summaryrefslogtreecommitdiffstats
path: root/kernel/rcu/tree_nocb.h
blob: 9e1c8caec5ceb983a0422954c0383a191643b3c2 (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
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
1750
1751
1752
1753
1754
1755
1756
1757
1758
/* SPDX-License-Identifier: GPL-2.0+ */
/*
 * Read-Copy Update mechanism for mutual exclusion (tree-based version)
 * Internal non-public definitions that provide either classic
 * or preemptible semantics.
 *
 * Copyright Red Hat, 2009
 * Copyright IBM Corporation, 2009
 * Copyright SUSE, 2021
 *
 * Author: Ingo Molnar <mingo@elte.hu>
 *	   Paul E. McKenney <paulmck@linux.ibm.com>
 *	   Frederic Weisbecker <frederic@kernel.org>
 */

#ifdef CONFIG_RCU_NOCB_CPU
static cpumask_var_t rcu_nocb_mask; /* CPUs to have callbacks offloaded. */
static bool __read_mostly rcu_nocb_poll;    /* Offload kthread are to poll. */
static inline int rcu_lockdep_is_held_nocb(struct rcu_data *rdp)
{
	return lockdep_is_held(&rdp->nocb_lock);
}

static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
{
	/* Race on early boot between thread creation and assignment */
	if (!rdp->nocb_cb_kthread || !rdp->nocb_gp_kthread)
		return true;

	if (current == rdp->nocb_cb_kthread || current == rdp->nocb_gp_kthread)
		if (in_task())
			return true;
	return false;
}

/*
 * Offload callback processing from the boot-time-specified set of CPUs
 * specified by rcu_nocb_mask.  For the CPUs in the set, there are kthreads
 * created that pull the callbacks from the corresponding CPU, wait for
 * a grace period to elapse, and invoke the callbacks.  These kthreads
 * are organized into GP kthreads, which manage incoming callbacks, wait for
 * grace periods, and awaken CB kthreads, and the CB kthreads, which only
 * invoke callbacks.  Each GP kthread invokes its own CBs.  The no-CBs CPUs
 * do a wake_up() on their GP kthread when they insert a callback into any
 * empty list, unless the rcu_nocb_poll boot parameter has been specified,
 * in which case each kthread actively polls its CPU.  (Which isn't so great
 * for energy efficiency, but which does reduce RCU's overhead on that CPU.)
 *
 * This is intended to be used in conjunction with Frederic Weisbecker's
 * adaptive-idle work, which would seriously reduce OS jitter on CPUs
 * running CPU-bound user-mode computations.
 *
 * Offloading of callbacks can also be used as an energy-efficiency
 * measure because CPUs with no RCU callbacks queued are more aggressive
 * about entering dyntick-idle mode.
 */


/*
 * Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters.
 * If the list is invalid, a warning is emitted and all CPUs are offloaded.
 */
static int __init rcu_nocb_setup(char *str)
{
	alloc_bootmem_cpumask_var(&rcu_nocb_mask);
	if (*str == '=') {
		if (cpulist_parse(++str, rcu_nocb_mask)) {
			pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
			cpumask_setall(rcu_nocb_mask);
		}
	}
	rcu_state.nocb_is_setup = true;
	return 1;
}
__setup("rcu_nocbs", rcu_nocb_setup);

static int __init parse_rcu_nocb_poll(char *arg)
{
	rcu_nocb_poll = true;
	return 0;
}
early_param("rcu_nocb_poll", parse_rcu_nocb_poll);

/*
 * Don't bother bypassing ->cblist if the call_rcu() rate is low.
 * After all, the main point of bypassing is to avoid lock contention
 * on ->nocb_lock, which only can happen at high call_rcu() rates.
 */
static int nocb_nobypass_lim_per_jiffy = 16 * 1000 / HZ;
module_param(nocb_nobypass_lim_per_jiffy, int, 0);

