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
12 //////////////////////////////////////////////////////////////////////////////
13 //
14 // Controlling CPU stall warnings, including delay calculation.
15
16 /* panic() on RCU Stall sysctl. */
17 int sysctl_panic_on_rcu_stall __read_mostly;
18 int sysctl_max_rcu_stall_to_panic __read_mostly;
19
20 #ifdef CONFIG_PROVE_RCU
21 #define RCU_STALL_DELAY_DELTA (5 * HZ)
22 #else
23 #define RCU_STALL_DELAY_DELTA 0
24 #endif
25 #define RCU_STALL_MIGHT_DIV 8
26 #define RCU_STALL_MIGHT_MIN (2 * HZ)
27
28 /* Limit-check stall timeouts specified at boottime and runtime. */
rcu_jiffies_till_stall_check(void)29 int rcu_jiffies_till_stall_check(void)
30 {
31 int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);
32
33 /*
34 * Limit check must be consistent with the Kconfig limits
35 * for CONFIG_RCU_CPU_STALL_TIMEOUT.
36 */
37 if (till_stall_check < 3) {
38 WRITE_ONCE(rcu_cpu_stall_timeout, 3);
39 till_stall_check = 3;
40 } else if (till_stall_check > 300) {
41 WRITE_ONCE(rcu_cpu_stall_timeout, 300);
42 till_stall_check = 300;
43 }
44 return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
45 }
46 EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);
47
48 /**
49 * rcu_gp_might_be_stalled - Is it likely that the grace period is stalled?
50 *
51 * Returns @true if the current grace period is sufficiently old that
52 * it is reasonable to assume that it might be stalled. This can be
53 * useful when deciding whether to allocate memory to enable RCU-mediated
54 * freeing on the one hand or just invoking synchronize_rcu() on the other.
55 * The latter is preferable when the grace period is stalled.
56 *
57 * Note that sampling of the .gp_start and .gp_seq fields must be done
58 * carefully to avoid false positives at the beginnings and ends of
59 * grace periods.
60 */
rcu_gp_might_be_stalled(void)61 bool rcu_gp_might_be_stalled(void)
62 {
63 unsigned long d = rcu_jiffies_till_stall_check() / RCU_STALL_MIGHT_DIV;
64 unsigned long j = jiffies;
65
66 if (d < RCU_STALL_MIGHT_MIN)
67 d = RCU_STALL_MIGHT_MIN;
68 smp_mb(); // jiffies before .gp_seq to avoid false positives.
69 if (!rcu_gp_in_progress())
70 return false;
71 // Long delays at this point avoids false positive, but a delay
72 // of ULONG_MAX/4 jiffies voids your no-false-positive warranty.
73 smp_mb(); // .gp_seq before second .gp_start
74 // And ditto here.
75 return !time_before(j, READ_ONCE(rcu_state.gp_start) + d);
76 }
77
78 /* Don't do RCU CPU stall warnings during long sysrq printouts. */
rcu_sysrq_start(void)79 void rcu_sysrq_start(void)
80 {
81 if (!rcu_cpu_stall_suppress)
82 rcu_cpu_stall_suppress = 2;
83 }
84
rcu_sysrq_end(void)85 void rcu_sysrq_end(void)
86 {
87 if (rcu_cpu_stall_suppress == 2)
88 rcu_cpu_stall_suppress = 0;
89 }
90
91 /* Don't print RCU CPU stall warnings during a kernel panic. */
rcu_panic(struct notifier_block * this,unsigned long ev,void * ptr)92 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
93 {
94 rcu_cpu_stall_suppress = 1;
95 return NOTIFY_DONE;
96 }
97
98 static struct notifier_block rcu_panic_block = {
99 .notifier_call = rcu_panic,
100 };
101
check_cpu_stall_init(void)102 static int __init check_cpu_stall_init(void)
103 {
104 atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
105 return 0;
106 }
107 early_initcall(check_cpu_stall_init);
108
109 /* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
panic_on_rcu_stall(void)110 static void panic_on_rcu_stall(void)
111 {
112 static int cpu_stall;
113
114 if (++cpu_stall < sysctl_max_rcu_stall_to_panic)
115 return;
116
117 if (sysctl_panic_on_rcu_stall)
118 panic("RCU Stall\n");
119 }
120
121 /**
122 * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
123 *
124 * To perform the reset request from the caller, disable stall detection until
125 * 3 fqs loops have passed. This is required to ensure a fresh jiffies is
126 * loaded. It should be safe to do from the fqs loop as enough timer
127 * interrupts and context switches should have passed.
