summaryrefslogtreecommitdiff
path: root/kernel/rcu/tree_stall.h
blob: 460efecd077be13940a94fe5a06197f25e6921a9 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
// SPDX-License-Identifier: GPL-2.0+
/*
 * RCU CPU stall warnings for normal RCU grace periods
 *
 * Copyright IBM Corporation, 2019
 *
 * Author: Paul E. McKenney <paulmck@linux.ibm.com>
 */

#include <linux/kvm_para.h>
#include <linux/rcu_notifier.h>

//////////////////////////////////////////////////////////////////////////////
//
// Controlling CPU stall warnings, including delay calculation.

/* panic() on RCU Stall sysctl. */
int sysctl_panic_on_rcu_stall __read_mostly;
int sysctl_max_rcu_stall_to_panic __read_mostly;

#ifdef CONFIG_PROVE_RCU
#define RCU_STALL_DELAY_DELTA		(5 * HZ)
#else
#define RCU_STALL_DELAY_DELTA		0
#endif
#define RCU_STALL_MIGHT_DIV		8
#define RCU_STALL_MIGHT_MIN		(2 * HZ)

int rcu_exp_jiffies_till_stall_check(void)
{
	int cpu_stall_timeout = READ_ONCE(rcu_exp_cpu_stall_timeout);
	int exp_stall_delay_delta = 0;
	int till_stall_check;

	// Zero says to use rcu_cpu_stall_timeout, but in milliseconds.
	if (!cpu_stall_timeout)
		cpu_stall_timeout = jiffies_to_msecs(rcu_jiffies_till_stall_check());

	// Limit check must be consistent with the Kconfig limits for
	// CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, so check the allowed range.
	// The minimum clamped value is "2UL", because at least one full
	// tick has to be guaranteed.
	till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 300UL * HZ);

	if (cpu_stall_timeout && jiffies_to_msecs(till_stall_check) != cpu_stall_timeout)
		WRITE_ONCE(rcu_exp_cpu_stall_timeout, jiffies_to_msecs(till_stall_check));

#ifdef CONFIG_PROVE_RCU
	/* Add extra ~25% out of till_stall_check. */
	exp_stall_delay_delta = ((till_stall_check * 25) / 100) + 1;
#endif

	return till_stall_check + exp_stall_delay_delta;
}
EXPORT_SYMBOL_GPL(rcu_exp_jiffies_till_stall_check);

/* Limit-check stall timeouts specified at boottime and runtime. */
int rcu_jiffies_till_stall_check(void)
{
	int till_stall_check = READ_ONCE(rcu_cpu_stall_timeout);

	/*
	 * Limit check must be consistent with the Kconfig limits
	 * for CONFIG_RCU_CPU_STALL_TIMEOUT.
	 */
	if (till_stall_check < 3) {
		WRITE_ONCE(rcu_cpu_stall_timeout, 3);
		till_stall_check = 3;
	} else if (till_stall_check > 300) {
		WRITE_ONCE(rcu_cpu_stall_timeout, 300);
		till_stall_check = 300;
	}
	return till_stall_check * HZ + RCU_STALL_DELAY_DELTA;
}
EXPORT_SYMBOL_GPL(rcu_jiffies_till_stall_check);

/**
 * rcu_gp_might_be_stalled - Is it likely that the grace period is stalled?
 *
 * Returns @true if the current grace period is sufficiently old that
 * it is reasonable to assume that it might be stalled.  This can be
 * useful when deciding whether to allocate memory to enable RCU-mediated
 * freeing on the one hand or just invoking synchronize_rcu() on the other.
 * The latter is preferable when the grace period is stalled.
 *
 * Note that sampling of the .gp_start and .gp_seq fields must be done
 * carefully to avoid false positives at the beginnings and ends of
 * grace periods.
 */
bool rcu_gp_might_be_stalled(void)
{
	unsigned long d = rcu_jiffies_till_stall_check() / RCU_STALL_MIGHT_DIV;
	unsigned long j = jiffies;

	if (d < RCU_STALL_MIGHT_MIN)
		d = RCU_STALL_MIGHT_MIN;
	smp_mb(); // jiffies before .gp_seq to avoid false positives.
	if (!rcu_gp_in_progress())
		return false;
	// Long delays at this point avoids false positive, but a delay
	// of ULONG_MAX/4 jiffies voids your no-false-positive warranty.
	smp_mb(); // .gp_seq before second .gp_start
	// And ditto here.
	return !time_before(j, READ_ONCE(rcu_state.gp_start) + d);
}

/* Don't do RCU CPU stall warnings during long sysrq printouts. */
void rcu_sysrq_start(void)
{
	if (!rcu_cpu_stall_suppress)
		rcu_cpu_stall_suppress = 2;
}

void rcu_sysrq_end(void)
{
	if (rcu_cpu_stall_suppress == 2)
		rcu_cpu_stall_suppress = 0;
}

/* Don't print RCU CPU stall warnings during a kernel panic. */
static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
{
	rcu_cpu_stall_suppress = 1;
	return NOTIFY_DONE;
}

static struct notifier_block rcu_panic_block = {
	.notifier_call = rcu_panic,
};

static int __init check_cpu_stall_init(void)
{
	atomic_notifier_chain_register(&panic_notifier_list, &rcu_panic_block);
	return 0;
}
early_initcall(check_cpu_stall_init);