/*
 * Acquire the specified rcu_data structure's ->nocb_bypass_lock.  If the
 * lock isn't immediately available, increment ->nocb_lock_contended to
 * flag the contention.
 */
static void rcu_nocb_bypass_lock(struct rcu_data *rdp)
	__acquires(&rdp->nocb_bypass_lock)
{
	lockdep_assert_irqs_disabled();
	if (raw_spin_trylock(&rdp->nocb_bypass_lock))
		return;
	atomic_inc(&rdp->nocb_lock_contended);
	WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
	smp_mb__after_atomic(); /* atomic_inc() before lock. */
	raw_spin_lock(&rdp->nocb_bypass_lock);
	smp_mb__before_atomic(); /* atomic_dec() after lock. */
	atomic_dec(&rdp->nocb_lock_contended);
}

/*
 * Spinwait until the specified rcu_data structure's ->nocb_lock is
 * not contended.  Please note that this is extremely special-purpose,
 * relying on the fact that at most two kthreads and one CPU contend for
 * this lock, and also that the two kthreads are guaranteed to have frequent
 * grace-period-duration time intervals between successive acquisitions
 * of the lock.  This allows us to use an extremely simple throttling
 * mechanism, and further to apply it only to the CPU doing floods of
 * call_rcu() invocations.  Don't try this at home!
 */
static void rcu_nocb_wait_contended(struct rcu_data *rdp)
{
	WARN_ON_ONCE(smp_processor_id() != rdp->cpu);
	while (WARN_ON_ONCE(atomic_read(&rdp->nocb_lock_contended)))
		cpu_relax();
}

/*
 * Conditionally acquire the specified rcu_data structure's
 * ->nocb_bypass_lock.
 */
static bool rcu_nocb_bypass_trylock(struct rcu_data *rdp)
{
	lockdep_assert_irqs_disabled();
	return raw_spin_trylock(&rdp->nocb_bypass_lock);
}

/*
 * Release the specified rcu_data structure's ->nocb_bypass_lock.
 */
static void rcu_nocb_bypass_unlock(struct rcu_data *rdp)
	__releases(&rdp->nocb_bypass_lock)
{
	lockdep_assert_irqs_disabled();
	raw_spin_unlock(&rdp->nocb_bypass_lock);
}

/*
 * Acquire the specified rcu_data structure's ->nocb_lock, but only
 * if it corresponds to a no-CBs CPU.
 */
static void rcu_nocb_lock(struct rcu_data *rdp)
{
	lockdep_assert_irqs_disabled();
	if (!rcu_rdp_is_offloaded(rdp))
		return;
	raw_spin_lock(&rdp->nocb_lock);
}

/*
 * Release the specified rcu_data structure's ->nocb_lock, but only
 * if it corresponds to a no-CBs CPU.
 */
static void rcu_nocb_unlock(struct rcu_data *rdp)
{
	if (rcu_rdp_is_offloaded(rdp)) {
		lockdep_assert_irqs_disabled();
		raw_spin_unlock(&rdp->nocb_lock);
	}
}

/*
 * Release the specified rcu_data structure's ->nocb_lock and restore
 * interrupts, but only if it corresponds to a no-CBs CPU.
 */
static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
				       unsigned long flags)
{
	if (rcu_rdp_is_offloaded(rdp)) {
		lockdep_assert_irqs_disabled();
		raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
	} else {
		local_irq_restore(flags);
	}
}

/* Lockdep check that ->cblist may be safely accessed. */
static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
{
	lockdep_assert_irqs_disabled();
	if (rcu_rdp_is_offloaded(rdp))
		lockdep_assert_held(&rdp->nocb_lock);
}

/*
 * Wake up any no-CBs CPUs' kthreads that were waiting on the just-ended
 * grace period.
 */
static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
{
	swake_up_all(sq);
}

static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
{
	return &rnp->nocb_gp_wq[rcu_seq_ctr(rnp->gp_seq) & 0x1];
}

static void rcu_init_one_nocb(struct rcu_node *rnp)
{
	init_swait_queue_head(&rnp->nocb_gp_wq[0]);
	init_swait_queue_head(&rnp->nocb_gp_wq[1]);
}

static bool __wake_nocb_gp(struct rcu_data *rdp_gp,
			   struct rcu_data *rdp,
			   bool force, unsigned long flags)
	__releases(rdp_gp->nocb_gp_lock)
{
	bool needwake = false;