128 *
129 * The caller must disable hard irqs.
130 */
rcu_cpu_stall_reset(void)131 void rcu_cpu_stall_reset(void)
132 {
133 WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 3);
134 WRITE_ONCE(rcu_state.jiffies_stall, ULONG_MAX);
135 }
136
137 //////////////////////////////////////////////////////////////////////////////
138 //
139 // Interaction with RCU grace periods
140
141 /* Start of new grace period, so record stall time (and forcing times). */
record_gp_stall_check_time(void)142 static void record_gp_stall_check_time(void)
143 {
144 unsigned long j = jiffies;
145 unsigned long j1;
146
147 WRITE_ONCE(rcu_state.gp_start, j);
148 j1 = rcu_jiffies_till_stall_check();
149 smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
150 WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 0);
151 WRITE_ONCE(rcu_state.jiffies_stall, j + j1);
152 rcu_state.jiffies_resched = j + j1 / 2;
153 rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
154 }
155
156 /* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
zero_cpu_stall_ticks(struct rcu_data * rdp)157 static void zero_cpu_stall_ticks(struct rcu_data *rdp)
158 {
159 rdp->ticks_this_gp = 0;
160 rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
161 WRITE_ONCE(rdp->last_fqs_resched, jiffies);
162 }
163
164 /*
165 * If too much time has passed in the current grace period, and if
166 * so configured, go kick the relevant kthreads.
167 */
rcu_stall_kick_kthreads(void)168 static void rcu_stall_kick_kthreads(void)
169 {
170 unsigned long j;
171
172 if (!READ_ONCE(rcu_kick_kthreads))
173 return;
174 j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
175 if (time_after(jiffies, j) && rcu_state.gp_kthread &&
176 (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
177 WARN_ONCE(1, "Kicking %s grace-period kthread\n",
178 rcu_state.name);
179 rcu_ftrace_dump(DUMP_ALL);
180 wake_up_process(rcu_state.gp_kthread);
181 WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
182 }
183 }
184
185 /*
186 * Handler for the irq_work request posted about halfway into the RCU CPU
187 * stall timeout, and used to detect excessive irq disabling. Set state
188 * appropriately, but just complain if there is unexpected state on entry.
189 */
rcu_iw_handler(struct irq_work * iwp)190 static void rcu_iw_handler(struct irq_work *iwp)
191 {
192 struct rcu_data *rdp;
193 struct rcu_node *rnp;
194
195 rdp = container_of(iwp, struct rcu_data, rcu_iw);
196 rnp = rdp->mynode;
197 raw_spin_lock_rcu_node(rnp);
198 if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
199 rdp->rcu_iw_gp_seq = rnp->gp_seq;
200 rdp->rcu_iw_pending = false;
201 }
202 raw_spin_unlock_rcu_node(rnp);
203 }
204
205 //////////////////////////////////////////////////////////////////////////////
206 //
207 // Printing RCU CPU stall warnings
208
209 #ifdef CONFIG_PREEMPT_RCU
210
211 /*
212 * Dump detailed information for all tasks blocking the current RCU
213 * grace period on the specified rcu_node structure.
214 */
rcu_print_detail_task_stall_rnp(struct rcu_node * rnp)215 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
216 {
217 unsigned long flags;
218 struct task_struct *t;
219
220 raw_spin_lock_irqsave_rcu_node(rnp, flags);
221 if (!rcu_preempt_blocked_readers_cgp(rnp)) {
222 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
223 return;
224 }
225 t = list_entry(rnp->gp_tasks->prev,
226 struct task_struct, rcu_node_entry);
227 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
228 /*
229 * We could be printing a lot while holding a spinlock.
230 * Avoid triggering hard lockup.
231 */
232 touch_nmi_watchdog();
233 sched_show_task(t);
234 }
235 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
236 }
237
238 // Communicate task state back to the RCU CPU stall warning request.
239 struct rcu_stall_chk_rdr {
240 int nesting;
241 union rcu_special rs;
242 bool on_blkd_list;
243 };
244
245 /*
246 * Report out the state of a not-running task that is stalling the
247 * current RCU grace period.
248 */
check_slow_task(struct task_struct * t,void * arg)249 static bool check_slow_task(struct task_struct *t, void *arg)
250 {
251 struct rcu_stall_chk_rdr *rscrp = arg;
252
253 if (task_curr(t))
254 return false; // It is running, so decline to inspect it.
