summaryrefslogtreecommitdiff
path: root/kernel/rcu/tree_exp.h
blob: 3b7abb58157df63b839e7d37752208db12a15a8b (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
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
/* SPDX-License-Identifier: GPL-2.0+ */
/*
 * RCU expedited grace periods
 *
 * Copyright IBM Corporation, 2016
 *
 * Authors: Paul E. McKenney <paulmck@linux.ibm.com>
 */

#include <linux/lockdep.h>

static void rcu_exp_handler(void *unused);
static int rcu_print_task_exp_stall(struct rcu_node *rnp);
static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp);

/*
 * Record the start of an expedited grace period.
 */
static void rcu_exp_gp_seq_start(void)
{
	rcu_seq_start(&rcu_state.expedited_sequence);
	rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_exp_snap);
}

/*
 * Return the value that the expedited-grace-period counter will have
 * at the end of the current grace period.
 */
static __maybe_unused unsigned long rcu_exp_gp_seq_endval(void)
{
	return rcu_seq_endval(&rcu_state.expedited_sequence);
}

/*
 * Record the end of an expedited grace period.
 */
static void rcu_exp_gp_seq_end(void)
{
	rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_exp_snap);
	rcu_seq_end(&rcu_state.expedited_sequence);
	smp_mb(); /* Ensure that consecutive grace periods serialize. */
}

/*
 * Take a snapshot of the expedited-grace-period counter, which is the
 * earliest value that will indicate that a full grace period has
 * elapsed since the current time.
 */
static unsigned long rcu_exp_gp_seq_snap(void)
{
	unsigned long s;

	smp_mb(); /* Caller's modifications seen first by other CPUs. */
	s = rcu_seq_snap(&rcu_state.expedited_sequence);
	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("snap"));
	return s;
}

/*
 * Given a counter snapshot from rcu_exp_gp_seq_snap(), return true
 * if a full expedited grace period has elapsed since that snapshot
 * was taken.
 */
static bool rcu_exp_gp_seq_done(unsigned long s)
{
	return rcu_seq_done(&rcu_state.expedited_sequence, s);
}

/*
 * Reset the ->expmaskinit values in the rcu_node tree to reflect any
 * recent CPU-online activity.  Note that these masks are not cleared
 * when CPUs go offline, so they reflect the union of all CPUs that have
 * ever been online.  This means that this function normally takes its
 * no-work-to-do fastpath.
 */
static void sync_exp_reset_tree_hotplug(void)
{
	bool done;
	unsigned long flags;
	unsigned long mask;
	unsigned long oldmask;
	int ncpus = smp_load_acquire(&rcu_state.ncpus); /* Order vs. locking. */
	struct rcu_node *rnp;
	struct rcu_node *rnp_up;

	/* If no new CPUs onlined since last time, nothing to do. */
	if (likely(ncpus == rcu_state.ncpus_snap))
		return;
	rcu_state.ncpus_snap = ncpus;

	/*
	 * Each pass through the following loop propagates newly onlined
	 * CPUs for the current rcu_node structure up the rcu_node tree.
	 */
	rcu_for_each_leaf_node(rnp) {
		raw_spin_lock_irqsave_rcu_node(rnp, flags);
		if (rnp->expmaskinit == rnp->expmaskinitnext) {
			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
			continue;  /* No new CPUs, nothing to do. */
		}

		/* Update this node's mask, track old value for propagation. */
		oldmask = rnp->expmaskinit;
		rnp->expmaskinit = rnp->expmaskinitnext;
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

		/* If was already nonzero, nothing to propagate. */
		if (oldmask)
			continue;

		/* Propagate the new CPU up the tree. */
		mask = rnp->grpmask;
		rnp_up = rnp->parent;
		done = false;
		while (rnp_up) {
			raw_spin_lock_irqsave_rcu_node(rnp_up, flags);
			if (rnp_up->expmaskinit)
				done = true;
			rnp_up->expmaskinit |= mask;
			raw_spin_unlock_irqrestore_rcu_node(rnp_up, flags);
			if (done)
				break;
			mask = rnp_up->grpmask;
			rnp_up = rnp_up->parent;
		}
	}
}

/*
 * Reset the ->expmask values in the rcu_node tree in preparation for
 * a new expedited grace period.
 */
static void __maybe_unused sync_exp_reset_tree(void)
{
	unsigned long flags;
	struct rcu_node *rnp;

	sync_exp_reset_tree_hotplug();
	rcu_for_each_node_breadth_first(rnp) {
		raw_spin_lock_irqsave_rcu_node(rnp, flags);
		WARN_ON_ONCE(rnp->expmask);
		WRITE_ONCE(rnp->expmask, rnp->expmaskinit);
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	}
}

/*
 * Return non-zero if there is no RCU expedited grace period in progress
 * for the specified rcu_node structure, in other words, if all CPUs and
 * tasks covered by the specified rcu_node structure have done their bit
 * for the current expedited grace period.
 */
static bool sync_rcu_exp_done(struct rcu_node *rnp)
{
	raw_lockdep_assert_held_rcu_node(rnp);
	return READ_ONCE(rnp->exp_tasks) == NULL &&
	       READ_ONCE(rnp->expmask) == 0;
}