	if (!READ_ONCE(rdp_gp->nocb_gp_kthread)) {
		raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
				    TPS("AlreadyAwake"));
		return false;
	}

	if (rdp_gp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) {
		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
		del_timer(&rdp_gp->nocb_timer);
	}

	if (force || READ_ONCE(rdp_gp->nocb_gp_sleep)) {
		WRITE_ONCE(rdp_gp->nocb_gp_sleep, false);
		needwake = true;
	}
	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
	if (needwake) {
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DoWake"));
		wake_up_process(rdp_gp->nocb_gp_kthread);
	}

	return needwake;
}

/*
 * Kick the GP kthread for this NOCB group.
 */
static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
{
	unsigned long flags;
	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;

	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
	return __wake_nocb_gp(rdp_gp, rdp, force, flags);
}

/*
 * LAZY_FLUSH_JIFFIES decides the maximum amount of time that
 * can elapse before lazy callbacks are flushed. Lazy callbacks
 * could be flushed much earlier for a number of other reasons
 * however, LAZY_FLUSH_JIFFIES will ensure no lazy callbacks are
 * left unsubmitted to RCU after those many jiffies.
 */
#define LAZY_FLUSH_JIFFIES (10 * HZ)
static unsigned long jiffies_till_flush = LAZY_FLUSH_JIFFIES;

#ifdef CONFIG_RCU_LAZY
// To be called only from test code.
void rcu_lazy_set_jiffies_till_flush(unsigned long jif)
{
	jiffies_till_flush = jif;
}
EXPORT_SYMBOL(rcu_lazy_set_jiffies_till_flush);

unsigned long rcu_lazy_get_jiffies_till_flush(void)
{
	return jiffies_till_flush;
}
EXPORT_SYMBOL(rcu_lazy_get_jiffies_till_flush);
#endif

/*
 * Arrange to wake the GP kthread for this NOCB group at some future
 * time when it is safe to do so.
 */
static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
			       const char *reason)
{
	unsigned long flags;
	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;

	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);

	/*
	 * Bypass wakeup overrides previous deferments. In case of
	 * callback storms, no need to wake up too early.
	 */
	if (waketype == RCU_NOCB_WAKE_LAZY &&
	    rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT) {
		mod_timer(&rdp_gp->nocb_timer, jiffies + jiffies_till_flush);
		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
	} else if (waketype == RCU_NOCB_WAKE_BYPASS) {
		mod_timer(&rdp_gp->nocb_timer, jiffies + 2);
		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
	} else {
		if (rdp_gp->nocb_defer_wakeup < RCU_NOCB_WAKE)
			mod_timer(&rdp_gp->nocb_timer, jiffies + 1);
		if (rdp_gp->nocb_defer_wakeup < waketype)
			WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
	}

	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);

	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, reason);
}

/*
 * Flush the ->nocb_bypass queue into ->cblist, enqueuing rhp if non-NULL.
 * However, if there is a callback to be enqueued and if ->nocb_bypass
 * proves to be initially empty, just return false because the no-CB GP
 * kthread may need to be awakened in this case.
 *
 * Return true if there was something to be flushed and it succeeded, otherwise
 * false.
 *
 * Note that this function always returns true if rhp is NULL.
 */
static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp_in,
				     unsigned long j, bool lazy)
{
	struct rcu_cblist rcl;
	struct rcu_head *rhp = rhp_in;

	WARN_ON_ONCE(!rcu_rdp_is_offloaded(rdp));
	rcu_lockdep_assert_cblist_protected(rdp);
	lockdep_assert_held(&rdp->nocb_bypass_lock);
	if (rhp && !rcu_cblist_n_cbs(&rdp->nocb_bypass)) {
		raw_spin_unlock(&rdp->nocb_bypass_lock);
		return false;
	}
	/* Note: ->cblist.len already accounts for ->nocb_bypass contents. */
	if (rhp)
		rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */

	/*
	 * If the new CB requested was a lazy one, queue it onto the main
	 * ->cblist so that we can take advantage of the grace-period that will
	 * happen regardless. But queue it onto the bypass list first so that
	 * the lazy CB is ordered with the existing CBs in the bypass list.
	 */
	if (lazy && rhp) {
		rcu_cblist_enqueue(&rdp->nocb_bypass, rhp);
		rhp = NULL;
	}
	rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp);
	WRITE_ONCE(rdp->lazy_len, 0);

	rcu_segcblist_insert_pend_cbs(&rdp->cblist, &rcl);
	WRITE_ONCE(rdp->nocb_bypass_first, j);
	rcu_nocb_bypass_unlock(rdp);
	return true;
}

/*
 * Flush the ->nocb_bypass queue into ->cblist, enqueuing rhp if non-NULL.
 * However, if there is a callback to be enqueued and if ->nocb_bypass
 * proves to be initially empty, just return false because the no-CB GP
 * kthread may need to be awakened in this case.
 *
 * Note that this function always returns true if rhp is NULL.
 */
static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
				  unsigned long j, bool lazy)
{
	if (!rcu_rdp_is_offloaded(rdp))
		return true;
	rcu_lockdep_assert_cblist_protected(rdp);
	rcu_nocb_bypass_lock(rdp);
	return rcu_nocb_do_flush_bypass(rdp, rhp, j, lazy);
}

/*
 * If the ->nocb_bypass_lock is immediately available, flush the
 * ->nocb_bypass queue into ->cblist.
 */
static void rcu_nocb_try_flush_bypass(struct rcu_data *rdp, unsigned long j)
{
	rcu_lockdep_assert_cblist_protected(rdp);
	if (!rcu_rdp_is_offloaded(rdp) ||
	    !rcu_nocb_bypass_trylock(rdp))
		return;
	WARN_ON_ONCE(!rcu_nocb_do_flush_bypass(rdp, NULL, j, false));
}

/*
 * See whether it is appropriate to use the ->nocb_bypass list in order
 * to control contention on ->nocb_lock.  A limited number of direct
 * enqueues are permitted into ->cblist per jiffy.  If ->nocb_bypass
 * is non-empty, further callbacks must be placed into ->nocb_bypass,
 * otherwise rcu_barrier() breaks.  Use rcu_nocb_flush_bypass() to switch
 * back to direct use of ->cblist.  However, ->nocb_bypass should not be
 * used if ->cblist is empty, because otherwise callbacks can be stranded
 * on ->nocb_bypass because we cannot count on the current CPU ever again
 * invoking call_rcu().  The general rule is that if ->nocb_bypass is
 * non-empty, the corresponding no-CBs grace-period kthread must not be
 * in an indefinite sleep state.
 *
 * Finally, it is not permitted to use the bypass during early boot,
 * as doing so would confuse the auto-initialization code.  Besides
 * which, there is no point in worrying about lock contention while
 * there is only one CPU in operation.
 */
static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
				bool *was_alldone, unsigned long flags,
				bool lazy)
{
	unsigned long c;
	unsigned long cur_gp_seq;
	unsigned long j = jiffies;
	long ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
	bool bypass_is_lazy = (ncbs == READ_ONCE(rdp->lazy_len));

	lockdep_assert_irqs_disabled();

	// Pure softirq/rcuc based processing: no bypassing, no
	// locking.
	if (!rcu_rdp_is_offloaded(rdp)) {
		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
		return false;
	}

	// In the process of (de-)offloading: no bypassing, but
	// locking.
	if (!rcu_segcblist_completely_offloaded(&rdp->cblist)) {
		rcu_nocb_lock(rdp);
		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
		return false; /* Not offloaded, no bypassing. */
	}

	// Don't use ->nocb_bypass during early boot.
	if (rcu_scheduler_active != RCU_SCHEDULER_RUNNING) {
		rcu_nocb_lock(rdp);
		WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
		return false;
	}

	// If we have advanced to a new jiffy, reset counts to allow
	// moving back from ->nocb_bypass to ->cblist.
	if (j == rdp->nocb_nobypass_last) {
		c = rdp->nocb_nobypass_count + 1;
	} else {
		WRITE_ONCE(rdp->nocb_nobypass_last, j);
		c = rdp->nocb_nobypass_count - nocb_nobypass_lim_per_jiffy;
		if (ULONG_CMP_LT(rdp->nocb_nobypass_count,
				 nocb_nobypass_lim_per_jiffy))
			c = 0;
		else if (c > nocb_nobypass_lim_per_jiffy)
			c = nocb_nobypass_lim_per_jiffy;
	}
	WRITE_ONCE(rdp->nocb_nobypass_count, c);