/* If so specified via sysctl, panic, yielding cleaner stall-warning output. */
static void panic_on_rcu_stall(void)
{
	static int cpu_stall;

	if (++cpu_stall < sysctl_max_rcu_stall_to_panic)
		return;

	if (sysctl_panic_on_rcu_stall)
		panic("RCU Stall\n");
}

/**
 * rcu_cpu_stall_reset - restart stall-warning timeout for current grace period
 *
 * To perform the reset request from the caller, disable stall detection until
 * 3 fqs loops have passed. This is required to ensure a fresh jiffies is
 * loaded.  It should be safe to do from the fqs loop as enough timer
 * interrupts and context switches should have passed.
 *
 * The caller must disable hard irqs.
 */
void rcu_cpu_stall_reset(void)
{
	WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 3);
	WRITE_ONCE(rcu_state.jiffies_stall, ULONG_MAX);
}

//////////////////////////////////////////////////////////////////////////////
//
// Interaction with RCU grace periods

/* Start of new grace period, so record stall time (and forcing times). */
static void record_gp_stall_check_time(void)
{
	unsigned long j = jiffies;
	unsigned long j1;

	WRITE_ONCE(rcu_state.gp_start, j);
	j1 = rcu_jiffies_till_stall_check();
	smp_mb(); // ->gp_start before ->jiffies_stall and caller's ->gp_seq.
	WRITE_ONCE(rcu_state.nr_fqs_jiffies_stall, 0);
	WRITE_ONCE(rcu_state.jiffies_stall, j + j1);
	rcu_state.jiffies_resched = j + j1 / 2;
	rcu_state.n_force_qs_gpstart = READ_ONCE(rcu_state.n_force_qs);
}

/* Zero ->ticks_this_gp and snapshot the number of RCU softirq handlers. */
static void zero_cpu_stall_ticks(struct rcu_data *rdp)
{
	rdp->ticks_this_gp = 0;
	rdp->softirq_snap = kstat_softirqs_cpu(RCU_SOFTIRQ, smp_processor_id());
	WRITE_ONCE(rdp->last_fqs_resched, jiffies);
}

/*
 * If too much time has passed in the current grace period, and if
 * so configured, go kick the relevant kthreads.
 */
static void rcu_stall_kick_kthreads(void)
{
	unsigned long j;

	if (!READ_ONCE(rcu_kick_kthreads))
		return;
	j = READ_ONCE(rcu_state.jiffies_kick_kthreads);
	if (time_after(jiffies, j) && rcu_state.gp_kthread &&
	    (rcu_gp_in_progress() || READ_ONCE(rcu_state.gp_flags))) {
		WARN_ONCE(1, "Kicking %s grace-period kthread\n",
			  rcu_state.name);
		rcu_ftrace_dump(DUMP_ALL);
		wake_up_process(rcu_state.gp_kthread);
		WRITE_ONCE(rcu_state.jiffies_kick_kthreads, j + HZ);
	}
}

/*
 * Handler for the irq_work request posted about halfway into the RCU CPU
 * stall timeout, and used to detect excessive irq disabling.  Set state
 * appropriately, but just complain if there is unexpected state on entry.
 */
static void rcu_iw_handler(struct irq_work *iwp)
{
	struct rcu_data *rdp;
	struct rcu_node *rnp;

	rdp = container_of(iwp, struct rcu_data, rcu_iw);
	rnp = rdp->mynode;
	raw_spin_lock_rcu_node(rnp);
	if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
		rdp->rcu_iw_gp_seq = rnp->gp_seq;
		rdp->rcu_iw_pending = false;
	}
	raw_spin_unlock_rcu_node(rnp);
}

//////////////////////////////////////////////////////////////////////////////
//
// Printing RCU CPU stall warnings

#ifdef CONFIG_PREEMPT_RCU

/*
 * Dump detailed information for all tasks blocking the current RCU
 * grace period on the specified rcu_node structure.
 */
static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
{
	unsigned long flags;
	struct task_struct *t;

	raw_spin_lock_irqsave_rcu_node(rnp, flags);
	if (!rcu_preempt_blocked_readers_cgp(rnp)) {
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return;
	}
	t = list_entry(rnp->gp_tasks->prev,
		       struct task_struct, rcu_node_entry);
	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
		/*
		 * We could be printing a lot while holding a spinlock.
		 * Avoid triggering hard lockup.
		 */
		touch_nmi_watchdog();
		sched_show_task(t);
	}
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}

// Communicate task state back to the RCU CPU stall warning request.
struct rcu_stall_chk_rdr {
	int nesting;
	union rcu_special rs;
	bool on_blkd_list;
};

/*
 * Report out the state of a not-running task that is stalling the
 * current RCU grace period.
 */
static int check_slow_task(struct task_struct *t, void *arg)
{
	struct rcu_stall_chk_rdr *rscrp = arg;

	if (task_curr(t))
		return -EBUSY; // It is running, so decline to inspect it.
	rscrp->nesting = t->rcu_read_lock_nesting;
	rscrp->rs = t->rcu_read_unlock_special;
	rscrp->on_blkd_list = !list_empty(&t->rcu_node_entry);
	return 0;
}