/*
 * Like sync_rcu_exp_done(), but where the caller does not hold the
 * rcu_node's ->lock.
 */
static bool sync_rcu_exp_done_unlocked(struct rcu_node *rnp)
{
	unsigned long flags;
	bool ret;

	raw_spin_lock_irqsave_rcu_node(rnp, flags);
	ret = sync_rcu_exp_done(rnp);
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

	return ret;
}


/*
 * Report the exit from RCU read-side critical section for the last task
 * that queued itself during or before the current expedited preemptible-RCU
 * grace period.  This event is reported either to the rcu_node structure on
 * which the task was queued or to one of that rcu_node structure's ancestors,
 * recursively up the tree.  (Calm down, calm down, we do the recursion
 * iteratively!)
 */
static void __rcu_report_exp_rnp(struct rcu_node *rnp,
				 bool wake, unsigned long flags)
	__releases(rnp->lock)
{
	unsigned long mask;

	raw_lockdep_assert_held_rcu_node(rnp);
	for (;;) {
		if (!sync_rcu_exp_done(rnp)) {
			if (!rnp->expmask)
				rcu_initiate_boost(rnp, flags);
			else
				raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
			break;
		}
		if (rnp->parent == NULL) {
			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
			if (wake) {
				smp_mb(); /* EGP done before wake_up(). */
				swake_up_one(&rcu_state.expedited_wq);
			}
			break;
		}
		mask = rnp->grpmask;
		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled */
		rnp = rnp->parent;
		raw_spin_lock_rcu_node(rnp); /* irqs already disabled */
		WARN_ON_ONCE(!(rnp->expmask & mask));
		WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
	}
}

/*
 * Report expedited quiescent state for specified node.  This is a
 * lock-acquisition wrapper function for __rcu_report_exp_rnp().
 */
static void __maybe_unused rcu_report_exp_rnp(struct rcu_node *rnp, bool wake)
{
	unsigned long flags;

	raw_spin_lock_irqsave_rcu_node(rnp, flags);
	__rcu_report_exp_rnp(rnp, wake, flags);
}

/*
 * Report expedited quiescent state for multiple CPUs, all covered by the
 * specified leaf rcu_node structure.
 */
static void rcu_report_exp_cpu_mult(struct rcu_node *rnp,
				    unsigned long mask, bool wake)
{
	int cpu;
	unsigned long flags;
	struct rcu_data *rdp;

	raw_spin_lock_irqsave_rcu_node(rnp, flags);
	if (!(rnp->expmask & mask)) {
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return;
	}
	WRITE_ONCE(rnp->expmask, rnp->expmask & ~mask);
	for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
		rdp = per_cpu_ptr(&rcu_data, cpu);
		if (!IS_ENABLED(CONFIG_NO_HZ_FULL) || !rdp->rcu_forced_tick_exp)
			continue;
		rdp->rcu_forced_tick_exp = false;
		tick_dep_clear_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
	}
	__rcu_report_exp_rnp(rnp, wake, flags); /* Releases rnp->lock. */
}

/*
 * Report expedited quiescent state for specified rcu_data (CPU).
 */
static void rcu_report_exp_rdp(struct rcu_data *rdp)
{
	WRITE_ONCE(rdp->cpu_no_qs.b.exp, false);
	rcu_report_exp_cpu_mult(rdp->mynode, rdp->grpmask, true);
}

/* Common code for work-done checking. */
static bool sync_exp_work_done(unsigned long s)
{
	if (rcu_exp_gp_seq_done(s)) {
		trace_rcu_exp_grace_period(rcu_state.name, s, TPS("done"));
		smp_mb(); /* Ensure test happens before caller kfree(). */
		return true;
	}
	return false;
}

/*
 * Funnel-lock acquisition for expedited grace periods.  Returns true
 * if some other task completed an expedited grace period that this task
 * can piggy-back on, and with no mutex held.  Otherwise, returns false
 * with the mutex held, indicating that the caller must actually do the
 * expedited grace period.
 */
static bool exp_funnel_lock(unsigned long s)
{
	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
	struct rcu_node *rnp = rdp->mynode;
	struct rcu_node *rnp_root = rcu_get_root();

	/* Low-contention fastpath. */
	if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s) &&
	    (rnp == rnp_root ||
	     ULONG_CMP_LT(READ_ONCE(rnp_root->exp_seq_rq), s)) &&
	    mutex_trylock(&rcu_state.exp_mutex))
		goto fastpath;

	/*
	 * Each pass through the following loop works its way up
	 * the rcu_node tree, returning if others have done the work or
	 * otherwise falls through to acquire ->exp_mutex.  The mapping
	 * from CPU to rcu_node structure can be inexact, as it is just
	 * promoting locality and is not strictly needed for correctness.
	 */
	for (; rnp != NULL; rnp = rnp->parent) {
		if (sync_exp_work_done(s))
			return true;

		/* Work not done, either wait here or go up. */
		spin_lock(&rnp->exp_lock);
		if (ULONG_CMP_GE(rnp->exp_seq_rq, s)) {