	// If there hasn't yet been all that many ->cblist enqueues
	// this jiffy, tell the caller to enqueue onto ->cblist.  But flush
	// ->nocb_bypass first.
	// Lazy CBs throttle this back and do immediate bypass queuing.
	if (rdp->nocb_nobypass_count < nocb_nobypass_lim_per_jiffy && !lazy) {
		rcu_nocb_lock(rdp);
		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
		if (*was_alldone)
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
					    TPS("FirstQ"));

		WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, j, false));
		WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
		return false; // Caller must enqueue the callback.
	}

	// If ->nocb_bypass has been used too long or is too full,
	// flush ->nocb_bypass to ->cblist.
	if ((ncbs && !bypass_is_lazy && j != READ_ONCE(rdp->nocb_bypass_first)) ||
	    (ncbs &&  bypass_is_lazy &&
	     (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + jiffies_till_flush))) ||
	    ncbs >= qhimark) {
		rcu_nocb_lock(rdp);
		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);

		if (!rcu_nocb_flush_bypass(rdp, rhp, j, lazy)) {
			if (*was_alldone)
				trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
						    TPS("FirstQ"));
			WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));
			return false; // Caller must enqueue the callback.
		}
		if (j != rdp->nocb_gp_adv_time &&
		    rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
		    rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
			rcu_advance_cbs_nowake(rdp->mynode, rdp);
			rdp->nocb_gp_adv_time = j;
		}

		// The flush succeeded and we moved CBs into the regular list.
		// Don't wait for the wake up timer as it may be too far ahead.
		// Wake up the GP thread now instead, if the cblist was empty.
		__call_rcu_nocb_wake(rdp, *was_alldone, flags);

		return true; // Callback already enqueued.
	}

	// We need to use the bypass.
	rcu_nocb_wait_contended(rdp);
	rcu_nocb_bypass_lock(rdp);
	ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
	rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
	rcu_cblist_enqueue(&rdp->nocb_bypass, rhp);

	if (lazy)
		WRITE_ONCE(rdp->lazy_len, rdp->lazy_len + 1);

	if (!ncbs) {
		WRITE_ONCE(rdp->nocb_bypass_first, j);
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("FirstBQ"));
	}
	rcu_nocb_bypass_unlock(rdp);
	smp_mb(); /* Order enqueue before wake. */
	// A wake up of the grace period kthread or timer adjustment
	// needs to be done only if:
	// 1. Bypass list was fully empty before (this is the first
	//    bypass list entry), or:
	// 2. Both of these conditions are met:
	//    a. The bypass list previously had only lazy CBs, and:
	//    b. The new CB is non-lazy.
	if (ncbs && (!bypass_is_lazy || lazy)) {
		local_irq_restore(flags);
	} else {
		// No-CBs GP kthread might be indefinitely asleep, if so, wake.
		rcu_nocb_lock(rdp); // Rare during call_rcu() flood.
		if (!rcu_segcblist_pend_cbs(&rdp->cblist)) {
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
					    TPS("FirstBQwake"));
			__call_rcu_nocb_wake(rdp, true, flags);
		} else {
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
					    TPS("FirstBQnoWake"));
			rcu_nocb_unlock_irqrestore(rdp, flags);
		}
	}
	return true; // Callback already enqueued.
}

/*
 * Awaken the no-CBs grace-period kthread if needed, either due to it
 * legitimately being asleep or due to overload conditions.
 *
 * If warranted, also wake up the kthread servicing this CPUs queues.
 */
static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
				 unsigned long flags)
				 __releases(rdp->nocb_lock)
{
	long bypass_len;
	unsigned long cur_gp_seq;
	unsigned long j;
	long lazy_len;
	long len;
	struct task_struct *t;