/*
 * Scan the current list of tasks blocked within RCU read-side critical
 * sections, printing out the tid of each of the first few of them.
 */
static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
	__releases(rnp->lock)
{
	int i = 0;
	int ndetected = 0;
	struct rcu_stall_chk_rdr rscr;
	struct task_struct *t;
	struct task_struct *ts[8];

	lockdep_assert_irqs_disabled();
	if (!rcu_preempt_blocked_readers_cgp(rnp)) {
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return 0;
	}
	pr_err("\tTasks blocked on level-%d rcu_node (CPUs %d-%d):",
	       rnp->level, rnp->grplo, rnp->grphi);
	t = list_entry(rnp->gp_tasks->prev,
		       struct task_struct, rcu_node_entry);
	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
		get_task_struct(t);
		ts[i++] = t;
		if (i >= ARRAY_SIZE(ts))
			break;
	}
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	while (i) {
		t = ts[--i];
		if (task_call_func(t, check_slow_task, &rscr))
			pr_cont(" P%d", t->pid);
		else
			pr_cont(" P%d/%d:%c%c%c%c",
				t->pid, rscr.nesting,
				".b"[rscr.rs.b.blocked],
				".q"[rscr.rs.b.need_qs],
				".e"[rscr.rs.b.exp_hint],
				".l"[rscr.on_blkd_list]);
		lockdep_assert_irqs_disabled();
		put_task_struct(t);
		ndetected++;
	}
	pr_cont("\n");
	return ndetected;
}

#else /* #ifdef CONFIG_PREEMPT_RCU */

/*
 * Because preemptible RCU does not exist, we never have to check for
 * tasks blocked within RCU read-side critical sections.
 */
static void rcu_print_detail_task_stall_rnp(struct rcu_node *rnp)
{
}

/*
 * Because preemptible RCU does not exist, we never have to check for
 * tasks blocked within RCU read-side critical sections.
 */
static int rcu_print_task_stall(struct rcu_node *rnp, unsigned long flags)
	__releases(rnp->lock)
{
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	return 0;
}
#endif /* #else #ifdef CONFIG_PREEMPT_RCU */

/*
 * Dump stacks of all tasks running on stalled CPUs.  First try using
 * NMIs, but fall back to manual remote stack tracing on architectures
 * that don't support NMI-based stack dumps.  The NMI-triggered stack
 * traces are more accurate because they are printed by the target CPU.
 */
static void rcu_dump_cpu_stacks(void)
{
	int cpu;
	unsigned long flags;
	struct rcu_node *rnp;

	rcu_for_each_leaf_node(rnp) {
		raw_spin_lock_irqsave_rcu_node(rnp, flags);
		for_each_leaf_node_possible_cpu(rnp, cpu)
			if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
				if (cpu_is_offline(cpu))
					pr_err("Offline CPU %d blocking current GP.\n", cpu);
				else
					dump_cpu_task(cpu);
			}
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	}
}

static const char * const gp_state_names[] = {
	[RCU_GP_IDLE] = "RCU_GP_IDLE",
	[RCU_GP_WAIT_GPS] = "RCU_GP_WAIT_GPS",
	[RCU_GP_DONE_GPS] = "RCU_GP_DONE_GPS",
	[RCU_GP_ONOFF] = "RCU_GP_ONOFF",
	[RCU_GP_INIT] = "RCU_GP_INIT",
	[RCU_GP_WAIT_FQS] = "RCU_GP_WAIT_FQS",
	[RCU_GP_DOING_FQS] = "RCU_GP_DOING_FQS",
	[RCU_GP_CLEANUP] = "RCU_GP_CLEANUP",
	[RCU_GP_CLEANED] = "RCU_GP_CLEANED",
};

/*
 * Convert a ->gp_state value to a character string.
 */
static const char *gp_state_getname(short gs)
{
	if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
		return "???";
	return gp_state_names[gs];
}

/* Is the RCU grace-period kthread being starved of CPU time? */
static bool rcu_is_gp_kthread_starving(unsigned long *jp)
{
	unsigned long j = jiffies - READ_ONCE(rcu_state.gp_activity);

	if (jp)
		*jp = j;
	return j > 2 * HZ;
}

static bool rcu_is_rcuc_kthread_starving(struct rcu_data *rdp, unsigned long *jp)
{
	int cpu;
	struct task_struct *rcuc;
	unsigned long j;

	rcuc = rdp->rcu_cpu_kthread_task;
	if (!rcuc)
		return false;

	cpu = task_cpu(rcuc);
	if (cpu_is_offline(cpu) || idle_cpu(cpu))
		return false;

	j = jiffies - READ_ONCE(rdp->rcuc_activity);

	if (jp)
		*jp = j;
	return j > 2 * HZ;
}

static void print_cpu_stat_info(int cpu)
{
	struct rcu_snap_record rsr, *rsrp;
	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
	struct kernel_cpustat *kcsp = &kcpustat_cpu(cpu);

	if (!rcu_cpu_stall_cputime)
		return;

	rsrp = &rdp->snap_record;
	if (rsrp->gp_seq != rdp->gp_seq)
		return;

	rsr.cputime_irq     = kcpustat_field(kcsp, CPUTIME_IRQ, cpu);
	rsr.cputime_softirq = kcpustat_field(kcsp, CPUTIME_SOFTIRQ, cpu);
	rsr.cputime_system  = kcpustat_field(kcsp, CPUTIME_SYSTEM, cpu);

	pr_err("\t         hardirqs   softirqs   csw/system\n");
	pr_err("\t number: %8ld %10d %12lld\n",
		kstat_cpu_irqs_sum(cpu) - rsrp->nr_hardirqs,
		kstat_cpu_softirqs_sum(cpu) - rsrp->nr_softirqs,
		nr_context_switches_cpu(cpu) - rsrp->nr_csw);
	pr_err("\tcputime: %8lld %10lld %12lld   ==> %d(ms)\n",
		div_u64(rsr.cputime_irq - rsrp->cputime_irq, NSEC_PER_MSEC),
		div_u64(rsr.cputime_softirq - rsrp->cputime_softirq, NSEC_PER_MSEC),
		div_u64(rsr.cputime_system - rsrp->cputime_system, NSEC_PER_MSEC),
		jiffies_to_msecs(jiffies - rsrp->jiffies));
}

