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/console.h>
11#include <linux/kvm_para.h>
12#include <linux/rcu_notifier.h>
13#include <linux/smp.h>
14
15//////////////////////////////////////////////////////////////////////////////
16//
17// Controlling CPU stall warnings, including delay calculation.
18
19/* panic() on RCU Stall sysctl. */
20int sysctl_panic_on_rcu_stall __read_mostly;
21int sysctl_max_rcu_stall_to_panic __read_mostly;
22
23#ifdef CONFIG_SYSFS
24
25static unsigned int rcu_stall_count;
26
27static ssize_t rcu_stall_count_show(struct kobject *kobj, struct kobj_attribute *attr,
28 char *page)
29{
30 return sysfs_emit(buf: page, fmt: "%u\n", rcu_stall_count);
31}
32
33static struct kobj_attribute rcu_stall_count_attr = __ATTR_RO(rcu_stall_count);
34
35static __init int kernel_rcu_stall_sysfs_init(void)
36{
37 sysfs_add_file_to_group(kobj: kernel_kobj, attr: &rcu_stall_count_attr.attr, NULL);
38 return 0;
39}
40
41late_initcall(kernel_rcu_stall_sysfs_init);
42
43#endif // CONFIG_SYSFS
44
45#ifdef CONFIG_PROVE_RCU
46#define RCU_STALL_DELAY_DELTA (5 * HZ)
47#else
48#define RCU_STALL_DELAY_DELTA 0
49#endif
50#define RCU_STALL_MIGHT_DIV 8
51#define RCU_STALL_MIGHT_MIN (2 * HZ)
52
53int rcu_exp_jiffies_till_stall_check(void)
54{
55 int cpu_stall_timeout = READ_ONCE(rcu_exp_cpu_stall_timeout);
56 int exp_stall_delay_delta = 0;
57 int till_stall_check;
58
59 // Zero says to use rcu_cpu_stall_timeout, but in milliseconds.
60 if (!cpu_stall_timeout)
61 cpu_stall_timeout = jiffies_to_msecs(j: rcu_jiffies_till_stall_check());
62
63 // Limit check must be consistent with the Kconfig limits for
64 // CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, so check the allowed range.
65 // The minimum clamped value is "2UL", because at least one full
66 // tick has to be guaranteed.
67 till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 300UL * HZ);
68
69 if (cpu_stall_timeout && jiffies_to_msecs(j: till_stall_check) != cpu_stall_timeout)
70 WRITE_ONCE(rcu_exp_cpu_stall_timeout, jiffies_to_msecs(till_stall_check));
71
72#ifdef CONFIG_PROVE_RCU
73 /* Add extra ~25% out of till_stall_check. */
74 exp_stall_delay_delta = ((till_stall_check * 25) / 100) + 1;
75#endif
76
77 return till_stall_check + exp_stall_delay_delta;
78}
79EXPORT_SYMBOL_GPL(rcu_exp_jiffies_till_stall_check);
80
81/* Limit-check stall timeouts specified at boottime and runtime. */
82int rcu_jiffies_till_stall_check(void)
83{
84 int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
85
86 /*
87 * Limit check must be consistent with the Kconfig limits
88 * for CONFIG_RCU_CPU_STALL_TIMEOUT.
89 */
90 if (till_stall_check < 3) {
91 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
92 till_stall_check = 3;
93 } else if (till_stall_check > 300) {
94 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
95 till_stall_check = 300;
96 }
97 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
98}
99EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
100
101/* Don't do RCU CPU stall warnings during long sysrq printouts. */
102void rcu_sysrq_start(void)
103{
104 if (!rcu_cpu_stall_suppress)
105 rcu_cpu_stall_suppress = 2;
106}
107
108void rcu_sysrq_end(void)
109{
110 if (rcu_cpu_stall_suppress == 2)
111 rcu_cpu_stall_suppress = 0;
112}
113
114/* Don't print RCU CPU stall warnings during a kernel panic. */
115static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
116{
117 rcu_cpu_stall_suppress = 1;
118 return NOTIFY_DONE;
119}
120
121static struct notifier_block rcu_panic_block = {
122 .notifier_call = rcu_panic,
123};
124
125static int __init check_cpu_stall_init(void)
126{
127 atomic_notifier_chain_register(nh: &panic_notifier_list, nb: &rcu_panic_block);
128 return 0;
129}
130early_initcall(check_cpu_stall_init);
131
132/* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
133static void panic_on_rcu_stall(void)
134{
135 static int cpu_stall;
136
137 if (++cpu_stall < sysctl_max_rcu_stall_to_panic)
138 return;
139
140 if (sysctl_panic_on_rcu_stall)
141 panic(fmt: "RCU Stall\n");
142}
143
144/**
145 * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
146 *
147 * To perform the reset request from the caller, disable stall detection until
148 * 3 fqs loops have passed. This is required to ensure a fresh jiffies is
149 * loaded. It should be safe to do from the fqs loop as enough timer
150 * interrupts and context switches should have passed.
151 *
152 * The caller must disable hard irqs.