			/* Someone else doing GP, so wait for them. */
			spin_unlock(&rnp->exp_lock);
			trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
						  rnp->grplo, rnp->grphi,
						  TPS("wait"));
			wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
				   sync_exp_work_done(s));
			return true;
		}
		WRITE_ONCE(rnp->exp_seq_rq, s); /* Followers can wait on us. */
		spin_unlock(&rnp->exp_lock);
		trace_rcu_exp_funnel_lock(rcu_state.name, rnp->level,
					  rnp->grplo, rnp->grphi, TPS("nxtlvl"));
	}
	mutex_lock(&rcu_state.exp_mutex);
fastpath:
	if (sync_exp_work_done(s)) {
		mutex_unlock(&rcu_state.exp_mutex);
		return true;
	}
	rcu_exp_gp_seq_start();
	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("start"));
	return false;
}

/*
 * Select the CPUs within the specified rcu_node that the upcoming
 * expedited grace period needs to wait for.
 */
static void __sync_rcu_exp_select_node_cpus(struct rcu_exp_work *rewp)
{
	int cpu;
	unsigned long flags;
	unsigned long mask_ofl_test;
	unsigned long mask_ofl_ipi;
	int ret;
	struct rcu_node *rnp = container_of(rewp, struct rcu_node, rew);

	raw_spin_lock_irqsave_rcu_node(rnp, flags);

	/* Each pass checks a CPU for identity, offline, and idle. */
	mask_ofl_test = 0;
	for_each_leaf_node_cpu_mask(rnp, cpu, rnp->expmask) {
		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
		unsigned long mask = rdp->grpmask;
		int snap;

		if (raw_smp_processor_id() == cpu ||
		    !(rnp->qsmaskinitnext & mask)) {
			mask_ofl_test |= mask;
		} else {
			snap = rcu_dynticks_snap(cpu);
			if (rcu_dynticks_in_eqs(snap))
				mask_ofl_test |= mask;
			else
				rdp->exp_dynticks_snap = snap;
		}
	}
	mask_ofl_ipi = rnp->expmask & ~mask_ofl_test;

	/*
	 * Need to wait for any blocked tasks as well.	Note that
	 * additional blocking tasks will also block the expedited GP
	 * until such time as the ->expmask bits are cleared.
	 */
	if (rcu_preempt_has_tasks(rnp))
		WRITE_ONCE(rnp->exp_tasks, rnp->blkd_tasks.next);
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);

	/* IPI the remaining CPUs for expedited quiescent state. */
	for_each_leaf_node_cpu_mask(rnp, cpu, mask_ofl_ipi) {
		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
		unsigned long mask = rdp->grpmask;

retry_ipi:
		if (rcu_dynticks_in_eqs_since(rdp, rdp->exp_dynticks_snap)) {
			mask_ofl_test |= mask;
			continue;
		}
		if (get_cpu() == cpu) {
			mask_ofl_test |= mask;
			put_cpu();
			continue;
		}
		ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
		put_cpu();
		/* The CPU will report the QS in response to the IPI. */
		if (!ret)
			continue;

		/* Failed, raced with CPU hotplug operation. */
		raw_spin_lock_irqsave_rcu_node(rnp, flags);
		if ((rnp->qsmaskinitnext & mask) &&
		    (rnp->expmask & mask)) {
			/* Online, so delay for a bit and try again. */
			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
			trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("selectofl"));
			schedule_timeout_idle(1);
			goto retry_ipi;
		}
		/* CPU really is offline, so we must report its QS. */
		if (rnp->expmask & mask)
			mask_ofl_test |= mask;
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	}
	/* Report quiescent states for those that went offline. */
	if (mask_ofl_test)
		rcu_report_exp_cpu_mult(rnp, mask_ofl_test, false);
}

static void rcu_exp_sel_wait_wake(unsigned long s);

#ifdef CONFIG_RCU_EXP_KTHREAD
static void sync_rcu_exp_select_node_cpus(struct kthread_work *wp)
{
	struct rcu_exp_work *rewp =
		container_of(wp, struct rcu_exp_work, rew_work);

	__sync_rcu_exp_select_node_cpus(rewp);
}

static inline bool rcu_gp_par_worker_started(void)
{
	return !!READ_ONCE(rcu_exp_par_gp_kworker);
}

static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
{
	kthread_init_work(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
	/*
	 * Use rcu_exp_par_gp_kworker, because flushing a work item from
	 * another work item on the same kthread worker can result in
	 * deadlock.
	 */
	kthread_queue_work(rcu_exp_par_gp_kworker, &rnp->rew.rew_work);
}

static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
{
	kthread_flush_work(&rnp->rew.rew_work);
}

/*
 * Work-queue handler to drive an expedited grace period forward.
 */
static void wait_rcu_exp_gp(struct kthread_work *wp)
{
	struct rcu_exp_work *rewp;

	rewp = container_of(wp, struct rcu_exp_work, rew_work);
	rcu_exp_sel_wait_wake(rewp->rew_s);
}

static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
{
	kthread_init_work(&rew->rew_work, wait_rcu_exp_gp);
	kthread_queue_work(rcu_exp_gp_kworker, &rew->rew_work);
}

static inline void synchronize_rcu_expedited_destroy_work(struct rcu_exp_work *rew)
{
}
#else /* !CONFIG_RCU_EXP_KTHREAD */
static void sync_rcu_exp_select_node_cpus(struct work_struct *wp)
{
	struct rcu_exp_work *rewp =
		container_of(wp, struct rcu_exp_work, rew_work);