	// If we are being polled or there is no kthread, just leave.
	t = READ_ONCE(rdp->nocb_gp_kthread);
	if (rcu_nocb_poll || !t) {
		rcu_nocb_unlock_irqrestore(rdp, flags);
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
				    TPS("WakeNotPoll"));
		return;
	}
	// Need to actually to a wakeup.
	len = rcu_segcblist_n_cbs(&rdp->cblist);
	bypass_len = rcu_cblist_n_cbs(&rdp->nocb_bypass);
	lazy_len = READ_ONCE(rdp->lazy_len);
	if (was_alldone) {
		rdp->qlen_last_fqs_check = len;
		// Only lazy CBs in bypass list
		if (lazy_len && bypass_len == lazy_len) {
			rcu_nocb_unlock_irqrestore(rdp, flags);
			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY,
					   TPS("WakeLazy"));
		} else if (!irqs_disabled_flags(flags)) {
			/* ... if queue was empty ... */
			rcu_nocb_unlock_irqrestore(rdp, flags);
			wake_nocb_gp(rdp, false);
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
					    TPS("WakeEmpty"));
		} else {
			rcu_nocb_unlock_irqrestore(rdp, flags);
			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE,
					   TPS("WakeEmptyIsDeferred"));
		}
	} else if (len > rdp->qlen_last_fqs_check + qhimark) {
		/* ... or if many callbacks queued. */
		rdp->qlen_last_fqs_check = len;
		j = jiffies;
		if (j != rdp->nocb_gp_adv_time &&
		    rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
		    rcu_seq_done(&rdp->mynode->gp_seq, cur_gp_seq)) {
			rcu_advance_cbs_nowake(rdp->mynode, rdp);
			rdp->nocb_gp_adv_time = j;
		}
		smp_mb(); /* Enqueue before timer_pending(). */
		if ((rdp->nocb_cb_sleep ||
		     !rcu_segcblist_ready_cbs(&rdp->cblist)) &&
		    !timer_pending(&rdp->nocb_timer)) {
			rcu_nocb_unlock_irqrestore(rdp, flags);
			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
					   TPS("WakeOvfIsDeferred"));
		} else {
			rcu_nocb_unlock_irqrestore(rdp, flags);
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
		}
	} else {
		rcu_nocb_unlock_irqrestore(rdp, flags);
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
	}
}

static int nocb_gp_toggle_rdp(struct rcu_data *rdp,
			       bool *wake_state)
{
	struct rcu_segcblist *cblist = &rdp->cblist;
	unsigned long flags;
	int ret;

	rcu_nocb_lock_irqsave(rdp, flags);
	if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED) &&
	    !rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP)) {
		/*
		 * Offloading. Set our flag and notify the offload worker.
		 * We will handle this rdp until it ever gets de-offloaded.
		 */
		rcu_segcblist_set_flags(cblist, SEGCBLIST_KTHREAD_GP);
		if (rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB))
			*wake_state = true;
		ret = 1;
	} else if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED) &&
		   rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP)) {
		/*
		 * De-offloading. Clear our flag and notify the de-offload worker.
		 * We will ignore this rdp until it ever gets re-offloaded.
		 */
		rcu_segcblist_clear_flags(cblist, SEGCBLIST_KTHREAD_GP);
		if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB))
			*wake_state = true;
		ret = 0;
	} else {
		WARN_ON_ONCE(1);
		ret = -1;
	}

	rcu_nocb_unlock_irqrestore(rdp, flags);

	return ret;
}

static void nocb_gp_sleep(struct rcu_data *my_rdp, int cpu)
{
	trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Sleep"));
	swait_event_interruptible_exclusive(my_rdp->nocb_gp_wq,
					!READ_ONCE(my_rdp->nocb_gp_sleep));
	trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("EndSleep"));
}

/*
 * No-CBs GP kthreads come here to wait for additional callbacks to show up
 * or for grace periods to end.
 */
static void nocb_gp_wait(struct rcu_data *my_rdp)
{
	bool bypass = false;
	int __maybe_unused cpu = my_rdp->cpu;
	unsigned long cur_gp_seq;
	unsigned long flags;
	bool gotcbs = false;
	unsigned long j = jiffies;
	bool lazy = false;
	bool needwait_gp = false; // This prevents actual uninitialized use.
	bool needwake;
	bool needwake_gp;
	struct rcu_data *rdp, *rdp_toggling = NULL;
	struct rcu_node *rnp;
	unsigned long wait_gp_seq = 0; // Suppress "use uninitialized" warning.
	bool wasempty = false;