153 */
154void rcu_cpu_stall_reset(void)
155{
156 WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 3);
157 WRITE_ONCE(rcu_state.jiffies_stall, ULONG_MAX);
158}
159
160//////////////////////////////////////////////////////////////////////////////
161//
162// Interaction with RCU grace periods
163
164/* Start of new grace period, so record stall time (and forcing times). */
165static void record_gp_stall_check_time(void)
166{
167 unsigned long j = jiffies;
168 unsigned long j1;
169
170 WRITE_ONCE(rcu_state.gp_start, j);
171 j1 = rcu_jiffies_till_stall_check();
172 smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
173 WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 0);
174 WRITE_ONCE(rcu_state.jiffies_stall, j + j1);
175 rcu_state.jiffies_resched = j + j1 / 2;
176 rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
177}
178
179/* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
180static void zero_cpu_stall_ticks(struct rcu_data *rdp)
181{
182 rdp->ticks_this_gp = 0;
183 rdp->softirq_snap = kstat_softirqs_cpu(irq: RCU_SOFTIRQ, smp_processor_id());
184 WRITE_ONCE(rdp->last_fqs_resched, jiffies);
185}
186
187/*
188 * If too much time has passed in the current grace period, and if
189 * so configured, go kick the relevant kthreads.
190 */
191static void rcu_stall_kick_kthreads(void)
192{
193 unsigned long j;
194
195 if (!READ_ONCE(rcu_kick_kthreads))
196 return;
197 j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
198 if (time_after(jiffies, j) && rcu_state.gp_kthread &&
199 (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
200 WARN_ONCE(1, "Kicking %s grace-period kthread\n",
201 rcu_state.name);
202 rcu_ftrace_dump(DUMP_ALL);
203 wake_up_process(tsk: rcu_state.gp_kthread);
204 WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
205 }
206}
207
208/*
209 * Handler for the irq_work request posted about halfway into the RCU CPU
210 * stall timeout, and used to detect excessive irq disabling. Set state
211 * appropriately, but just complain if there is unexpected state on entry.
212 */
213static void rcu_iw_handler(struct irq_work *iwp)
214{
215 struct rcu_data *rdp;
216 struct rcu_node *rnp;
217
218 rdp = container_of(iwp, struct rcu_data, rcu_iw);
219 rnp = rdp->mynode;
220 raw_spin_lock_rcu_node(rnp);
221 if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
222 rdp->rcu_iw_gp_seq = rnp->gp_seq;
223 rdp->rcu_iw_pending = false;
224 }
225 raw_spin_unlock_rcu_node(rnp);
226}
227
228//////////////////////////////////////////////////////////////////////////////
229//
230// Printing RCU CPU stall warnings
231
232#ifdef CONFIG_PREEMPT_RCU
233
234/*
235 * Dump detailed information for all tasks blocking the current RCU
236 * grace period on the specified rcu_node structure.
237 */
238static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
239{
240 unsigned long flags;
241 struct task_struct *t;
242
243 raw_spin_lock_irqsave_rcu_node(rnp, flags);
244 if (!rcu_preempt_blocked_readers_cgp(rnp)) {
245 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
246 return;
247 }
248 t = list_entry(rnp->gp_tasks->prev,
249 struct task_struct, rcu_node_entry);
250 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
251 /*
252 * We could be printing a lot while holding a spinlock.
253 * Avoid triggering hard lockup.
254 */
255 touch_nmi_watchdog();
256 sched_show_task(p: t);
257 }
258 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
259}
260
261// Communicate task state back to the RCU CPU stall warning request.
262struct rcu_stall_chk_rdr {
263 int nesting;
264 union rcu_special rs;
265 bool on_blkd_list;
266};
267
268/*
269 * Report out the state of a not-running task that is stalling the
270 * current RCU grace period.
271 */
272static int check_slow_task(struct task_struct *t, void *arg)
273{
274 struct rcu_stall_chk_rdr *rscrp = arg;
275
276 if (task_curr(p: t))
277 return -EBUSY; // It is running, so decline to inspect it.
278 rscrp->nesting = t->rcu_read_lock_nesting;
279 rscrp->rs = t->rcu_read_unlock_special;
280 rscrp->on_blkd_list = !list_empty(head: &t->rcu_node_entry);
281 return 0;
282}
283
284/*
285 * Scan the current list of tasks blocked within RCU read-side critical
286 * sections, printing out the tid of each of the first few of them.