/*
 * Print out diagnostic information for the specified stalled CPU.
 *
 * If the specified CPU is aware of the current RCU grace period, then
 * print the number of scheduling clock interrupts the CPU has taken
 * during the time that it has been aware.  Otherwise, print the number
 * of RCU grace periods that this CPU is ignorant of, for example, "1"
 * if the CPU was aware of the previous grace period.
 *
 * Also print out idle info.
 */
static void print_cpu_stall_info(int cpu)
{
	unsigned long delta;
	bool falsepositive;
	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
	char *ticks_title;
	unsigned long ticks_value;
	bool rcuc_starved;
	unsigned long j;
	char buf[32];

	/*
	 * We could be printing a lot while holding a spinlock.  Avoid
	 * triggering hard lockup.
	 */
	touch_nmi_watchdog();

	ticks_value = rcu_seq_ctr(rcu_state.gp_seq - rdp->gp_seq);
	if (ticks_value) {
		ticks_title = "GPs behind";
	} else {
		ticks_title = "ticks this GP";
		ticks_value = rdp->ticks_this_gp;
	}
	delta = rcu_seq_ctr(rdp->mynode->gp_seq - rdp->rcu_iw_gp_seq);
	falsepositive = rcu_is_gp_kthread_starving(NULL) &&
			rcu_dynticks_in_eqs(rcu_dynticks_snap(cpu));
	rcuc_starved = rcu_is_rcuc_kthread_starving(rdp, &j);
	if (rcuc_starved)
		// Print signed value, as negative values indicate a probable bug.
		snprintf(buf, sizeof(buf), " rcuc=%ld jiffies(starved)", j);
	pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%04x/%ld/%#lx softirq=%u/%u fqs=%ld%s%s\n",
	       cpu,
	       "O."[!!cpu_online(cpu)],
	       "o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
	       "N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
	       !IS_ENABLED(CONFIG_IRQ_WORK) ? '?' :
			rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' :
				"!."[!delta],
	       ticks_value, ticks_title,
	       rcu_dynticks_snap(cpu) & 0xffff,
	       ct_dynticks_nesting_cpu(cpu), ct_dynticks_nmi_nesting_cpu(cpu),
	       rdp->softirq_snap, kstat_softirqs_cpu(RCU_SOFTIRQ, cpu),
	       data_race(rcu_state.n_force_qs) - rcu_state.n_force_qs_gpstart,
	       rcuc_starved ? buf : "",
	       falsepositive ? " (false positive?)" : "");

	print_cpu_stat_info(cpu);
}

/* Complain about starvation of grace-period kthread.  */
static void rcu_check_gp_kthread_starvation(void)
{
	int cpu;
	struct task_struct *gpk = rcu_state.gp_kthread;
	unsigned long j;

	if (rcu_is_gp_kthread_starving(&j)) {
		cpu = gpk ? task_cpu(gpk) : -1;
		pr_err("%s kthread starved for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x ->cpu=%d\n",
		       rcu_state.name, j,
		       (long)rcu_seq_current(&rcu_state.gp_seq),
		       data_race(READ_ONCE(rcu_state.gp_flags)),
		       gp_state_getname(rcu_state.gp_state),
		       data_race(READ_ONCE(rcu_state.gp_state)),
		       gpk ? data_race(READ_ONCE(gpk->__state)) : ~0, cpu);
		if (gpk) {
			struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);

			pr_err("\tUnless %s kthread gets sufficient CPU time, OOM is now expected behavior.\n", rcu_state.name);
			pr_err("RCU grace-period kthread stack dump:\n");
			sched_show_task(gpk);
			if (cpu_is_offline(cpu)) {
				pr_err("RCU GP kthread last ran on offline CPU %d.\n", cpu);
			} else if (!(data_race(READ_ONCE(rdp->mynode->qsmask)) & rdp->grpmask)) {
				pr_err("Stack dump where RCU GP kthread last ran:\n");
				dump_cpu_task(cpu);
			}
			wake_up_process(gpk);
		}
	}
}

/* Complain about missing wakeups from expired fqs wait timer */
static void rcu_check_gp_kthread_expired_fqs_timer(void)
{
	struct task_struct *gpk = rcu_state.gp_kthread;
	short gp_state;
	unsigned long jiffies_fqs;
	int cpu;

	/*
	 * Order reads of .gp_state and .jiffies_force_qs.
	 * Matching smp_wmb() is present in rcu_gp_fqs_loop().
	 */
	gp_state = smp_load_acquire(&rcu_state.gp_state);
	jiffies_fqs = READ_ONCE(rcu_state.jiffies_force_qs);

	if (gp_state == RCU_GP_WAIT_FQS &&
	    time_after(jiffies, jiffies_fqs + RCU_STALL_MIGHT_MIN) &&
	    gpk && !READ_ONCE(gpk->on_rq)) {
		cpu = task_cpu(gpk);
		pr_err("%s kthread timer wakeup didn't happen for %ld jiffies! g%ld f%#x %s(%d) ->state=%#x\n",
		       rcu_state.name, (jiffies - jiffies_fqs),
		       (long)rcu_seq_current(&rcu_state.gp_seq),
		       data_race(READ_ONCE(rcu_state.gp_flags)), // Diagnostic read
		       gp_state_getname(RCU_GP_WAIT_FQS), RCU_GP_WAIT_FQS,
		       data_race(READ_ONCE(gpk->__state)));
		pr_err("\tPossible timer handling issue on cpu=%d timer-softirq=%u\n",
		       cpu, kstat_softirqs_cpu(TIMER_SOFTIRQ, cpu));
	}
}

static void print_other_cpu_stall(unsigned long gp_seq, unsigned long gps)
{
	int cpu;
	unsigned long flags;
	unsigned long gpa;
	unsigned long j;
	int ndetected = 0;
	struct rcu_node *rnp;
	long totqlen = 0;

	lockdep_assert_irqs_disabled();