255 rscrp->nesting = t->rcu_read_lock_nesting;
256 rscrp->rs = t->rcu_read_unlock_special;
257 rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
258 return true;
259 }
260
261 /*
262 * Scan the current list of tasks blocked within RCU read-side critical
263 * sections, printing out the tid of each of the first few of them.
264 */
rcu_print_task_stall(struct rcu_node * rnp,unsigned long flags)265 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
266 __releases(rnp->lock)
267 {
268 int i = 0;
269 int ndetected = 0;
270 struct rcu_stall_chk_rdr rscr;
271 struct task_struct *t;
272 struct task_struct *ts[8];
273
274 lockdep_assert_irqs_disabled();
275 if (!rcu_preempt_blocked_readers_cgp(rnp)) {
276 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
277 return 0;
278 }
279 pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
280 rnp->level, rnp->grplo, rnp->grphi);
281 t = list_entry(rnp->gp_tasks->prev,
282 struct task_struct, rcu_node_entry);
283 list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
284 get_task_struct(t);
285 ts[i++] = t;
286 if (i >= ARRAY_SIZE(ts))
287 break;
288 }
289 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
290 while (i) {
291 t = ts[--i];
292 if (!try_invoke_on_locked_down_task(t, check_slow_task, &rscr))
293 pr_cont(" P%d", t->pid);
294 else
295 pr_cont(" P%d/%d:%c%c%c%c",
296 t->pid, rscr.nesting,
297 ".b"[rscr.rs.b.blocked],
298 ".q"[rscr.rs.b.need_qs],
299 ".e"[rscr.rs.b.exp_hint],
300 ".l"[rscr.on_blkd_list]);
301 lockdep_assert_irqs_disabled();
302 put_task_struct(t);
303 ndetected++;
304 }
305 pr_cont("\n");
306 return ndetected;
307 }
308
309 #else /* #ifdef CONFIG_PREEMPT_RCU */
310
311 /*
312 * Because preemptible RCU does not exist, we never have to check for
313 * tasks blocked within RCU read-side critical sections.
314 */
rcu_print_detail_task_stall_rnp(struct rcu_node * rnp)315 static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
316 {
317 }
318
319 /*
320 * Because preemptible RCU does not exist, we never have to check for
321 * tasks blocked within RCU read-side critical sections.
322 */
rcu_print_task_stall(struct rcu_node * rnp,unsigned long flags)323 static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
324 __releases(rnp->lock)
325 {
326 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
327 return 0;
328 }
329 #endif /* #else #ifdef CONFIG_PREEMPT_RCU */
330
331 /*
332 * Dump stacks of all tasks running on stalled CPUs. First try using
333 * NMIs, but fall back to manual remote stack tracing on architectures
334 * that don't support NMI-based stack dumps. The NMI-triggered stack
335 * traces are more accurate because they are printed by the target CPU.
336 */
rcu_dump_cpu_stacks(void)337 static void rcu_dump_cpu_stacks(void)
338 {
339 int cpu;
340 unsigned long flags;
341 struct rcu_node *rnp;
342
343 rcu_for_each_leaf_node(rnp) {
344 raw_spin_lock_irqsave_rcu_node(rnp, flags);
345 for_each_leaf_node_possible_cpu(rnp, cpu)
346 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
347 if (cpu_is_offline(cpu))
348 pr_err("Offline CPU %d blocking current GP.\n", cpu);
349 else if (!trigger_single_cpu_backtrace(cpu))
350 dump_cpu_task(cpu);
351 }
352 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
353 }
354 }
355
356 #ifdef CONFIG_RCU_FAST_NO_HZ
357
print_cpu_stall_fast_no_hz(char * cp,int cpu)358 static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
359 {
360 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
361
362 sprintf(cp, "last_accelerate: %04lx/%04lx dyntick_enabled: %d",
363 rdp->last_accelerate & 0xffff, jiffies & 0xffff,
364 !!rdp->tick_nohz_enabled_snap);
365 }
366
367 #else /* #ifdef CONFIG_RCU_FAST_NO_HZ */
368
print_cpu_stall_fast_no_hz(char * cp,int cpu)369 static void print_cpu_stall_fast_no_hz(char *cp, int cpu)
370 {
371 *cp = '\0';
372 }
373
374 #endif /* #else #ifdef CONFIG_RCU_FAST_NO_HZ */
375
376 static const char * const gp_state_names[] = {
377 [RCU_GP_IDLE] = "RCU_GP_IDLE",
378 [RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS",
379 [RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS",
380 [RCU_GP_ONOFF] = "RCU_GP_ONOFF",
381 [RCU_GP_INIT] = "RCU_GP_INIT",
382 [RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS",
383 [RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS",
384 [RCU_GP_CLEANUP] = "RCU_GP_CLEANUP",
385 [RCU_GP_CLEANED] = "RCU_GP_CLEANED",
386 };
387
388 /*
389 * Convert a ->gp_state value to a character string.