287 */
288static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
289 __releases(rnp->lock)
290{
291 int i = 0;
292 int ndetected = 0;
293 struct rcu_stall_chk_rdr rscr;
294 struct task_struct *t;
295 struct task_struct *ts[8];
296
297 lockdep_assert_irqs_disabled();
298 if (!rcu_preempt_blocked_readers_cgp(rnp)) {
299 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
300 return 0;
301 }
302 pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
303 rnp->level, rnp->grplo, rnp->grphi);
304 t = list_entry(rnp->gp_tasks->prev,
305 struct task_struct, rcu_node_entry);
306 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
307 get_task_struct(t);
308 ts[i++] = t;
309 if (i >= ARRAY_SIZE(ts))
310 break;
311 }
312 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
313 while (i) {
314 t = ts[--i];
315 if (task_call_func(p: t, func: check_slow_task, arg: &rscr))
316 pr_cont(" P%d", t->pid);
317 else
318 pr_cont(" P%d/%d:%c%c%c%c",
319 t->pid, rscr.nesting,
320 ".b"[rscr.rs.b.blocked],
321 ".q"[rscr.rs.b.need_qs],
322 ".e"[rscr.rs.b.exp_hint],
323 ".l"[rscr.on_blkd_list]);
324 lockdep_assert_irqs_disabled();
325 put_task_struct(t);
326 ndetected++;
327 }
328 pr_cont("\n");
329 return ndetected;
330}
331
332#else /* #ifdef CONFIG_PREEMPT_RCU */
333
334/*
335 * Because preemptible RCU does not exist, we never have to check for
336 * tasks blocked within RCU read-side critical sections.
337 */
338static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
339{
340}
341
342/*
343 * Because preemptible RCU does not exist, we never have to check for
344 * tasks blocked within RCU read-side critical sections.
345 */
346static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
347 __releases(rnp->lock)
348{
349 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
350 return 0;
351}
352#endif /* #else #ifdef CONFIG_PREEMPT_RCU */
353
354/*
355 * Dump stacks of all tasks running on stalled CPUs. First try using
356 * NMIs, but fall back to manual remote stack tracing on architectures
357 * that don't support NMI-based stack dumps. The NMI-triggered stack
358 * traces are more accurate because they are printed by the target CPU.
359 */
360static void rcu_dump_cpu_stacks(unsigned long gp_seq)
361{
362 int cpu;
363 unsigned long flags;
364 struct rcu_node *rnp;
365
366 rcu_for_each_leaf_node(rnp) {
367 printk_deferred_enter();
368 for_each_leaf_node_possible_cpu(rnp, cpu) {
369 if (gp_seq != data_race(rcu_state.gp_seq)) {
370 printk_deferred_exit();
371 pr_err("INFO: Stall ended during stack backtracing.\n");
372 return;
373 }
374 if (!(data_race(rnp->qsmask) & leaf_node_cpu_bit(rnp, cpu)))
375 continue;
376 raw_spin_lock_irqsave_rcu_node(rnp, flags);
377 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
378 if (cpu_is_offline(cpu))
379 pr_err("Offline CPU %d blocking current GP.\n", cpu);
380 else
381 dump_cpu_task(cpu);
382 }
383 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
384 }
385 printk_deferred_exit();
386 }
387}
388
389static const char * const gp_state_names[] = {
390 [RCU_GP_IDLE] = "RCU_GP_IDLE",
391 [RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS",
392 [RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS",
393 [RCU_GP_ONOFF] = "RCU_GP_ONOFF",
394 [RCU_GP_INIT] = "RCU_GP_INIT",
395 [RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS",
396 [RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS",
397 [RCU_GP_CLEANUP] = "RCU_GP_CLEANUP",
398 [RCU_GP_CLEANED] = "RCU_GP_CLEANED",
399};
400
401/*
402 * Convert a ->gp_state value to a character string.
403 */
404static const char *gp_state_getname(short gs)
405{
406 if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
407 return "???";
408 return gp_state_names[gs];
409}
410
411/* Is the RCU grace-period kthread being starved of CPU time? */
412static bool rcu_is_gp_kthread_starving(unsigned long *jp)
413{
414 unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity);
415
416 if (jp)
417 *jp = j;
418 return j > 2 * HZ;
419}
420
421static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp)
422{
423 int cpu;
424 struct task_struct *rcuc;
425 unsigned long j;
426
427 rcuc = rdp->rcu_cpu_kthread_task;
428 if (!rcuc)
429 return false;
430
431 cpu = task_cpu(p: rcuc);
432 if (cpu_is_offline(cpu) || idle_cpu(cpu))
433 return false;
434
435 j = jiffies - READ_ONCE(rdp->rcuc_activity);
436
437 if (jp)
438 *jp = j;
439 return j > 2 * HZ;
440}
441
442static void print_cpu_stat_info(int cpu)
443{
444 struct rcu_snap_record rsr, *rsrp;
445 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
446 struct kernel_cpustat *kcsp = &kcpustat_cpu(cpu);
447
448 if (!rcu_cpu_stall_cputime)
449 return;
450
451 rsrp = &rdp->snap_record;
452 if (rsrp->gp_seq != rdp->gp_seq)
453 return;
454
455 rsr.cputime_irq = kcpustat_field(kcpustat: kcsp, usage: CPUTIME_IRQ, cpu);
456 rsr.cputime_softirq = kcpustat_field(kcpustat: kcsp, usage: CPUTIME_SOFTIRQ, cpu);
457 rsr.cputime_system = kcpustat_field(kcpustat: kcsp, usage: CPUTIME_SYSTEM, cpu);
458
459 pr_err("\t hardirqs softirqs csw/system\n");
460 pr_err("\t number: %8lld %10d %12lld\n",
461 kstat_cpu_irqs_sum(cpu) + arch_irq_stat_cpu(cpu) - rsrp->nr_hardirqs,
462 kstat_cpu_softirqs_sum(cpu) - rsrp->nr_softirqs,
463 nr_context_switches_cpu(cpu) - rsrp->nr_csw);
464 pr_err("\tcputime: %8lld %10lld %12lld ==> %d(ms)\n",
465 div_u64(rsr.cputime_irq - rsrp->cputime_irq, NSEC_PER_MSEC),
466 div_u64(rsr.cputime_softirq - rsrp->cputime_softirq, NSEC_PER_MSEC),
467 div_u64(rsr.cputime_system - rsrp->cputime_system, NSEC_PER_MSEC),
468 jiffies_to_msecs(jiffies - rsrp->jiffies));
469}
470
471/*
472 * Print out diagnostic information for the specified stalled CPU.