	__sync_rcu_exp_select_node_cpus(rewp);
}

static inline bool rcu_gp_par_worker_started(void)
{
	return !!READ_ONCE(rcu_par_gp_wq);
}

static inline void sync_rcu_exp_select_cpus_queue_work(struct rcu_node *rnp)
{
	int cpu = find_next_bit(&rnp->ffmask, BITS_PER_LONG, -1);

	INIT_WORK(&rnp->rew.rew_work, sync_rcu_exp_select_node_cpus);
	/* If all offline, queue the work on an unbound CPU. */
	if (unlikely(cpu > rnp->grphi - rnp->grplo))
		cpu = WORK_CPU_UNBOUND;
	else
		cpu += rnp->grplo;
	queue_work_on(cpu, rcu_par_gp_wq, &rnp->rew.rew_work);
}

static inline void sync_rcu_exp_select_cpus_flush_work(struct rcu_node *rnp)
{
	flush_work(&rnp->rew.rew_work);
}

/*
 * Work-queue handler to drive an expedited grace period forward.
 */
static void wait_rcu_exp_gp(struct work_struct *wp)
{
	struct rcu_exp_work *rewp;

	rewp = container_of(wp, struct rcu_exp_work, rew_work);
	rcu_exp_sel_wait_wake(rewp->rew_s);
}

static inline void synchronize_rcu_expedited_queue_work(struct rcu_exp_work *rew)
{
	INIT_WORK_ONSTACK(&rew->rew_work, wait_rcu_exp_gp);
	queue_work(rcu_gp_wq, &rew->rew_work);
}

static inline void synchronize_rcu_expedited_destroy_work(struct rcu_exp_work *rew)
{
	destroy_work_on_stack(&rew->rew_work);
}
#endif /* CONFIG_RCU_EXP_KTHREAD */

/*
 * Select the nodes that the upcoming expedited grace period needs
 * to wait for.
 */
static void sync_rcu_exp_select_cpus(void)
{
	struct rcu_node *rnp;

	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("reset"));
	sync_exp_reset_tree();
	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("select"));

	/* Schedule work for each leaf rcu_node structure. */
	rcu_for_each_leaf_node(rnp) {
		rnp->exp_need_flush = false;
		if (!READ_ONCE(rnp->expmask))
			continue; /* Avoid early boot non-existent wq. */
		if (!rcu_gp_par_worker_started() ||
		    rcu_scheduler_active != RCU_SCHEDULER_RUNNING ||
		    rcu_is_last_leaf_node(rnp)) {
			/* No worker started yet or last leaf, do direct call. */
			sync_rcu_exp_select_node_cpus(&rnp->rew.rew_work);
			continue;
		}
		sync_rcu_exp_select_cpus_queue_work(rnp);
		rnp->exp_need_flush = true;
	}

	/* Wait for jobs (if any) to complete. */
	rcu_for_each_leaf_node(rnp)
		if (rnp->exp_need_flush)
			sync_rcu_exp_select_cpus_flush_work(rnp);
}

/*
 * Wait for the expedited grace period to elapse, within time limit.
 * If the time limit is exceeded without the grace period elapsing,
 * return false, otherwise return true.
 */
static bool synchronize_rcu_expedited_wait_once(long tlimit)
{
	int t;
	struct rcu_node *rnp_root = rcu_get_root();

	t = swait_event_timeout_exclusive(rcu_state.expedited_wq,
					  sync_rcu_exp_done_unlocked(rnp_root),
					  tlimit);
	// Workqueues should not be signaled.
	if (t > 0 || sync_rcu_exp_done_unlocked(rnp_root))
		return true;
	WARN_ON(t < 0);  /* workqueues should not be signaled. */
	return false;
}

/*
 * Wait for the expedited grace period to elapse, issuing any needed
 * RCU CPU stall warnings along the way.
 */
static void synchronize_rcu_expedited_wait(void)
{
	int cpu;
	unsigned long j;
	unsigned long jiffies_stall;
	unsigned long jiffies_start;
	unsigned long mask;
	int ndetected;
	struct rcu_data *rdp;
	struct rcu_node *rnp;
	struct rcu_node *rnp_root = rcu_get_root();
	unsigned long flags;

	trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
	jiffies_stall = rcu_exp_jiffies_till_stall_check();
	jiffies_start = jiffies;
	if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) {
		if (synchronize_rcu_expedited_wait_once(1))
			return;
		rcu_for_each_leaf_node(rnp) {
			raw_spin_lock_irqsave_rcu_node(rnp, flags);
			mask = READ_ONCE(rnp->expmask);
			for_each_leaf_node_cpu_mask(rnp, cpu, mask) {
				rdp = per_cpu_ptr(&rcu_data, cpu);
				if (rdp->rcu_forced_tick_exp)
					continue;
				rdp->rcu_forced_tick_exp = true;
				if (cpu_online(cpu))
					tick_dep_set_cpu(cpu, TICK_DEP_BIT_RCU_EXP);
			}
			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		}
		j = READ_ONCE(jiffies_till_first_fqs);
		if (synchronize_rcu_expedited_wait_once(j + HZ))
			return;
	}