390 */
gp_state_getname(short gs)391 static const char *gp_state_getname(short gs)
392 {
393 if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
394 return "???";
395 return gp_state_names[gs];
396 }
397
398 /* Is the RCU grace-period kthread being starved of CPU time? */
rcu_is_gp_kthread_starving(unsigned long * jp)399 static bool rcu_is_gp_kthread_starving(unsigned long *jp)
400 {
401 unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity);
402
403 if (jp)
404 *jp = j;
405 return j > 2 * HZ;
406 }
407
408 /*
409 * Print out diagnostic information for the specified stalled CPU.
410 *
411 * If the specified CPU is aware of the current RCU grace period, then
412 * print the number of scheduling clock interrupts the CPU has taken
413 * during the time that it has been aware. Otherwise, print the number
414 * of RCU grace periods that this CPU is ignorant of, for example, "1"
415 * if the CPU was aware of the previous grace period.
416 *
417 * Also print out idle and (if CONFIG_RCU_FAST_NO_HZ) idle-entry info.
418 */
print_cpu_stall_info(int cpu)419 static void print_cpu_stall_info(int cpu)
420 {
421 unsigned long delta;
422 bool falsepositive;
423 char fast_no_hz[72];
424 struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
425 char *ticks_title;
426 unsigned long ticks_value;
427
428 /*
429 * We could be printing a lot while holding a spinlock. Avoid
430 * triggering hard lockup.
431 */
432 touch_nmi_watchdog();
433
434 ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
435 if (ticks_value) {
436 ticks_title = "GPs behind";
437 } else {
438 ticks_title = "ticks this GP";
439 ticks_value = rdp->ticks_this_gp;
440 }
441 print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
442 delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
443 falsepositive = rcu_is_gp_kthread_starving(NULL) &&
444 rcu_dynticks_in_eqs(rcu_dynticks_snap(rdp));
445 pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%ld/%#lx softirq=%u/%u fqs=%ld %s%s\n",
446 cpu,
447 "O."[!!cpu_online(cpu)],
448 "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
449 "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
450 !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
451 rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
452 "!."[!delta],
453 ticks_value, ticks_title,
454 rcu_dynticks_snap(rdp) & 0xfff,
455 rdp->dynticks_nesting, rdp->dynticks_nmi_nesting,
456 rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
457 data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
458 fast_no_hz,
459 falsepositive ? " (false positive?)" : "");
460 }
461
462 /* Complain about starvation of grace-period kthread. */
rcu_check_gp_kthread_starvation(void)463 static void rcu_check_gp_kthread_starvation(void)
464 {
465 int cpu;
466 struct task_struct *gpk = rcu_state.gp_kthread;
467 unsigned long j;
468
469 if (rcu_is_gp_kthread_starving(&j)) {
470 cpu = gpk ? task_cpu(gpk) : -1;
471 pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n",
472 rcu_state.name, j,
473 (long)rcu_seq_current(&rcu_state.gp_seq),
474 data_race(READ_ONCE(rcu_state.gp_flags)),
475 gp_state_getname(rcu_state.gp_state),
476 data_race(READ_ONCE(rcu_state.gp_state)),
477 gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu);
478 if (gpk) {
479 pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
480 pr_err("RCU grace-period kthread stack dump:\n");
481 sched_show_task(gpk);
482 if (cpu >= 0) {
483 if (cpu_is_offline(cpu)) {
484 pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
485 } else {
486 pr_err("Stack dump where RCU GP kthread last ran:\n");
487 if (!trigger_single_cpu_backtrace(cpu))
488 dump_cpu_task(cpu);
489 }
490 }
491 wake_up_process(gpk);
492 }
493 }
494 }
495
496 /* Complain about missing wakeups from expired fqs wait timer */
rcu_check_gp_kthread_expired_fqs_timer(void)497 static void rcu_check_gp_kthread_expired_fqs_timer(void)
498 {
499 struct task_struct *gpk = rcu_state.gp_kthread;
500 short gp_state;
501 unsigned long jiffies_fqs;
502 int cpu;
503
504 /*
505 * Order reads of .gp_state and .jiffies_force_qs.