473 *
474 * If the specified CPU is aware of the current RCU grace period, then
475 * print the number of scheduling clock interrupts the CPU has taken
476 * during the time that it has been aware. Otherwise, print the number
477 * of RCU grace periods that this CPU is ignorant of, for example, "1"
478 * if the CPU was aware of the previous grace period.
479 *
480 * Also print out idle info.
481 */
482static void print_cpu_stall_info(int cpu)
483{
484 unsigned long delta;
485 bool falsepositive;
486 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
487 char *ticks_title;
488 unsigned long ticks_value;
489 bool rcuc_starved;
490 unsigned long j;
491 char buf[32];
492
493 /*
494 * We could be printing a lot while holding a spinlock. Avoid
495 * triggering hard lockup.
496 */
497 touch_nmi_watchdog();
498
499 ticks_value = rcu_seq_ctr(s: rcu_state.gp_seq - rdp->gp_seq);
500 if (ticks_value) {
501 ticks_title = "GPs behind";
502 } else {
503 ticks_title = "ticks this GP";
504 ticks_value = rdp->ticks_this_gp;
505 }
506 delta = rcu_seq_ctr(s: rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
507 falsepositive = rcu_is_gp_kthread_starving(NULL) &&
508 rcu_watching_snap_in_eqs(snap: ct_rcu_watching_cpu(cpu));
509 rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, jp: &j);
510 if (rcuc_starved)
511 // Print signed value, as negative values indicate a probable bug.
512 snprintf(buf, size: sizeof(buf), fmt: " rcuc=%ld jiffies(starved)", j);
513 pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%04x/%ld/%#lx softirq=%u/%u fqs=%ld%s%s\n",
514 cpu,
515 "O."[!!cpu_online(cpu)],
516 "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
517 "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
518 !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
519 rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
520 "!."[!delta],
521 ticks_value, ticks_title,
522 ct_rcu_watching_cpu(cpu) & 0xffff,
523 ct_nesting_cpu(cpu), ct_nmi_nesting_cpu(cpu),
524 rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
525 data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
526 rcuc_starved ? buf : "",
527 falsepositive ? " (false positive?)" : "");
528
529 print_cpu_stat_info(cpu);
530}
531
532/* Complain about starvation of grace-period kthread. */
533static void rcu_check_gp_kthread_starvation(void)
534{
535 int cpu;
536 struct task_struct *gpk = rcu_state.gp_kthread;
537 unsigned long j;
538
539 if (rcu_is_gp_kthread_starving(jp: &j)) {
540 cpu = gpk ? task_cpu(p: gpk) : -1;
541 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n",
542 rcu_state.name, j,
543 (long)rcu_seq_current(&rcu_state.gp_seq),
544 data_race(READ_ONCE(rcu_state.gp_flags)),
545 gp_state_getname(rcu_state.gp_state),
546 data_race(READ_ONCE(rcu_state.gp_state)),
547 gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu);
548 if (gpk) {
549 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
550
551 pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
552 pr_err("RCU grace-period kthread stack dump:\n");
553 sched_show_task(p: gpk);
554 if (cpu_is_offline(cpu)) {
555 pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
556 } else if (!(data_race(READ_ONCE(rdp->mynode->qsmask)) & rdp->grpmask)) {
557 pr_err("Stack dump where RCU GP kthread last ran:\n");
558 dump_cpu_task(cpu);
559 }
560 wake_up_process(tsk: gpk);
561 }
562 }
563}
564
565/* Complain about missing wakeups from expired fqs wait timer */
566static void rcu_check_gp_kthread_expired_fqs_timer(void)
567{
568 struct task_struct *gpk = rcu_state.gp_kthread;
569 short gp_state;
570 unsigned long jiffies_fqs;
571 int cpu;
572
573 /*
574 * Order reads of .gp_state and .jiffies_force_qs.
575 * Matching smp_wmb() is present in rcu_gp_fqs_loop().