	for (;;) {
		if (synchronize_rcu_expedited_wait_once(jiffies_stall))
			return;
		if (rcu_stall_is_suppressed())
			continue;
		trace_rcu_stall_warning(rcu_state.name, TPS("ExpeditedStall"));
		pr_err("INFO: %s detected expedited stalls on CPUs/tasks: {",
		       rcu_state.name);
		ndetected = 0;
		rcu_for_each_leaf_node(rnp) {
			ndetected += rcu_print_task_exp_stall(rnp);
			for_each_leaf_node_possible_cpu(rnp, cpu) {
				struct rcu_data *rdp;

				mask = leaf_node_cpu_bit(rnp, cpu);
				if (!(READ_ONCE(rnp->expmask) & mask))
					continue;
				ndetected++;
				rdp = per_cpu_ptr(&rcu_data, cpu);
				pr_cont(" %d-%c%c%c%c", cpu,
					"O."[!!cpu_online(cpu)],
					"o."[!!(rdp->grpmask & rnp->expmaskinit)],
					"N."[!!(rdp->grpmask & rnp->expmaskinitnext)],
					"D."[!!(rdp->cpu_no_qs.b.exp)]);
			}
		}
		pr_cont(" } %lu jiffies s: %lu root: %#lx/%c\n",
			jiffies - jiffies_start, rcu_state.expedited_sequence,
			data_race(rnp_root->expmask),
			".T"[!!data_race(rnp_root->exp_tasks)]);
		if (ndetected) {
			pr_err("blocking rcu_node structures (internal RCU debug):");
			rcu_for_each_node_breadth_first(rnp) {
				if (rnp == rnp_root)
					continue; /* printed unconditionally */
				if (sync_rcu_exp_done_unlocked(rnp))
					continue;
				pr_cont(" l=%u:%d-%d:%#lx/%c",
					rnp->level, rnp->grplo, rnp->grphi,
					data_race(rnp->expmask),
					".T"[!!data_race(rnp->exp_tasks)]);
			}
			pr_cont("\n");
		}
		rcu_for_each_leaf_node(rnp) {
			for_each_leaf_node_possible_cpu(rnp, cpu) {
				mask = leaf_node_cpu_bit(rnp, cpu);
				if (!(READ_ONCE(rnp->expmask) & mask))
					continue;
				preempt_disable(); // For smp_processor_id() in dump_cpu_task().
				dump_cpu_task(cpu);
				preempt_enable();
			}
			rcu_exp_print_detail_task_stall_rnp(rnp);
		}
		jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
		panic_on_rcu_stall();
	}
}

/*
 * Wait for the current expedited grace period to complete, and then
 * wake up everyone who piggybacked on the just-completed expedited
 * grace period.  Also update all the ->exp_seq_rq counters as needed
 * in order to avoid counter-wrap problems.
 */
static void rcu_exp_wait_wake(unsigned long s)
{
	struct rcu_node *rnp;

	synchronize_rcu_expedited_wait();

	// Switch over to wakeup mode, allowing the next GP to proceed.
	// End the previous grace period only after acquiring the mutex
	// to ensure that only one GP runs concurrently with wakeups.
	mutex_lock(&rcu_state.exp_wake_mutex);
	rcu_exp_gp_seq_end();
	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("end"));

	rcu_for_each_node_breadth_first(rnp) {
		if (ULONG_CMP_LT(READ_ONCE(rnp->exp_seq_rq), s)) {
			spin_lock(&rnp->exp_lock);
			/* Recheck, avoid hang in case someone just arrived. */
			if (ULONG_CMP_LT(rnp->exp_seq_rq, s))
				WRITE_ONCE(rnp->exp_seq_rq, s);
			spin_unlock(&rnp->exp_lock);
		}
		smp_mb(); /* All above changes before wakeup. */
		wake_up_all(&rnp->exp_wq[rcu_seq_ctr(s) & 0x3]);
	}
	trace_rcu_exp_grace_period(rcu_state.name, s, TPS("endwake"));
	mutex_unlock(&rcu_state.exp_wake_mutex);
}

/*
 * Common code to drive an expedited grace period forward, used by
 * workqueues and mid-boot-time tasks.
 */
static void rcu_exp_sel_wait_wake(unsigned long s)
{
	/* Initialize the rcu_node tree in preparation for the wait. */
	sync_rcu_exp_select_cpus();