506 * Matching smp_wmb() is present in rcu_gp_fqs_loop().
507 */
508 gp_state = smp_load_acquire(&rcu_state.gp_state);
509 jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs);
510
511 if (gp_state == RCU_GP_WAIT_FQS &&
512 time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) &&
513 gpk && !READ_ONCE(gpk->on_rq)) {
514 cpu = task_cpu(gpk);
515 pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n",
516 rcu_state.name, (jiffies - jiffies_fqs),
517 (long)rcu_seq_current(&rcu_state.gp_seq),
518 data_race(rcu_state.gp_flags),
519 gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
520 data_race(READ_ONCE(gpk->__state)));
521 pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",
522 cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu));
523 }
524 }
525
print_other_cpu_stall(unsigned long gp_seq,unsigned long gps)526 static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
527 {
528 int cpu;
529 unsigned long flags;
530 unsigned long gpa;
531 unsigned long j;
532 int ndetected = 0;
533 struct rcu_node *rnp;
534 long totqlen = 0;
535
536 lockdep_assert_irqs_disabled();
537
538 /* Kick and suppress, if so configured. */
539 rcu_stall_kick_kthreads();
540 if (rcu_stall_is_suppressed())
541 return;
542
543 /*
544 * OK, time to rat on our buddy...
545 * See Documentation/RCU/stallwarn.rst for info on how to debug
546 * RCU CPU stall warnings.
547 */
548 trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
549 pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
550 rcu_for_each_leaf_node(rnp) {
551 raw_spin_lock_irqsave_rcu_node(rnp, flags);
552 if (rnp->qsmask != 0) {
553 for_each_leaf_node_possible_cpu(rnp, cpu)
554 if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
555 print_cpu_stall_info(cpu);
556 ndetected++;
557 }
558 }
559 ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
560 lockdep_assert_irqs_disabled();
561 }
562
563 for_each_possible_cpu(cpu)
564 totqlen += rcu_get_n_cbs_cpu(cpu);
565 pr_cont("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu)\n",
566 smp_processor_id(), (long)(jiffies - gps),
567 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
568 if (ndetected) {
569 rcu_dump_cpu_stacks();
570
571 /* Complain about tasks blocking the grace period. */
572 rcu_for_each_leaf_node(rnp)
573 rcu_print_detail_task_stall_rnp(rnp);
574 } else {
575 if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
576 pr_err("INFO: Stall ended before state dump start\n");
577 } else {
578 j = jiffies;
579 gpa = data_race(READ_ONCE(rcu_state.gp_activity));
580 pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
581 rcu_state.name, j - gpa, j, gpa,
582 data_race(READ_ONCE(jiffies_till_next_fqs)),
583 data_race(READ_ONCE(rcu_get_root()->qsmask)));
584 }
585 }
586 /* Rewrite if needed in case of slow consoles. */
587 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
588 WRITE_ONCE(rcu_state.jiffies_stall,
589 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
590
591 rcu_check_gp_kthread_expired_fqs_timer();
592 rcu_check_gp_kthread_starvation();
593
594 panic_on_rcu_stall();
595
596 rcu_force_quiescent_state(); /* Kick them all. */
597 }
598
print_cpu_stall(unsigned long gps)599 static void print_cpu_stall(unsigned long gps)
600 {
601 int cpu;
602 unsigned long flags;
603 struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
604 struct rcu_node *rnp = rcu_get_root();
605 long totqlen = 0;
606
607 lockdep_assert_irqs_disabled();
608
609 /* Kick and suppress, if so configured. */
610 rcu_stall_kick_kthreads();
611 if (rcu_stall_is_suppressed())
612 return;
613
614 /*
615 * OK, time to rat on ourselves...
616 * See Documentation/RCU/stallwarn.rst for info on how to debug
617 * RCU CPU stall warnings.
618 */
619 trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
620 pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
621 raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
622 print_cpu_stall_info(smp_processor_id());
623 raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
624 for_each_possible_cpu(cpu)
625 totqlen += rcu_get_n_cbs_cpu(cpu);
626 pr_cont("\t(t=%lu jiffies g=%ld q=%lu)\n",
627 jiffies - gps,
628 (long)rcu_seq_current(&rcu_state.gp_seq), totqlen);
629
630 rcu_check_gp_kthread_expired_fqs_timer();
631 rcu_check_gp_kthread_starvation();
632
633 rcu_dump_cpu_stacks();
634
635 raw_spin_lock_irqsave_rcu_node(rnp, flags);
636 /* Rewrite if needed in case of slow consoles. */
637 if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
638 WRITE_ONCE(rcu_state.jiffies_stall,
639 jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
640 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
641
642 panic_on_rcu_stall();
643
644 /*
645 * Attempt to revive the RCU machinery by forcing a context switch.