576 */
577 gp_state = smp_load_acquire(&rcu_state.gp_state);
578 jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs);
579
580 if (gp_state == RCU_GP_WAIT_FQS &&
581 time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) &&
582 gpk && !READ_ONCE(gpk->on_rq)) {
583 cpu = task_cpu(p: gpk);
584 pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n",
585 rcu_state.name, (jiffies - jiffies_fqs),
586 (long)rcu_seq_current(&rcu_state.gp_seq),
587 data_race(READ_ONCE(rcu_state.gp_flags)), // Diagnostic read
588 gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
589 data_race(READ_ONCE(gpk->__state)));
590 pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",
591 cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu));
592 }
593}
594
595static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
596{
597 int cpu;
598 unsigned long flags;
599 unsigned long gpa;
600 unsigned long j;
601 int ndetected = 0;
602 struct rcu_node *rnp;
603 long totqlen = 0;
604
605 lockdep_assert_irqs_disabled();
606
607 /* Kick and suppress, if so configured. */
608 rcu_stall_kick_kthreads();
609 if (rcu_stall_is_suppressed())
610 return;
611
612 nbcon_cpu_emergency_enter();
613
614 /*
615 * OK, time to rat on our buddy...
616 * See Documentation/RCU/stallwarn.rst for info on how to debug
617 * RCU CPU stall warnings.
618 */
619 trace_rcu_stall_warning(rcuname: rcu_state.name, TPS("StallDetected"));
620 pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
621 rcu_for_each_leaf_node(rnp) {
622 raw_spin_lock_irqsave_rcu_node(rnp, flags);
623 if (rnp->qsmask != 0) {
624 for_each_leaf_node_possible_cpu(rnp, cpu)
625 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
626 print_cpu_stall_info(cpu);
627 ndetected++;
628 }
629 }
630 ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
631 lockdep_assert_irqs_disabled();
632 }
633
634 for_each_possible_cpu(cpu)
635 totqlen += rcu_get_n_cbs_cpu(cpu);
636 pr_err("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n",
637 smp_processor_id(), (long)(jiffies - gps),
638 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen,
639 data_race(rcu_state.n_online_cpus)); // Diagnostic read
640 if (ndetected) {
641 rcu_dump_cpu_stacks(gp_seq);
642
643 /* Complain about tasks blocking the grace period. */
644 rcu_for_each_leaf_node(rnp)
645 rcu_print_detail_task_stall_rnp(rnp);
646 } else {
647 if (rcu_seq_current(sp: &rcu_state.gp_seq) != gp_seq) {
648 pr_err("INFO: Stall ended before state dump start\n");
649 } else {
650 j = jiffies;
651 gpa = data_race(READ_ONCE(rcu_state.gp_activity));
652 pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
653 rcu_state.name, j - gpa, j, gpa,
654 data_race(READ_ONCE(jiffies_till_next_fqs)),
655 data_race(READ_ONCE(rcu_get_root()->qsmask)));
656 }
657 }
658 /* Rewrite if needed in case of slow consoles. */
659 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
660 WRITE_ONCE(rcu_state.jiffies_stall,
661 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
662
663 rcu_check_gp_kthread_expired_fqs_timer();
664 rcu_check_gp_kthread_starvation();
665
666 nbcon_cpu_emergency_exit();
667
668 panic_on_rcu_stall();
669
670 rcu_force_quiescent_state(); /* Kick them all. */
671}
672
673static void print_cpu_stall(unsigned long gp_seq, unsigned long gps)
674{
675 int cpu;
676 unsigned long flags;
677 struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
678 struct rcu_node *rnp = rcu_get_root();
679 long totqlen = 0;
680
681 lockdep_assert_irqs_disabled();
682
683 /* Kick and suppress, if so configured. */
684 rcu_stall_kick_kthreads();
685 if (rcu_stall_is_suppressed())
686 return;
687
688 nbcon_cpu_emergency_enter();
689
690 /*
691 * OK, time to rat on ourselves...
692 * See Documentation/RCU/stallwarn.rst for info on how to debug
693 * RCU CPU stall warnings.
694 */
695 trace_rcu_stall_warning(rcuname: rcu_state.name, TPS("SelfDetected"));
696 pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
697 raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
698 print_cpu_stall_info(smp_processor_id());
699 raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
700 for_each_possible_cpu(cpu)
701 totqlen += rcu_get_n_cbs_cpu(cpu);
702 pr_err("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n",
703 jiffies - gps,
704 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen,
705 data_race(rcu_state.n_online_cpus)); // Diagnostic read
706
707 rcu_check_gp_kthread_expired_fqs_timer();
708 rcu_check_gp_kthread_starvation();
709
710 rcu_dump_cpu_stacks(gp_seq);
711
712 raw_spin_lock_irqsave_rcu_node(rnp, flags);
713 /* Rewrite if needed in case of slow consoles. */
714 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
715 WRITE_ONCE(rcu_state.jiffies_stall,
716 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
717 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
718
719 nbcon_cpu_emergency_exit();
720
721 panic_on_rcu_stall();
722
723 /*
724 * Attempt to revive the RCU machinery by forcing a context switch.
725 *
726 * A context switch would normally allow the RCU state machine to make
727 * progress and it could be we're stuck in kernel space without context
728 * switches for an entirely unreasonable amount of time.