	/* Wait and clean up, including waking everyone. */
	rcu_exp_wait_wake(s);
}

#ifdef CONFIG_PREEMPT_RCU

/*
 * Remote handler for smp_call_function_single().  If there is an
 * RCU read-side critical section in effect, request that the
 * next rcu_read_unlock() record the quiescent state up the
 * ->expmask fields in the rcu_node tree.  Otherwise, immediately
 * report the quiescent state.
 */
static void rcu_exp_handler(void *unused)
{
	int depth = rcu_preempt_depth();
	unsigned long flags;
	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
	struct rcu_node *rnp = rdp->mynode;
	struct task_struct *t = current;

	/*
	 * First, the common case of not being in an RCU read-side
	 * critical section.  If also enabled or idle, immediately
	 * report the quiescent state, otherwise defer.
	 */
	if (!depth) {
		if (!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)) ||
		    rcu_is_cpu_rrupt_from_idle()) {
			rcu_report_exp_rdp(rdp);
		} else {
			WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
			set_tsk_need_resched(t);
			set_preempt_need_resched();
		}
		return;
	}

	/*
	 * Second, the less-common case of being in an RCU read-side
	 * critical section.  In this case we can count on a future
	 * rcu_read_unlock().  However, this rcu_read_unlock() might
	 * execute on some other CPU, but in that case there will be
	 * a future context switch.  Either way, if the expedited
	 * grace period is still waiting on this CPU, set ->deferred_qs
	 * so that the eventual quiescent state will be reported.
	 * Note that there is a large group of race conditions that
	 * can have caused this quiescent state to already have been
	 * reported, so we really do need to check ->expmask.
	 */
	if (depth > 0) {
		raw_spin_lock_irqsave_rcu_node(rnp, flags);
		if (rnp->expmask & rdp->grpmask) {
			WRITE_ONCE(rdp->cpu_no_qs.b.exp, true);
			t->rcu_read_unlock_special.b.exp_hint = true;
		}
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return;
	}

	// Finally, negative nesting depth should not happen.
	WARN_ON_ONCE(1);
}

/* PREEMPTION=y, so no PREEMPTION=n expedited grace period to clean up after. */
static void sync_sched_exp_online_cleanup(int cpu)
{
}

/*
 * Scan the current list of tasks blocked within RCU read-side critical
 * sections, printing out the tid of each that is blocking the current
 * expedited grace period.
 */
static int rcu_print_task_exp_stall(struct rcu_node *rnp)
{
	unsigned long flags;
	int ndetected = 0;
	struct task_struct *t;

	raw_spin_lock_irqsave_rcu_node(rnp, flags);
	if (!rnp->exp_tasks) {
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return 0;
	}
	t = list_entry(rnp->exp_tasks->prev,
		       struct task_struct, rcu_node_entry);
	list_for_each_entry_continue(t, &rnp->blkd_tasks, rcu_node_entry) {
		pr_cont(" P%d", t->pid);
		ndetected++;
	}
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	return ndetected;
}

/*
 * Scan the current list of tasks blocked within RCU read-side critical
 * sections, dumping the stack of each that is blocking the current
 * expedited grace period.
 */
static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
{
	unsigned long flags;
	struct task_struct *t;

	if (!rcu_exp_stall_task_details)
		return;
	raw_spin_lock_irqsave_rcu_node(rnp, flags);
	if (!READ_ONCE(rnp->exp_tasks)) {
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return;
	}
	t = list_entry(rnp->exp_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);
}

#else /* #ifdef CONFIG_PREEMPT_RCU */

/* Request an expedited quiescent state. */
static void rcu_exp_need_qs(void)
{
	__this_cpu_write(rcu_data.cpu_no_qs.b.exp, true);
	/* Store .exp before .rcu_urgent_qs. */
	smp_store_release(this_cpu_ptr(&rcu_data.rcu_urgent_qs), true);
	set_tsk_need_resched(current);
	set_preempt_need_resched();
}

/* Invoked on each online non-idle CPU for expedited quiescent state. */
static void rcu_exp_handler(void *unused)
{
	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
	struct rcu_node *rnp = rdp->mynode;
	bool preempt_bh_enabled = !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));

	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
	    __this_cpu_read(rcu_data.cpu_no_qs.b.exp))
		return;
	if (rcu_is_cpu_rrupt_from_idle() ||
	    (IS_ENABLED(CONFIG_PREEMPT_COUNT) && preempt_bh_enabled)) {
		rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
		return;
	}
	rcu_exp_need_qs();
}

/* Send IPI for expedited cleanup if needed at end of CPU-hotplug operation. */
static void sync_sched_exp_online_cleanup(int cpu)
{
	unsigned long flags;
	int my_cpu;
	struct rcu_data *rdp;
	int ret;
	struct rcu_node *rnp;

	rdp = per_cpu_ptr(&rcu_data, cpu);
	rnp = rdp->mynode;
	my_cpu = get_cpu();
	/* Quiescent state either not needed or already requested, leave. */
	if (!(READ_ONCE(rnp->expmask) & rdp->grpmask) ||
	    READ_ONCE(rdp->cpu_no_qs.b.exp)) {
		put_cpu();
		return;
	}
	/* Quiescent state needed on current CPU, so set it up locally. */
	if (my_cpu == cpu) {
		local_irq_save(flags);
		rcu_exp_need_qs();
		local_irq_restore(flags);
		put_cpu();
		return;
	}
	/* Quiescent state needed on some other CPU, send IPI. */
	ret = smp_call_function_single(cpu, rcu_exp_handler, NULL, 0);
	put_cpu();
	WARN_ON_ONCE(ret);
}