	/*
	 * Each pass through the following loop checks for CBs and for the
	 * nearest grace period (if any) to wait for next.  The CB kthreads
	 * and the global grace-period kthread are awakened if needed.
	 */
	WARN_ON_ONCE(my_rdp->nocb_gp_rdp != my_rdp);
	/*
	 * An rcu_data structure is removed from the list after its
	 * CPU is de-offloaded and added to the list before that CPU is
	 * (re-)offloaded.  If the following loop happens to be referencing
	 * that rcu_data structure during the time that the corresponding
	 * CPU is de-offloaded and then immediately re-offloaded, this
	 * loop's rdp pointer will be carried to the end of the list by
	 * the resulting pair of list operations.  This can cause the loop
	 * to skip over some of the rcu_data structures that were supposed
	 * to have been scanned.  Fortunately a new iteration through the
	 * entire loop is forced after a given CPU's rcu_data structure
	 * is added to the list, so the skipped-over rcu_data structures
	 * won't be ignored for long.
	 */
	list_for_each_entry(rdp, &my_rdp->nocb_head_rdp, nocb_entry_rdp) {
		long bypass_ncbs;
		bool flush_bypass = false;
		long lazy_ncbs;

		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check"));
		rcu_nocb_lock_irqsave(rdp, flags);
		lockdep_assert_held(&rdp->nocb_lock);
		bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
		lazy_ncbs = READ_ONCE(rdp->lazy_len);

		if (bypass_ncbs && (lazy_ncbs == bypass_ncbs) &&
		    (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + jiffies_till_flush) ||
		     bypass_ncbs > 2 * qhimark)) {
			flush_bypass = true;
		} else if (bypass_ncbs && (lazy_ncbs != bypass_ncbs) &&
		    (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + 1) ||
		     bypass_ncbs > 2 * qhimark)) {
			flush_bypass = true;
		} else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
			rcu_nocb_unlock_irqrestore(rdp, flags);
			continue; /* No callbacks here, try next. */
		}

		if (flush_bypass) {
			// Bypass full or old, so flush it.
			(void)rcu_nocb_try_flush_bypass(rdp, j);
			bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
			lazy_ncbs = READ_ONCE(rdp->lazy_len);
		}

		if (bypass_ncbs) {
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
					    bypass_ncbs == lazy_ncbs ? TPS("Lazy") : TPS("Bypass"));
			if (bypass_ncbs == lazy_ncbs)
				lazy = true;
			else
				bypass = true;
		}
		rnp = rdp->mynode;

		// Advance callbacks if helpful and low contention.
		needwake_gp = false;
		if (!rcu_segcblist_restempty(&rdp->cblist,
					     RCU_NEXT_READY_TAIL) ||
		    (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq) &&
		     rcu_seq_done(&rnp->gp_seq, cur_gp_seq))) {
			raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
			needwake_gp = rcu_advance_cbs(rnp, rdp);
			wasempty = rcu_segcblist_restempty(&rdp->cblist,
							   RCU_NEXT_READY_TAIL);
			raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */
		}
		// Need to wait on some grace period?
		WARN_ON_ONCE(wasempty &&
			     !rcu_segcblist_restempty(&rdp->cblist,
						      RCU_NEXT_READY_TAIL));
		if (rcu_segcblist_nextgp(&rdp->cblist, &cur_gp_seq)) {
			if (!needwait_gp ||
			    ULONG_CMP_LT(cur_gp_seq, wait_gp_seq))
				wait_gp_seq = cur_gp_seq;
			needwait_gp = true;
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
					    TPS("NeedWaitGP"));
		}
		if (rcu_segcblist_ready_cbs(&rdp->cblist)) {
			needwake = rdp->nocb_cb_sleep;
			WRITE_ONCE(rdp->nocb_cb_sleep, false);
			smp_mb(); /* CB invocation -after- GP end. */
		} else {
			needwake = false;
		}
		rcu_nocb_unlock_irqrestore(rdp, flags);
		if (needwake) {
			swake_up_one(&rdp->nocb_cb_wq);
			gotcbs = true;
		}
		if (needwake_gp)
			rcu_gp_kthread_wake();
	}

	my_rdp->nocb_gp_bypass = bypass;
	my_rdp->nocb_gp_gp = needwait_gp;
	my_rdp->nocb_gp_seq = needwait_gp ? wait_gp_seq : 0;