729 */
730 set_tsk_need_resched(current);
731 set_preempt_need_resched();
732}
733
734static bool csd_lock_suppress_rcu_stall;
735module_param(csd_lock_suppress_rcu_stall, bool, 0644);
736
737static void check_cpu_stall(struct rcu_data *rdp)
738{
739 bool self_detected;
740 unsigned long gs1;
741 unsigned long gs2;
742 unsigned long gps;
743 unsigned long j;
744 unsigned long jn;
745 unsigned long js;
746 struct rcu_node *rnp;
747
748 lockdep_assert_irqs_disabled();
749 if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
750 !rcu_gp_in_progress())
751 return;
752 rcu_stall_kick_kthreads();
753
754 /*
755 * Check if it was requested (via rcu_cpu_stall_reset()) that the FQS
756 * loop has to set jiffies to ensure a non-stale jiffies value. This
757 * is required to have good jiffies value after coming out of long
758 * breaks of jiffies updates. Not doing so can cause false positives.
759 */
760 if (READ_ONCE(rcu_state.nr_fqs_jiffies_stall) > 0)
761 return;
762
763 j = jiffies;
764
765 /*
766 * Lots of memory barriers to reject false positives.
767 *
768 * The idea is to pick up rcu_state.gp_seq, then
769 * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
770 * another copy of rcu_state.gp_seq. These values are updated in
771 * the opposite order with memory barriers (or equivalent) during
772 * grace-period initialization and cleanup. Now, a false positive
773 * can occur if we get an new value of rcu_state.gp_start and a old
774 * value of rcu_state.jiffies_stall. But given the memory barriers,
775 * the only way that this can happen is if one grace period ends
776 * and another starts between these two fetches. This is detected
777 * by comparing the second fetch of rcu_state.gp_seq with the
778 * previous fetch from rcu_state.gp_seq.
779 *
780 * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
781 * and rcu_state.gp_start suffice to forestall false positives.
782 */
783 gs1 = READ_ONCE(rcu_state.gp_seq);
784 smp_rmb(); /* Pick up ->gp_seq first... */
785 js = READ_ONCE(rcu_state.jiffies_stall);
786 smp_rmb(); /* ...then ->jiffies_stall before the rest... */
787 gps = READ_ONCE(rcu_state.gp_start);
788 smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
789 gs2 = READ_ONCE(rcu_state.gp_seq);
790 if (gs1 != gs2 ||
791 ULONG_CMP_LT(j, js) ||
792 ULONG_CMP_GE(gps, js) ||
793 !rcu_seq_state(s: gs2))
794 return; /* No stall or GP completed since entering function. */
795 rnp = rdp->mynode;
796 jn = jiffies + ULONG_MAX / 2;
797 self_detected = READ_ONCE(rnp->qsmask) & rdp->grpmask;
798 if (rcu_gp_in_progress() &&
799 (self_detected || ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) &&
800 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
801 /*
802 * If a virtual machine is stopped by the host it can look to
803 * the watchdog like an RCU stall. Check to see if the host
804 * stopped the vm.
805 */
806 if (kvm_check_and_clear_guest_paused())
807 return;
808
809#ifdef CONFIG_SYSFS
810 ++rcu_stall_count;
811#endif
812
813 rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_NORM, v: (void *)j - gps);
814 if (READ_ONCE(csd_lock_suppress_rcu_stall) && csd_lock_is_stuck()) {
815 pr_err("INFO: %s detected stall, but suppressed full report due to a stuck CSD-lock.\n", rcu_state.name);
816 } else if (self_detected) {
817 /* We haven't checked in, so go dump stack. */
818 print_cpu_stall(gp_seq: gs2, gps);
819 } else {
820 /* They had a few time units to dump stack, so complain. */
821 print_other_cpu_stall(gp_seq: gs2, gps);
822 }
823
824 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
825 rcu_ftrace_dump(DUMP_ALL);
826
827 if (READ_ONCE(rcu_state.jiffies_stall) == jn) {
828 jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
829 WRITE_ONCE(rcu_state.jiffies_stall, jn);
830 }
831 }
832}
833
834//////////////////////////////////////////////////////////////////////////////
835//
836// RCU forward-progress mechanisms, including for callback invocation.
837
838
839/*
840 * Check to see if a failure to end RCU priority inversion was due to
841 * a CPU not passing through a quiescent state. When this happens, there
842 * is nothing that RCU priority boosting can do to help, so we shouldn't
843 * count this as an RCU priority boosting failure. A return of true says
844 * RCU priority boosting is to blame, and false says otherwise. If false
845 * is returned, the first of the CPUs to blame is stored through cpup.
846 * If there was no CPU blocking the current grace period, but also nothing
847 * in need of being boosted, *cpup is set to -1. This can happen in case
848 * of vCPU preemption while the last CPU is reporting its quiscent state,
849 * for example.
850 *
851 * If cpup is NULL, then a lockless quick check is carried out, suitable
852 * for high-rate usage. On the other hand, if cpup is non-NULL, each
853 * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
854 */
855bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
856{
857 bool atb = false;
858 int cpu;
859 unsigned long flags;
860 struct rcu_node *rnp;
861
862 rcu_for_each_leaf_node(rnp) {
863 if (!cpup) {
864 if (data_race(READ_ONCE(rnp->qsmask))) {
865 return false;
866 } else {
867 if (READ_ONCE(rnp->gp_tasks))
868 atb = true;
869 continue;
870 }
871 }
872 *cpup = -1;
873 raw_spin_lock_irqsave_rcu_node(rnp, flags);
874 if (rnp->gp_tasks)
875 atb = true;
876 if (!rnp->qsmask) {
877 // No CPUs without quiescent states for this rnp.