646 *
647 * A context switch would normally allow the RCU state machine to make
648 * progress and it could be we're stuck in kernel space without context
649 * switches for an entirely unreasonable amount of time.
650 */
651 set_tsk_need_resched(current);
652 set_preempt_need_resched();
653 }
654
check_cpu_stall(struct rcu_data * rdp)655 static void check_cpu_stall(struct rcu_data *rdp)
656 {
657 bool didstall = false;
658 unsigned long gs1;
659 unsigned long gs2;
660 unsigned long gps;
661 unsigned long j;
662 unsigned long jn;
663 unsigned long js;
664 struct rcu_node *rnp;
665
666 lockdep_assert_irqs_disabled();
667 if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
668 !rcu_gp_in_progress())
669 return;
670 rcu_stall_kick_kthreads();
671
672 /*
673 * Check if it was requested (via rcu_cpu_stall_reset()) that the FQS
674 * loop has to set jiffies to ensure a non-stale jiffies value. This
675 * is required to have good jiffies value after coming out of long
676 * breaks of jiffies updates. Not doing so can cause false positives.
677 */
678 if (READ_ONCE(rcu_state.nr_fqs_jiffies_stall) > 0)
679 return;
680
681 j = jiffies;
682
683 /*
684 * Lots of memory barriers to reject false positives.
685 *
686 * The idea is to pick up rcu_state.gp_seq, then
687 * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
688 * another copy of rcu_state.gp_seq. These values are updated in
689 * the opposite order with memory barriers (or equivalent) during
690 * grace-period initialization and cleanup. Now, a false positive
691 * can occur if we get an new value of rcu_state.gp_start and a old
692 * value of rcu_state.jiffies_stall. But given the memory barriers,
693 * the only way that this can happen is if one grace period ends
694 * and another starts between these two fetches. This is detected
695 * by comparing the second fetch of rcu_state.gp_seq with the
696 * previous fetch from rcu_state.gp_seq.
697 *
698 * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
699 * and rcu_state.gp_start suffice to forestall false positives.
700 */
701 gs1 = READ_ONCE(rcu_state.gp_seq);
702 smp_rmb(); /* Pick up ->gp_seq first... */
703 js = READ_ONCE(rcu_state.jiffies_stall);
704 smp_rmb(); /* ...then ->jiffies_stall before the rest... */
705 gps = READ_ONCE(rcu_state.gp_start);
706 smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
707 gs2 = READ_ONCE(rcu_state.gp_seq);
708 if (gs1 != gs2 ||
709 ULONG_CMP_LT(j, js) ||
710 ULONG_CMP_GE(gps, js))
711 return; /* No stall or GP completed since entering function. */
712 rnp = rdp->mynode;
713 jn = jiffies + ULONG_MAX / 2;
714 if (rcu_gp_in_progress() &&
715 (READ_ONCE(rnp->qsmask) & rdp->grpmask) &&
716 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
717
718 /*
719 * If a virtual machine is stopped by the host it can look to
720 * the watchdog like an RCU stall. Check to see if the host
721 * stopped the vm.
722 */
723 if (kvm_check_and_clear_guest_paused())
724 return;
725
726 /* We haven't checked in, so go dump stack. */
727 print_cpu_stall(gps);
728 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
729 rcu_ftrace_dump(DUMP_ALL);
730 didstall = true;
731
732 } else if (rcu_gp_in_progress() &&
733 ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY) &&
734 cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
735
736 /*
737 * If a virtual machine is stopped by the host it can look to
738 * the watchdog like an RCU stall. Check to see if the host
739 * stopped the vm.
740 */
741 if (kvm_check_and_clear_guest_paused())
742 return;
743
744 /* They had a few time units to dump stack, so complain. */
745 print_other_cpu_stall(gs2, gps);
746 if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
747 rcu_ftrace_dump(DUMP_ALL);
748 didstall = true;
749 }
750 if (didstall && READ_ONCE(rcu_state.jiffies_stall) == jn) {
751 jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
752 WRITE_ONCE(rcu_state.jiffies_stall, jn);
753 }
754 }
755
756 //////////////////////////////////////////////////////////////////////////////
757 //
758 // RCU forward-progress mechanisms, including of callback invocation.