	/* Kick and suppress, if so configured. */
	rcu_stall_kick_kthreads();
	if (rcu_stall_is_suppressed())
		return;

	/*
	 * OK, time to rat on our buddy...
	 * See Documentation/RCU/stallwarn.rst for info on how to debug
	 * RCU CPU stall warnings.
	 */
	trace_rcu_stall_warning(rcu_state.name, TPS("StallDetected"));
	pr_err("INFO: %s detected stalls on CPUs/tasks:\n", rcu_state.name);
	rcu_for_each_leaf_node(rnp) {
		raw_spin_lock_irqsave_rcu_node(rnp, flags);
		if (rnp->qsmask != 0) {
			for_each_leaf_node_possible_cpu(rnp, cpu)
				if (rnp->qsmask & leaf_node_cpu_bit(rnp, cpu)) {
					print_cpu_stall_info(cpu);
					ndetected++;
				}
		}
		ndetected += rcu_print_task_stall(rnp, flags); // Releases rnp->lock.
		lockdep_assert_irqs_disabled();
	}

	for_each_possible_cpu(cpu)
		totqlen += rcu_get_n_cbs_cpu(cpu);
	pr_err("\t(detected by %d, t=%ld jiffies, g=%ld, q=%lu ncpus=%d)\n",
	       smp_processor_id(), (long)(jiffies - gps),
	       (long)rcu_seq_current(&rcu_state.gp_seq), totqlen,
	       data_race(rcu_state.n_online_cpus)); // Diagnostic read
	if (ndetected) {
		rcu_dump_cpu_stacks();

		/* Complain about tasks blocking the grace period. */
		rcu_for_each_leaf_node(rnp)
			rcu_print_detail_task_stall_rnp(rnp);
	} else {
		if (rcu_seq_current(&rcu_state.gp_seq) != gp_seq) {
			pr_err("INFO: Stall ended before state dump start\n");
		} else {
			j = jiffies;
			gpa = data_race(READ_ONCE(rcu_state.gp_activity));
			pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld, root ->qsmask %#lx\n",
			       rcu_state.name, j - gpa, j, gpa,
			       data_race(READ_ONCE(jiffies_till_next_fqs)),
			       data_race(READ_ONCE(rcu_get_root()->qsmask)));
		}
	}
	/* Rewrite if needed in case of slow consoles. */
	if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
		WRITE_ONCE(rcu_state.jiffies_stall,
			   jiffies + 3 * rcu_jiffies_till_stall_check() + 3);

	rcu_check_gp_kthread_expired_fqs_timer();
	rcu_check_gp_kthread_starvation();

	panic_on_rcu_stall();

	rcu_force_quiescent_state();  /* Kick them all. */
}

static void print_cpu_stall(unsigned long gps)
{
	int cpu;
	unsigned long flags;
	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
	struct rcu_node *rnp = rcu_get_root();
	long totqlen = 0;

	lockdep_assert_irqs_disabled();

	/* Kick and suppress, if so configured. */
	rcu_stall_kick_kthreads();
	if (rcu_stall_is_suppressed())
		return;

	/*
	 * OK, time to rat on ourselves...
	 * See Documentation/RCU/stallwarn.rst for info on how to debug
	 * RCU CPU stall warnings.
	 */
	trace_rcu_stall_warning(rcu_state.name, TPS("SelfDetected"));
	pr_err("INFO: %s self-detected stall on CPU\n", rcu_state.name);
	raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
	print_cpu_stall_info(smp_processor_id());
	raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
	for_each_possible_cpu(cpu)
		totqlen += rcu_get_n_cbs_cpu(cpu);
	pr_err("\t(t=%lu jiffies g=%ld q=%lu ncpus=%d)\n",
		jiffies - gps,
		(long)rcu_seq_current(&rcu_state.gp_seq), totqlen,
		data_race(rcu_state.n_online_cpus)); // Diagnostic read

	rcu_check_gp_kthread_expired_fqs_timer();
	rcu_check_gp_kthread_starvation();

	rcu_dump_cpu_stacks();

	raw_spin_lock_irqsave_rcu_node(rnp, flags);
	/* Rewrite if needed in case of slow consoles. */
	if (ULONG_CMP_GE(jiffies, READ_ONCE(rcu_state.jiffies_stall)))
		WRITE_ONCE(rcu_state.jiffies_stall,
			   jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

	panic_on_rcu_stall();

	/*
	 * Attempt to revive the RCU machinery by forcing a context switch.
	 *
	 * A context switch would normally allow the RCU state machine to make
	 * progress and it could be we're stuck in kernel space without context
	 * switches for an entirely unreasonable amount of time.
	 */
	set_tsk_need_resched(current);
	set_preempt_need_resched();
}

static void check_cpu_stall(struct rcu_data *rdp)
{
	bool self_detected;
	unsigned long gs1;
	unsigned long gs2;
	unsigned long gps;
	unsigned long j;
	unsigned long jn;
	unsigned long js;
	struct rcu_node *rnp;

	lockdep_assert_irqs_disabled();
	if ((rcu_stall_is_suppressed() && !READ_ONCE(rcu_kick_kthreads)) ||
	    !rcu_gp_in_progress())
		return;
	rcu_stall_kick_kthreads();

