gp_in_progress() ||
968 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
969 READ_ONCE(rnp_root->gp_seq_needed)) ||
970 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
971 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
972 atomic_read(&warned)) {
973 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
974 return;
975 }
976 /* Hold onto the leaf lock to make others see warned==1. */
977
978 if (rnp_root != rnp)
979 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
980 j = jiffies;
981 if (rcu_gp_in_progress() ||
982 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
983 READ_ONCE(rnp_root->gp_seq_needed)) ||
984 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
985 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
986 atomic_xchg(&warned, 1)) {
987 if (rnp_root != rnp)
988 /* irqs remain disabled. */
989 raw_spin_unlock_rcu_node(rnp_root);
990 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
991 return;
992 }
993 WARN_ON(1);
994 if (rnp_root != rnp)
995 raw_spin_unlock_rcu_node(rnp_root);
996 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
997 show_rcu_gp_kthreads();
998 }
999
1000 /*
1001 * Do a forward-progress check for rcutorture. This is normally invoked
1002 * due to an OOM event. The argument "j" gives the time period during
1003 * which rcutorture would like progress to have been made.
1004 */
1005 void rcu_fwd_progress_check(unsigned long j)
1006 {
1007 unsigned long cbs;
1008 int cpu;
1009 unsigned long max_cbs = 0;
1010 int max_cpu = -1;
1011 struct rcu_data *rdp;
1012
1013 if (rcu_gp_in_progress()) {
1014 pr_info("%s: GP age %lu jiffies\n",
1015 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start)));
1016 show_rcu_gp_kthreads();
1017 } else {
1018 pr_info("%s: Last GP end %lu jiffies ago\n",
1019 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end)));
1020 preempt_disable();
1021 rdp = this_cpu_ptr(&rcu_data);
1022 rcu_check_gp_start_stall(rdp->mynode, rdp, j);
1023 preempt_enable();
1024 }
1025 for_each_possible_cpu(cpu) {
1026 cbs = rcu_get_n_cbs_cpu(cpu);
1027 if (!cbs)
1028 continue;
1029 if (max_cpu < 0)
1030 pr_info("%s: callbacks", __func__);
1031 pr_cont(" %d: %lu", cpu, cbs);
1032 if (cbs <= max_cbs)
1033 continue;
1034 max_cbs = cbs;
1035 max_cpu = cpu;
1036 }
1037 if (max_cpu >= 0)
1038 pr_cont("\n");
1039 }
1040 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
1041
1042 /* Commandeer a sysrq key to dump RCU's tree. */
1043 static bool sysrq_rcu;
1044 module_param(sysrq_rcu, bool, 0444);
1045
1046 /* Dump grace-period-request information due to commandeered sysrq. */
1047 static void sysrq_show_rcu(u8 key)
1048 {
1049 show_rcu_gp_kthreads();
1050 }
1051
1052 static const struct sysrq_key_op sysrq_rcudump_op = {
1053 .handler = sysrq_show_rcu,
1054 .help_msg = "show-rcu(y)",
1055 .action_msg = "Show RCU tree",
1056 .enable_mask = SYSRQ_ENABLE_DUMP,
1057 };
1058
1059 static int __init rcu_sysrq_init(void)
1060 {
1061 if (sysrq_rcu)
1062 return register_sysrq_key('y', &sysrq_rcudump_op);
1063 return 0;
1064 }
1065 early_initcall(rcu_sysrq_init);
1066
1067 #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
1068
1069 //////////////////////////////////////////////////////////////////////////////
1070 //
1071 // RCU CPU stall-warning notifiers
1072
1073 static ATOMIC_NOTIFIER_HEAD(rcu_cpu_stall_notifier_list);
1074
1075 /**
1076 * rcu_stall_chain_notifier_register - Add an RCU CPU stall notifier
1077 * @n: Entry to add.
1078 *
1079 * Adds an RCU CPU stall notifier to an atomic notifier chain.
1080 * The @action passed to a notifier will be @RCU_STALL_NOTIFY_NORM or
1081 * friends. The @data will be the duration of the stalled grace period,
1082 * in jiffies, coerced to a void* pointer.
1083 *
1084 * Returns 0 on success, %-EEXIST on error.
1085 */
1086 int rcu_stall_chain_notifier_register(struct notifier_block *n)
1087 {
1088 int rcsn = rcu_cpu_stall_notifiers;
1089
1090 WARN(1, "Adding %pS() to RCU stall notifier list (%s).\n", n->notifier_call,
1091 rcsn ? "possibly suppressing RCU CPU stall warnings" : "failed, so all is well");
1092 if (rcsn)
1093 return atomic_notifier_chain_register(&rcu_cpu_stall_notifier_list, n);
1094 return -EEXIST;
1095 }
1096 EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_register);
1097
1098 /**
1099 * rcu_stall_chain_notifier_unregister - Remove an RCU CPU stall notifier
1100 * @n: Entry to add.
1101 *
1102 * Removes an RCU CPU stall notifier from an atomic notifier chain.
1103 *
1104 * Returns zero on success, %-ENOENT on failure.
1105 */
1106 int rcu_stall_chain_notifier_unregister(struct notifier_block *n)
1107 {
1108 return atomic_notifier_chain_unregister(&rcu_cpu_stall_notifier_list, n);
1109 }
1110 EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_unregister);
1111
1112 /*
1113 * rcu_stall_notifier_call_chain - Call functions in an RCU CPU stall notifier chain
1114 * @val: Value passed unmodified to notifier function
1115 * @v: Pointer passed unmodified to notifier function
1116 *
1117 * Calls each function in the RCU CPU stall notifier chain in turn, which
1118 * is an atomic call chain. See atomic_notifier_call_chain() for more
1119 * information.
1120 *
1121 * This is for use within RCU, hence the omission of the extra asterisk
1122 * to indicate a non-kerneldoc format header comment.
1123 */
1124 int rcu_stall_notifier_call_chain(unsigned long val, void *v)
1125 {
1126 return atomic_notifier_call_chain(&rcu_cpu_stall_notifier_list, val, v);
1127 }
1128
1129 #endif // #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
1130
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