759
760
761 /*
762 * Check to see if a failure to end RCU priority inversion was due to
763 * a CPU not passing through a quiescent state. When this happens, there
764 * is nothing that RCU priority boosting can do to help, so we shouldn't
765 * count this as an RCU priority boosting failure. A return of true says
766 * RCU priority boosting is to blame, and false says otherwise. If false
767 * is returned, the first of the CPUs to blame is stored through cpup.
768 * If there was no CPU blocking the current grace period, but also nothing
769 * in need of being boosted, *cpup is set to -1. This can happen in case
770 * of vCPU preemption while the last CPU is reporting its quiscent state,
771 * for example.
772 *
773 * If cpup is NULL, then a lockless quick check is carried out, suitable
774 * for high-rate usage. On the other hand, if cpup is non-NULL, each
775 * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
776 */
rcu_check_boost_fail(unsigned long gp_state,int * cpup)777 bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
778 {
779 bool atb = false;
780 int cpu;
781 unsigned long flags;
782 struct rcu_node *rnp;
783
784 rcu_for_each_leaf_node(rnp) {
785 if (!cpup) {
786 if (data_race(READ_ONCE(rnp->qsmask))) {
787 return false;
788 } else {
789 if (READ_ONCE(rnp->gp_tasks))
790 atb = true;
791 continue;
792 }
793 }
794 *cpup = -1;
795 raw_spin_lock_irqsave_rcu_node(rnp, flags);
796 if (rnp->gp_tasks)
797 atb = true;
798 if (!rnp->qsmask) {
799 // No CPUs without quiescent states for this rnp.
800 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
801 continue;
802 }
803 // Find the first holdout CPU.
804 for_each_leaf_node_possible_cpu(rnp, cpu) {
805 if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) {
806 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
807 *cpup = cpu;
808 return false;
809 }
810 }
811 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
812 }
813 // Can't blame CPUs, so must blame RCU priority boosting.
814 return atb;
815 }
816 EXPORT_SYMBOL_GPL(rcu_check_boost_fail);
817
818 /*
819 * Show the state of the grace-period kthreads.
820 */
show_rcu_gp_kthreads(void)821 void show_rcu_gp_kthreads(void)
822 {
823 unsigned long cbs = 0;
824 int cpu;
825 unsigned long j;
826 unsigned long ja;
827 unsigned long jr;
828 unsigned long js;
829 unsigned long jw;
830 struct rcu_data *rdp;
831 struct rcu_node *rnp;
832 struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
833
834 j = jiffies;
835 ja = j - data_race(READ_ONCE(rcu_state.gp_activity));
836 jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity));
837 js = j - data_race(READ_ONCE(rcu_state.gp_start));
838 jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time));
839 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",
840 rcu_state.name, gp_state_getname(rcu_state.gp_state),
841 data_race(READ_ONCE(rcu_state.gp_state)),
842 t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU,
843 js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)),
844 (long)data_race(READ_ONCE(rcu_state.gp_seq)),
845 (long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)),
846 data_race(READ_ONCE(rcu_state.gp_max)),
847 data_race(READ_ONCE(rcu_state.gp_flags)));
848 rcu_for_each_node_breadth_first(rnp) {
849 if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
850 !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
851 !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
852 continue;
853 pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
854 rnp->grplo, rnp->grphi,
855 (long)data_race(READ_ONCE(rnp->gp_seq)),
856 (long)data_race(READ_ONCE(rnp->gp_seq_needed)),
857 data_race(READ_ONCE(rnp->qsmask)),
858 ".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
859 ".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
860 ".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],
861 ".G"[!!data_race(READ_ONCE(rnp->gp_tasks))],
862 data_race(READ_ONCE(rnp->n_boosts)));
863 if (!rcu_is_leaf_node(rnp))
864 continue;
865 for_each_leaf_node_possible_cpu(rnp, cpu) {
866 rdp = per_cpu_ptr(&rcu_data, cpu);
867 if (READ_ONCE(rdp->gpwrap) ||
868 ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
869 READ_ONCE(rdp->gp_seq_needed)))
870 continue;
871 pr_info("\tcpu %d ->gp_seq_needed %ld\n",
872 cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed)));
873 }
874 }
875 for_each_possible_cpu(cpu) {
876 rdp = per_cpu_ptr(&rcu_data, cpu);
877 cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
878 if (rcu_segcblist_is_offloaded(&rdp->cblist))
879 show_rcu_nocb_state(rdp);
880 }
881 pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
882 show_rcu_tasks_gp_kthreads();
883 }
884 EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);
885
886 /*
887 * This function checks for grace-period requests that fail to motivate
888 * RCU to come out of its idle mode.