	/*
	 * Check if it was requested (via rcu_cpu_stall_reset()) that the FQS
	 * loop has to set jiffies to ensure a non-stale jiffies value. This
	 * is required to have good jiffies value after coming out of long
	 * breaks of jiffies updates. Not doing so can cause false positives.
	 */
	if (READ_ONCE(rcu_state.nr_fqs_jiffies_stall) > 0)
		return;

	j = jiffies;

	/*
	 * Lots of memory barriers to reject false positives.
	 *
	 * The idea is to pick up rcu_state.gp_seq, then
	 * rcu_state.jiffies_stall, then rcu_state.gp_start, and finally
	 * another copy of rcu_state.gp_seq.  These values are updated in
	 * the opposite order with memory barriers (or equivalent) during
	 * grace-period initialization and cleanup.  Now, a false positive
	 * can occur if we get an new value of rcu_state.gp_start and a old
	 * value of rcu_state.jiffies_stall.  But given the memory barriers,
	 * the only way that this can happen is if one grace period ends
	 * and another starts between these two fetches.  This is detected
	 * by comparing the second fetch of rcu_state.gp_seq with the
	 * previous fetch from rcu_state.gp_seq.
	 *
	 * Given this check, comparisons of jiffies, rcu_state.jiffies_stall,
	 * and rcu_state.gp_start suffice to forestall false positives.
	 */
	gs1 = READ_ONCE(rcu_state.gp_seq);
	smp_rmb(); /* Pick up ->gp_seq first... */
	js = READ_ONCE(rcu_state.jiffies_stall);
	smp_rmb(); /* ...then ->jiffies_stall before the rest... */
	gps = READ_ONCE(rcu_state.gp_start);
	smp_rmb(); /* ...and finally ->gp_start before ->gp_seq again. */
	gs2 = READ_ONCE(rcu_state.gp_seq);
	if (gs1 != gs2 ||
	    ULONG_CMP_LT(j, js) ||
	    ULONG_CMP_GE(gps, js))
		return; /* No stall or GP completed since entering function. */
	rnp = rdp->mynode;
	jn = jiffies + ULONG_MAX / 2;
	self_detected = READ_ONCE(rnp->qsmask) & rdp->grpmask;
	if (rcu_gp_in_progress() &&
	    (self_detected || ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) &&
	    cmpxchg(&rcu_state.jiffies_stall, js, jn) == js) {
		/*
		 * If a virtual machine is stopped by the host it can look to
		 * the watchdog like an RCU stall. Check to see if the host
		 * stopped the vm.
		 */
		if (kvm_check_and_clear_guest_paused())
			return;

		rcu_stall_notifier_call_chain(RCU_STALL_NOTIFY_NORM, (void *)j - gps);
		if (self_detected) {
			/* We haven't checked in, so go dump stack. */
			print_cpu_stall(gps);
		} else {
			/* They had a few time units to dump stack, so complain. */
			print_other_cpu_stall(gs2, gps);
		}

		if (READ_ONCE(rcu_cpu_stall_ftrace_dump))
			rcu_ftrace_dump(DUMP_ALL);

		if (READ_ONCE(rcu_state.jiffies_stall) == jn) {
			jn = jiffies + 3 * rcu_jiffies_till_stall_check() + 3;
			WRITE_ONCE(rcu_state.jiffies_stall, jn);
		}
	}
}

//////////////////////////////////////////////////////////////////////////////
//
// RCU forward-progress mechanisms, including for callback invocation.


/*
 * Check to see if a failure to end RCU priority inversion was due to
 * a CPU not passing through a quiescent state.  When this happens, there
 * is nothing that RCU priority boosting can do to help, so we shouldn't
 * count this as an RCU priority boosting failure.  A return of true says
 * RCU priority boosting is to blame, and false says otherwise.  If false
 * is returned, the first of the CPUs to blame is stored through cpup.
 * If there was no CPU blocking the current grace period, but also nothing
 * in need of being boosted, *cpup is set to -1.  This can happen in case
 * of vCPU preemption while the last CPU is reporting its quiscent state,
 * for example.
 *
 * If cpup is NULL, then a lockless quick check is carried out, suitable
 * for high-rate usage.  On the other hand, if cpup is non-NULL, each
 * rcu_node structure's ->lock is acquired, ruling out high-rate usage.
 */
bool rcu_check_boost_fail(unsigned long gp_state, int *cpup)
{
	bool atb = false;
	int cpu;
	unsigned long flags;
	struct rcu_node *rnp;

	rcu_for_each_leaf_node(rnp) {
		if (!cpup) {
			if (data_race(READ_ONCE(rnp->qsmask))) {
				return false;
			} else {
				if (READ_ONCE(rnp->gp_tasks))
					atb = true;
				continue;
			}
		}
		*cpup = -1;
		raw_spin_lock_irqsave_rcu_node(rnp, flags);
		if (rnp->gp_tasks)
			atb = true;
		if (!rnp->qsmask) {
			// No CPUs without quiescent states for this rnp.
			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
			continue;
		}
		// Find the first holdout CPU.
		for_each_leaf_node_possible_cpu(rnp, cpu) {
			if (rnp->qsmask & (1UL << (cpu - rnp->grplo))) {
				raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
				*cpup = cpu;
				return false;
			}
		}
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	}
	// Can't blame CPUs, so must blame RCU priority boosting.
	return atb;
}
EXPORT_SYMBOL_GPL(rcu_check_boost_fail);