/*
 * Because preemptible RCU does not exist, we never have to check for
 * tasks blocked within RCU read-side critical sections that are
 * blocking the current expedited grace period.
 */
static int rcu_print_task_exp_stall(struct rcu_node *rnp)
{
	return 0;
}

/*
 * Because preemptible RCU does not exist, we never have to print out
 * tasks blocked within RCU read-side critical sections that are blocking
 * the current expedited grace period.
 */
static void rcu_exp_print_detail_task_stall_rnp(struct rcu_node *rnp)
{
}

#endif /* #else #ifdef CONFIG_PREEMPT_RCU */

/**
 * synchronize_rcu_expedited - Brute-force RCU grace period
 *
 * Wait for an RCU grace period, but expedite it.  The basic idea is to
 * IPI all non-idle non-nohz online CPUs.  The IPI handler checks whether
 * the CPU is in an RCU critical section, and if so, it sets a flag that
 * causes the outermost rcu_read_unlock() to report the quiescent state
 * for RCU-preempt or asks the scheduler for help for RCU-sched.  On the
 * other hand, if the CPU is not in an RCU read-side critical section,
 * the IPI handler reports the quiescent state immediately.
 *
 * Although this is a great improvement over previous expedited
 * implementations, it is still unfriendly to real-time workloads, so is
 * thus not recommended for any sort of common-case code.  In fact, if
 * you are using synchronize_rcu_expedited() in a loop, please restructure
 * your code to batch your updates, and then use a single synchronize_rcu()
 * instead.
 *
 * This has the same semantics as (but is more brutal than) synchronize_rcu().
 */
void synchronize_rcu_expedited(void)
{
	bool boottime = (rcu_scheduler_active == RCU_SCHEDULER_INIT);
	unsigned long flags;
	struct rcu_exp_work rew;
	struct rcu_node *rnp;
	unsigned long s;

	RCU_LOCKDEP_WARN(lock_is_held(&rcu_bh_lock_map) ||
			 lock_is_held(&rcu_lock_map) ||
			 lock_is_held(&rcu_sched_lock_map),
			 "Illegal synchronize_rcu_expedited() in RCU read-side critical section");

	/* Is the state is such that the call is a grace period? */
	if (rcu_blocking_is_gp()) {
		// Note well that this code runs with !PREEMPT && !SMP.
		// In addition, all code that advances grace periods runs
		// at process level.  Therefore, this expedited GP overlaps
		// with other expedited GPs only by being fully nested within
		// them, which allows reuse of ->gp_seq_polled_exp_snap.
		rcu_poll_gp_seq_start_unlocked(&rcu_state.gp_seq_polled_exp_snap);
		rcu_poll_gp_seq_end_unlocked(&rcu_state.gp_seq_polled_exp_snap);

		local_irq_save(flags);
		WARN_ON_ONCE(num_online_cpus() > 1);
		rcu_state.expedited_sequence += (1 << RCU_SEQ_CTR_SHIFT);
		local_irq_restore(flags);
		return;  // Context allows vacuous grace periods.
	}

	/* If expedited grace periods are prohibited, fall back to normal. */
	if (rcu_gp_is_normal()) {
		wait_rcu_gp(call_rcu_hurry);
		return;
	}

	/* Take a snapshot of the sequence number.  */
	s = rcu_exp_gp_seq_snap();
	if (exp_funnel_lock(s))
		return;  /* Someone else did our work for us. */

	/* Ensure that load happens before action based on it. */
	if (unlikely(boottime)) {
		/* Direct call during scheduler init and early_initcalls(). */
		rcu_exp_sel_wait_wake(s);
	} else {
		/* Marshall arguments & schedule the expedited grace period. */
		rew.rew_s = s;
		synchronize_rcu_expedited_queue_work(&rew);
	}

	/* Wait for expedited grace period to complete. */
	rnp = rcu_get_root();
	wait_event(rnp->exp_wq[rcu_seq_ctr(s) & 0x3],
		   sync_exp_work_done(s));
	smp_mb(); /* Work actions happen before return. */

	/* Let the next expedited grace period start. */
	mutex_unlock(&rcu_state.exp_mutex);

	if (likely(!boottime))
		synchronize_rcu_expedited_destroy_work(&rew);
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

/*
 * Ensure that start_poll_synchronize_rcu_expedited() has the expedited
 * RCU grace periods that it needs.
 */
static void sync_rcu_do_polled_gp(struct work_struct *wp)
{
	unsigned long flags;
	int i = 0;
	struct rcu_node *rnp = container_of(wp, struct rcu_node, exp_poll_wq);
	unsigned long s;

	raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
	s = rnp->exp_seq_poll_rq;
	rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
	raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
	if (s == RCU_GET_STATE_COMPLETED)
		return;
	while (!poll_state_synchronize_rcu(s)) {
		synchronize_rcu_expedited();
		if (i == 10 || i == 20)
			pr_info("%s: i = %d s = %lx gp_seq_polled = %lx\n", __func__, i, s, READ_ONCE(rcu_state.gp_seq_polled));
		i++;
	}
	raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
	s = rnp->exp_seq_poll_rq;
	if (poll_state_synchronize_rcu(s))
		rnp->exp_seq_poll_rq = RCU_GET_STATE_COMPLETED;
	raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);
}