878 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
879 continue;
880 }
881 // Find the first holdout CPU.
882 for_each_leaf_node_possible_cpu(rnp, cpu) {
883 if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) {
884 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
885 *cpup = cpu;
886 return false;
887 }
888 }
889 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
890 }
891 // Can't blame CPUs, so must blame RCU priority boosting.
892 return atb;
893}
894EXPORT_SYMBOL_GPL(rcu_check_boost_fail);
895
896/*
897 * Show the state of the grace-period kthreads.
898 */
899void show_rcu_gp_kthreads(void)
900{
901 unsigned long cbs = 0;
902 int cpu;
903 unsigned long j;
904 unsigned long ja;
905 unsigned long jr;
906 unsigned long js;
907 unsigned long jw;
908 struct rcu_data *rdp;
909 struct rcu_node *rnp;
910 struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
911
912 j = jiffies;
913 ja = j - data_race(READ_ONCE(rcu_state.gp_activity));
914 jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity));
915 js = j - data_race(READ_ONCE(rcu_state.gp_start));
916 jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time));
917 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",
918 rcu_state.name, gp_state_getname(rcu_state.gp_state),
919 data_race(READ_ONCE(rcu_state.gp_state)),
920 t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU,
921 js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)),
922 (long)data_race(READ_ONCE(rcu_state.gp_seq)),
923 (long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)),
924 data_race(READ_ONCE(rcu_state.gp_max)),
925 data_race(READ_ONCE(rcu_state.gp_flags)));
926 rcu_for_each_node_breadth_first(rnp) {
927 if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
928 !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
929 !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
930 continue;
931 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
932 rnp->grplo, rnp->grphi,
933 (long)data_race(READ_ONCE(rnp->gp_seq)),
934 (long)data_race(READ_ONCE(rnp->gp_seq_needed)),
935 data_race(READ_ONCE(rnp->qsmask)),
936 ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
937 ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
938 ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],
939 ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))],
940 data_race(READ_ONCE(rnp->n_boosts)));
941 if (!rcu_is_leaf_node(rnp))
942 continue;
943 for_each_leaf_node_possible_cpu(rnp, cpu) {
944 rdp = per_cpu_ptr(&rcu_data, cpu);
945 if (READ_ONCE(rdp->gpwrap) ||
946 ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
947 READ_ONCE(rdp->gp_seq_needed)))
948 continue;
949 pr_info("\tcpu %d ->gp_seq_needed %ld\n",
950 cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed)));
951 }
952 }
953 for_each_possible_cpu(cpu) {
954 rdp = per_cpu_ptr(&rcu_data, cpu);
955 cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
956 if (rcu_segcblist_is_offloaded(rsclp: &rdp->cblist))
957 show_rcu_nocb_state(rdp);
958 }
959 pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
960 show_rcu_tasks_gp_kthreads();
961}
962EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
963
964/*
965 * This function checks for grace-period requests that fail to motivate
966 * RCU to come out of its idle mode.
967 */
968static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
969 const unsigned long gpssdelay)
970{
971 unsigned long flags;
972 unsigned long j;
973 struct rcu_node *rnp_root = rcu_get_root();
974 static atomic_t warned = ATOMIC_INIT(0);
975
976 if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
977 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
978 READ_ONCE(rnp_root->gp_seq_needed)) ||
979 !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
980 return;
981 j = jiffies; /* Expensive access, and in common case don't get here. */
982 if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
983 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
984 atomic_read(v: &warned))
985 return;
986
987 raw_spin_lock_irqsave_rcu_node(rnp, flags);
988 j = jiffies;
989 if (rcu_gp_in_progress() ||
990 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
991 READ_ONCE(rnp_root->gp_seq_needed)) ||
992 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
993 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
994 atomic_read(v: &warned)) {
995 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
996 return;
997 }
998 /* Hold onto the leaf lock to make others see warned==1. */
999
1000 if (rnp_root != rnp)
1001 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
1002 j = jiffies;
1003 if (rcu_gp_in_progress() ||
1004 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
1005 READ_ONCE(rnp_root->gp_seq_needed)) ||
1006 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
1007 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
1008 atomic_xchg(v: &warned, new: 1)) {
1009 if (rnp_root != rnp)
1010 /* irqs remain disabled. */
1011 raw_spin_unlock_rcu_node(rnp_root);
1012 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1013 return;
1014 }
1015 WARN_ON(1);
1016 if (rnp_root != rnp)
1017 raw_spin_unlock_rcu_node(rnp_root);
1018 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
1019 show_rcu_gp_kthreads();
1020}
1021
1022/*
1023 * Do a forward-progress check for rcutorture. This is normally invoked
1024 * due to an OOM event. The argument "j" gives the time period during
1025 * which rcutorture would like progress to have been made.