/*
 * Show the state of the grace-period kthreads.
 */
void show_rcu_gp_kthreads(void)
{
	unsigned long cbs = 0;
	int cpu;
	unsigned long j;
	unsigned long ja;
	unsigned long jr;
	unsigned long js;
	unsigned long jw;
	struct rcu_data *rdp;
	struct rcu_node *rnp;
	struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);

	j = jiffies;
	ja = j - data_race(READ_ONCE(rcu_state.gp_activity));
	jr = j - data_race(READ_ONCE(rcu_state.gp_req_activity));
	js = j - data_race(READ_ONCE(rcu_state.gp_start));
	jw = j - data_race(READ_ONCE(rcu_state.gp_wake_time));
	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",
		rcu_state.name, gp_state_getname(rcu_state.gp_state),
		data_race(READ_ONCE(rcu_state.gp_state)),
		t ? data_race(READ_ONCE(t->__state)) : 0x1ffff, t ? t->rt_priority : 0xffU,
		js, ja, jr, jw, (long)data_race(READ_ONCE(rcu_state.gp_wake_seq)),
		(long)data_race(READ_ONCE(rcu_state.gp_seq)),
		(long)data_race(READ_ONCE(rcu_get_root()->gp_seq_needed)),
		data_race(READ_ONCE(rcu_state.gp_max)),
		data_race(READ_ONCE(rcu_state.gp_flags)));
	rcu_for_each_node_breadth_first(rnp) {
		if (ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq), READ_ONCE(rnp->gp_seq_needed)) &&
		    !data_race(READ_ONCE(rnp->qsmask)) && !data_race(READ_ONCE(rnp->boost_tasks)) &&
		    !data_race(READ_ONCE(rnp->exp_tasks)) && !data_race(READ_ONCE(rnp->gp_tasks)))
			continue;
		pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld ->qsmask %#lx %c%c%c%c ->n_boosts %ld\n",
			rnp->grplo, rnp->grphi,
			(long)data_race(READ_ONCE(rnp->gp_seq)),
			(long)data_race(READ_ONCE(rnp->gp_seq_needed)),
			data_race(READ_ONCE(rnp->qsmask)),
			".b"[!!data_race(READ_ONCE(rnp->boost_kthread_task))],
			".B"[!!data_race(READ_ONCE(rnp->boost_tasks))],
			".E"[!!data_race(READ_ONCE(rnp->exp_tasks))],
			".G"[!!data_race(READ_ONCE(rnp->gp_tasks))],
			data_race(READ_ONCE(rnp->n_boosts)));
		if (!rcu_is_leaf_node(rnp))
			continue;
		for_each_leaf_node_possible_cpu(rnp, cpu) {
			rdp = per_cpu_ptr(&rcu_data, cpu);
			if (READ_ONCE(rdp->gpwrap) ||
			    ULONG_CMP_GE(READ_ONCE(rcu_state.gp_seq),
					 READ_ONCE(rdp->gp_seq_needed)))
				continue;
			pr_info("\tcpu %d ->gp_seq_needed %ld\n",
				cpu, (long)data_race(READ_ONCE(rdp->gp_seq_needed)));
		}
	}
	for_each_possible_cpu(cpu) {
		rdp = per_cpu_ptr(&rcu_data, cpu);
		cbs += data_race(READ_ONCE(rdp->n_cbs_invoked));
		if (rcu_segcblist_is_offloaded(&rdp->cblist))
			show_rcu_nocb_state(rdp);
	}
	pr_info("RCU callbacks invoked since boot: %lu\n", cbs);
	show_rcu_tasks_gp_kthreads();
}
EXPORT_SYMBOL_GPL(show_rcu_gp_kthreads);

/*
 * This function checks for grace-period requests that fail to motivate
 * RCU to come out of its idle mode.
 */
static void rcu_check_gp_start_stall(struct rcu_node *rnp, struct rcu_data *rdp,
				     const unsigned long gpssdelay)
{
	unsigned long flags;
	unsigned long j;
	struct rcu_node *rnp_root = rcu_get_root();
	static atomic_t warned = ATOMIC_INIT(0);

	if (!IS_ENABLED(CONFIG_PROVE_RCU) || rcu_gp_in_progress() ||
	    ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
			 READ_ONCE(rnp_root->gp_seq_needed)) ||
	    !smp_load_acquire(&rcu_state.gp_kthread)) // Get stable kthread.
		return;
	j = jiffies; /* Expensive access, and in common case don't get here. */
	if (time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
	    time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
	    atomic_read(&warned))
		return;

	raw_spin_lock_irqsave_rcu_node(rnp, flags);
	j = jiffies;
	if (rcu_gp_in_progress() ||
	    ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
			 READ_ONCE(rnp_root->gp_seq_needed)) ||
	    time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
	    time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
	    atomic_read(&warned)) {
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return;
	}
	/* Hold onto the leaf lock to make others see warned==1. */

	if (rnp_root != rnp)
		raw_spin_lock_rcu_node(rnp_root); /* irqs already disabled. */
	j = jiffies;
	if (rcu_gp_in_progress() ||
	    ULONG_CMP_GE(READ_ONCE(rnp_root->gp_seq),
			 READ_ONCE(rnp_root->gp_seq_needed)) ||
	    time_before(j, READ_ONCE(rcu_state.gp_req_activity) + gpssdelay) ||
	    time_before(j, READ_ONCE(rcu_state.gp_activity) + gpssdelay) ||
	    atomic_xchg(&warned, 1)) {
		if (rnp_root != rnp)
			/* irqs remain disabled. */
			raw_spin_unlock_rcu_node(rnp_root);
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return;
	}
	WARN_ON(1);
	if (rnp_root != rnp)
		raw_spin_unlock_rcu_node(rnp_root);
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	show_rcu_gp_kthreads();
}