/**
 * start_poll_synchronize_rcu_expedited - Snapshot current RCU state and start expedited grace period
 *
 * Returns a cookie to pass to a call to cond_synchronize_rcu(),
 * cond_synchronize_rcu_expedited(), or poll_state_synchronize_rcu(),
 * allowing them to determine whether or not any sort of grace period has
 * elapsed in the meantime.  If the needed expedited grace period is not
 * already slated to start, initiates that grace period.
 */
unsigned long start_poll_synchronize_rcu_expedited(void)
{
	unsigned long flags;
	struct rcu_data *rdp;
	struct rcu_node *rnp;
	unsigned long s;

	s = get_state_synchronize_rcu();
	rdp = per_cpu_ptr(&rcu_data, raw_smp_processor_id());
	rnp = rdp->mynode;
	if (rcu_init_invoked())
		raw_spin_lock_irqsave(&rnp->exp_poll_lock, flags);
	if (!poll_state_synchronize_rcu(s)) {
		if (rcu_init_invoked()) {
			rnp->exp_seq_poll_rq = s;
			queue_work(rcu_gp_wq, &rnp->exp_poll_wq);
		}
	}
	if (rcu_init_invoked())
		raw_spin_unlock_irqrestore(&rnp->exp_poll_lock, flags);

	return s;
}
EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited);

/**
 * start_poll_synchronize_rcu_expedited_full - Take a full snapshot and start expedited grace period
 * @rgosp: Place to put snapshot of grace-period state
 *
 * Places the normal and expedited grace-period states in rgosp.  This
 * state value can be passed to a later call to cond_synchronize_rcu_full()
 * or poll_state_synchronize_rcu_full() to determine whether or not a
 * grace period (whether normal or expedited) has elapsed in the meantime.
 * If the needed expedited grace period is not already slated to start,
 * initiates that grace period.
 */
void start_poll_synchronize_rcu_expedited_full(struct rcu_gp_oldstate *rgosp)
{
	get_state_synchronize_rcu_full(rgosp);
	(void)start_poll_synchronize_rcu_expedited();
}
EXPORT_SYMBOL_GPL(start_poll_synchronize_rcu_expedited_full);

/**
 * cond_synchronize_rcu_expedited - Conditionally wait for an expedited RCU grace period
 *
 * @oldstate: value from get_state_synchronize_rcu(), start_poll_synchronize_rcu(), or start_poll_synchronize_rcu_expedited()
 *
 * If any type of full RCU grace period has elapsed since the earlier
 * call to get_state_synchronize_rcu(), start_poll_synchronize_rcu(),
 * or start_poll_synchronize_rcu_expedited(), just return.  Otherwise,
 * invoke synchronize_rcu_expedited() to wait for a full grace period.
 *
 * Yes, this function does not take counter wrap into account.
 * But counter wrap is harmless.  If the counter wraps, we have waited for
 * more than 2 billion grace periods (and way more on a 64-bit system!),
 * so waiting for a couple of additional grace periods should be just fine.
 *
 * This function provides the same memory-ordering guarantees that
 * would be provided by a synchronize_rcu() that was invoked at the call
 * to the function that provided @oldstate and that returned at the end
 * of this function.
 */
void cond_synchronize_rcu_expedited(unsigned long oldstate)
{
	if (!poll_state_synchronize_rcu(oldstate))
		synchronize_rcu_expedited();
}
EXPORT_SYMBOL_GPL(cond_synchronize_rcu_expedited);

/**
 * cond_synchronize_rcu_expedited_full - Conditionally wait for an expedited RCU grace period
 * @rgosp: value from get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(), or start_poll_synchronize_rcu_expedited_full()
 *
 * If a full RCU grace period has elapsed since the call to
 * get_state_synchronize_rcu_full(), start_poll_synchronize_rcu_full(),
 * or start_poll_synchronize_rcu_expedited_full() from which @rgosp was
 * obtained, just return.  Otherwise, invoke synchronize_rcu_expedited()
 * to wait for a full grace period.
 *
 * Yes, this function does not take counter wrap into account.
 * But counter wrap is harmless.  If the counter wraps, we have waited for
 * more than 2 billion grace periods (and way more on a 64-bit system!),
 * so waiting for a couple of additional grace periods should be just fine.
 *
 * This function provides the same memory-ordering guarantees that
 * would be provided by a synchronize_rcu() that was invoked at the call
 * to the function that provided @rgosp and that returned at the end of
 * this function.
 */
void cond_synchronize_rcu_expedited_full(struct rcu_gp_oldstate *rgosp)
{
	if (!poll_state_synchronize_rcu_full(rgosp))
		synchronize_rcu_expedited();
}
EXPORT_SYMBOL_GPL(cond_synchronize_rcu_expedited_full);