889 */
rcu_check_gp_start_stall(struct rcu_node * rnp,struct rcu_data * rdp,const unsigned long gpssdelay)890 static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
891 const unsigned long gpssdelay)
892 {
893 unsigned long flags;
894 unsigned long j;
895 struct rcu_node *rnp_root = rcu_get_root();
896 static atomic_t warned = ATOMIC_INIT(0);
897
898 if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
899 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
900 READ_ONCE(rnp_root->gp_seq_needed)) ||
901 !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
902 return;
903 j = jiffies; /* Expensive access, and in common case don't get here. */
904 if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
905 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
906 atomic_read(&warned))
907 return;
908
909 raw_spin_lock_irqsave_rcu_node(rnp, flags);
910 j = jiffies;
911 if (rcu_gp_in_progress() ||
912 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
913 READ_ONCE(rnp_root->gp_seq_needed)) ||
914 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
915 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
916 atomic_read(&warned)) {
917 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
918 return;
919 }
920 /* Hold onto the leaf lock to make others see warned==1. */
921
922 if (rnp_root != rnp)
923 raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
924 j = jiffies;
925 if (rcu_gp_in_progress() ||
926 ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
927 READ_ONCE(rnp_root->gp_seq_needed)) ||
928 time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
929 time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
930 atomic_xchg(&warned, 1)) {
931 if (rnp_root != rnp)
932 /* irqs remain disabled. */
933 raw_spin_unlock_rcu_node(rnp_root);
934 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
935 return;
936 }
937 WARN_ON(1);
938 if (rnp_root != rnp)
939 raw_spin_unlock_rcu_node(rnp_root);
940 raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
941 show_rcu_gp_kthreads();
942 }
943
944 /*
945 * Do a forward-progress check for rcutorture. This is normally invoked
946 * due to an OOM event. The argument "j" gives the time period during
947 * which rcutorture would like progress to have been made.
948 */
rcu_fwd_progress_check(unsigned long j)949 void rcu_fwd_progress_check(unsigned long j)
950 {
951 unsigned long cbs;
952 int cpu;
953 unsigned long max_cbs = 0;
954 int max_cpu = -1;
955 struct rcu_data *rdp;
956
957 if (rcu_gp_in_progress()) {
958 pr_info("%s: GP age %lu jiffies\n",
959 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start)));
960 show_rcu_gp_kthreads();
961 } else {
962 pr_info("%s: Last GP end %lu jiffies ago\n",
963 __func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end)));
964 preempt_disable();
965 rdp = this_cpu_ptr(&rcu_data);
966 rcu_check_gp_start_stall(rdp->mynode, rdp, j);
967 preempt_enable();
968 }
969 for_each_possible_cpu(cpu) {
970 cbs = rcu_get_n_cbs_cpu(cpu);
971 if (!cbs)
972 continue;
973 if (max_cpu < 0)
974 pr_info("%s: callbacks", __func__);
975 pr_cont(" %d: %lu", cpu, cbs);
976 if (cbs <= max_cbs)
977 continue;
978 max_cbs = cbs;
979 max_cpu = cpu;
980 }
981 if (max_cpu >= 0)
982 pr_cont("\n");
983 }
984 EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);
985
986 /* Commandeer a sysrq key to dump RCU's tree. */
987 static bool sysrq_rcu;
988 module_param(sysrq_rcu, bool, 0444);
989
990 /* Dump grace-period-request information due to commandeered sysrq. */
sysrq_show_rcu(int key)991 static void sysrq_show_rcu(int key)
992 {
993 show_rcu_gp_kthreads();
994 }
995
996 static const struct sysrq_key_op sysrq_rcudump_op = {
997 .handler = sysrq_show_rcu,
998 .help_msg = "show-rcu(y)",
999 .action_msg = "Show RCU tree",
1000 .enable_mask = SYSRQ_ENABLE_DUMP,
1001 };
1002
rcu_sysrq_init(void)1003 static int __init rcu_sysrq_init(void)
1004 {
1005 if (sysrq_rcu)
1006 return register_sysrq_key('y', &sysrq_rcudump_op);
1007 return 0;
1008 }
1009 early_initcall(rcu_sysrq_init);
1010