/*
 * Do a forward-progress check for rcutorture.  This is normally invoked
 * due to an OOM event.  The argument "j" gives the time period during
 * which rcutorture would like progress to have been made.
 */
void rcu_fwd_progress_check(unsigned long j)
{
	unsigned long cbs;
	int cpu;
	unsigned long max_cbs = 0;
	int max_cpu = -1;
	struct rcu_data *rdp;

	if (rcu_gp_in_progress()) {
		pr_info("%s: GP age %lu jiffies\n",
			__func__, jiffies - data_race(READ_ONCE(rcu_state.gp_start)));
		show_rcu_gp_kthreads();
	} else {
		pr_info("%s: Last GP end %lu jiffies ago\n",
			__func__, jiffies - data_race(READ_ONCE(rcu_state.gp_end)));
		preempt_disable();
		rdp = this_cpu_ptr(&rcu_data);
		rcu_check_gp_start_stall(rdp->mynode, rdp, j);
		preempt_enable();
	}
	for_each_possible_cpu(cpu) {
		cbs = rcu_get_n_cbs_cpu(cpu);
		if (!cbs)
			continue;
		if (max_cpu < 0)
			pr_info("%s: callbacks", __func__);
		pr_cont(" %d: %lu", cpu, cbs);
		if (cbs <= max_cbs)
			continue;
		max_cbs = cbs;
		max_cpu = cpu;
	}
	if (max_cpu >= 0)
		pr_cont("\n");
}
EXPORT_SYMBOL_GPL(rcu_fwd_progress_check);

/* Commandeer a sysrq key to dump RCU's tree. */
static bool sysrq_rcu;
module_param(sysrq_rcu, bool, 0444);

/* Dump grace-period-request information due to commandeered sysrq. */
static void sysrq_show_rcu(u8 key)
{
	show_rcu_gp_kthreads();
}

static const struct sysrq_key_op sysrq_rcudump_op = {
	.handler = sysrq_show_rcu,
	.help_msg = "show-rcu(y)",
	.action_msg = "Show RCU tree",
	.enable_mask = SYSRQ_ENABLE_DUMP,
};

static int __init rcu_sysrq_init(void)
{
	if (sysrq_rcu)
		return register_sysrq_key('y', &sysrq_rcudump_op);
	return 0;
}
early_initcall(rcu_sysrq_init);

#ifdef CONFIG_RCU_CPU_STALL_NOTIFIER

//////////////////////////////////////////////////////////////////////////////
//
// RCU CPU stall-warning notifiers

static ATOMIC_NOTIFIER_HEAD(rcu_cpu_stall_notifier_list);

/**
 * rcu_stall_chain_notifier_register - Add an RCU CPU stall notifier
 * @n: Entry to add.
 *
 * Adds an RCU CPU stall notifier to an atomic notifier chain.
 * The @action passed to a notifier will be @RCU_STALL_NOTIFY_NORM or
 * friends.  The @data will be the duration of the stalled grace period,
 * in jiffies, coerced to a void* pointer.
 *
 * Returns 0 on success, %-EEXIST on error.
 */
int rcu_stall_chain_notifier_register(struct notifier_block *n)
{
	int rcsn = rcu_cpu_stall_notifiers;

	WARN(1, "Adding %pS() to RCU stall notifier list (%s).\n", n->notifier_call,
	     rcsn ? "possibly suppressing RCU CPU stall warnings" : "failed, so all is well");
	if (rcsn)
		return atomic_notifier_chain_register(&rcu_cpu_stall_notifier_list, n);
	return -EEXIST;
}
EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_register);

/**
 * rcu_stall_chain_notifier_unregister - Remove an RCU CPU stall notifier
 * @n: Entry to add.
 *
 * Removes an RCU CPU stall notifier from an atomic notifier chain.
 *
 * Returns zero on success, %-ENOENT on failure.
 */
int rcu_stall_chain_notifier_unregister(struct notifier_block *n)
{
	return atomic_notifier_chain_unregister(&rcu_cpu_stall_notifier_list, n);
}
EXPORT_SYMBOL_GPL(rcu_stall_chain_notifier_unregister);

/*
 * rcu_stall_notifier_call_chain - Call functions in an RCU CPU stall notifier chain
 * @val: Value passed unmodified to notifier function
 * @v: Pointer passed unmodified to notifier function
 *
 * Calls each function in the RCU CPU stall notifier chain in turn, which
 * is an atomic call chain.  See atomic_notifier_call_chain() for more
 * information.
 *
 * This is for use within RCU, hence the omission of the extra asterisk
 * to indicate a non-kerneldoc format header comment.
 */
int rcu_stall_notifier_call_chain(unsigned long val, void *v)
{
	return atomic_notifier_call_chain(&rcu_cpu_stall_notifier_list, val, v);
}

#endif // #ifdef CONFIG_RCU_CPU_STALL_NOTIFIER