1026 */
1027void rcu_fwd_progress_check(unsigned long j)
1028{
1029 unsigned long cbs;
1030 int cpu;
1031 unsigned long max_cbs = 0;
1032 int max_cpu = -1;
1033 struct rcu_data *rdp;
1034
1035 if (rcu_gp_in_progress()) {
1036 pr_info("%s: GP age %lu jiffies\n",
1037 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start)));
1038 show_rcu_gp_kthreads();
1039 } else {
1040 pr_info("%s: Last GP end %lu jiffies ago\n",
1041 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end)));
1042 preempt_disable();
1043 rdp = this_cpu_ptr(&rcu_data);
1044 rcu_check_gp_start_stall(rnp: rdp->mynode, rdp, gpssdelay: j);
1045 preempt_enable();
1046 }
1047 for_each_possible_cpu(cpu) {
1048 cbs = rcu_get_n_cbs_cpu(cpu);
1049 if (!cbs)
1050 continue;
1051 if (max_cpu < 0)
1052 pr_info("%s: callbacks", __func__);
1053 pr_cont(" %d: %lu", cpu, cbs);
1054 if (cbs <= max_cbs)
1055 continue;
1056 max_cbs = cbs;
1057 max_cpu = cpu;
1058 }
1059 if (max_cpu >= 0)
1060 pr_cont("\n");
1061}
1062EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
1063
1064/* Commandeer a sysrq key to dump RCU's tree. */
1065static bool sysrq_rcu;
1066module_param(sysrq_rcu, bool, 0444);
1067
1068/* Dump grace-period-request information due to commandeered sysrq. */
1069static void sysrq_show_rcu(u8 key)
1070{
1071 show_rcu_gp_kthreads();
1072}
1073
1074static const struct sysrq_key_op sysrq_rcudump_op = {
1075 .handler = sysrq_show_rcu,
1076 .help_msg = "show-rcu(y)",
1077 .action_msg = "Show RCU tree",
1078 .enable_mask = SYSRQ_ENABLE_DUMP,
1079};
1080
1081static int __init rcu_sysrq_init(void)
1082{
1083 if (sysrq_rcu)
1084 return register_sysrq_key(key: 'y', op: &sysrq_rcudump_op);
1085 return 0;
1086}
1087early_initcall(rcu_sysrq_init);
1088
1089#ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
1090
1091//////////////////////////////////////////////////////////////////////////////
1092//
1093// RCU CPU stall-warning notifiers
1094
1095static ATOMIC_NOTIFIER_HEAD(rcu_cpu_stall_notifier_list);
1096
1097/**
1098 * rcu_stall_chain_notifier_register - Add an RCU CPU stall notifier
1099 * @n: Entry to add.
1100 *
1101 * Adds an RCU CPU stall notifier to an atomic notifier chain.
1102 * The @action passed to a notifier will be @RCU_STALL_NOTIFY_NORM or
1103 * friends. The @data will be the duration of the stalled grace period,
1104 * in jiffies, coerced to a void* pointer.
1105 *
1106 * Returns 0 on success, %-EEXIST on error.
1107 */
1108int rcu_stall_chain_notifier_register(struct notifier_block *n)
1109{
1110 int rcsn = rcu_cpu_stall_notifiers;
1111
1112 WARN(1, "Adding %pS() to RCU stall notifier list (%s).\n", n->notifier_call,
1113 rcsn ? "possibly suppressing RCU CPU stall warnings" : "failed, so all is well");
1114 if (rcsn)
1115 return atomic_notifier_chain_register(nh: &rcu_cpu_stall_notifier_list, nb: n);
1116 return -EEXIST;
1117}
1118EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_register);
1119
1120/**
1121 * rcu_stall_chain_notifier_unregister - Remove an RCU CPU stall notifier
1122 * @n: Entry to add.
1123 *
1124 * Removes an RCU CPU stall notifier from an atomic notifier chain.
1125 *
1126 * Returns zero on success, %-ENOENT on failure.
1127 */
1128int rcu_stall_chain_notifier_unregister(struct notifier_block *n)
1129{
1130 return atomic_notifier_chain_unregister(nh: &rcu_cpu_stall_notifier_list, nb: n);
1131}
1132EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_unregister);
1133
1134/*
1135 * rcu_stall_notifier_call_chain - Call functions in an RCU CPU stall notifier chain
1136 * @val: Value passed unmodified to notifier function
1137 * @v: Pointer passed unmodified to notifier function
1138 *
1139 * Calls each function in the RCU CPU stall notifier chain in turn, which
1140 * is an atomic call chain. See atomic_notifier_call_chain() for more
1141 * information.
1142 *
1143 * This is for use within RCU, hence the omission of the extra asterisk
1144 * to indicate a non-kerneldoc format header comment.
1145 */
1146int rcu_stall_notifier_call_chain(unsigned long val, void *v)
1147{
1148 return atomic_notifier_call_chain(nh: &rcu_cpu_stall_notifier_list, val, v);
1149}
1150
1151#endif // #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER
1152

source code of linux/kernel/rcu/tree_stall.h