	// At least one child with non-empty ->nocb_bypass, so set
	// timer in order to avoid stranding its callbacks.
	if (!rcu_nocb_poll) {
		// If bypass list only has lazy CBs. Add a deferred lazy wake up.
		if (lazy && !bypass) {
			wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_LAZY,
					TPS("WakeLazyIsDeferred"));
		// Otherwise add a deferred bypass wake up.
		} else if (bypass) {
			wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_BYPASS,
					TPS("WakeBypassIsDeferred"));
		}
	}

	if (rcu_nocb_poll) {
		/* Polling, so trace if first poll in the series. */
		if (gotcbs)
			trace_rcu_nocb_wake(rcu_state.name, cpu, TPS("Poll"));
		if (list_empty(&my_rdp->nocb_head_rdp)) {
			raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
			if (!my_rdp->nocb_toggling_rdp)
				WRITE_ONCE(my_rdp->nocb_gp_sleep, true);
			raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
			/* Wait for any offloading rdp */
			nocb_gp_sleep(my_rdp, cpu);
		} else {
			schedule_timeout_idle(1);
		}
	} else if (!needwait_gp) {
		/* Wait for callbacks to appear. */
		nocb_gp_sleep(my_rdp, cpu);
	} else {
		rnp = my_rdp->mynode;
		trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("StartWait"));
		swait_event_interruptible_exclusive(
			rnp->nocb_gp_wq[rcu_seq_ctr(wait_gp_seq) & 0x1],
			rcu_seq_done(&rnp->gp_seq, wait_gp_seq) ||
			!READ_ONCE(my_rdp->nocb_gp_sleep));
		trace_rcu_this_gp(rnp, my_rdp, wait_gp_seq, TPS("EndWait"));
	}

	if (!rcu_nocb_poll) {
		raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
		// (De-)queue an rdp to/from the group if its nocb state is changing
		rdp_toggling = my_rdp->nocb_toggling_rdp;
		if (rdp_toggling)
			my_rdp->nocb_toggling_rdp = NULL;

		if (my_rdp->nocb_defer_wakeup > RCU_NOCB_WAKE_NOT) {
			WRITE_ONCE(my_rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
			del_timer(&my_rdp->nocb_timer);
		}
		WRITE_ONCE(my_rdp->nocb_gp_sleep, true);
		raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
	} else {
		rdp_toggling = READ_ONCE(my_rdp->nocb_toggling_rdp);
		if (rdp_toggling) {
			/*
			 * Paranoid locking to make sure nocb_toggling_rdp is well
			 * reset *before* we (re)set SEGCBLIST_KTHREAD_GP or we could
			 * race with another round of nocb toggling for this rdp.
			 * Nocb locking should prevent from that already but we stick
			 * to paranoia, especially in rare path.
			 */
			raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
			my_rdp->nocb_toggling_rdp = NULL;
			raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
		}
	}

	if (rdp_toggling) {
		bool wake_state = false;
		int ret;

		ret = nocb_gp_toggle_rdp(rdp_toggling, &wake_state);
		if (ret == 1)
			list_add_tail(&rdp_toggling->nocb_entry_rdp, &my_rdp->nocb_head_rdp);
		else if (ret == 0)
			list_del(&rdp_toggling->nocb_entry_rdp);
		if (wake_state)
			swake_up_one(&rdp_toggling->nocb_state_wq);
	}

	my_rdp->nocb_gp_seq = -1;
	WARN_ON(signal_pending(current));
}

/*
 * No-CBs grace-period-wait kthread.  There is one of these per group
 * of CPUs, but only once at least one CPU in that group has come online
 * at least once since boot.  This kthread checks for newly posted
 * callbacks from any of the CPUs it is responsible for, waits for a
 * grace period, then awakens all of the rcu_nocb_cb_kthread() instances
 * that then have callback-invocation work to do.
 */
static int rcu_nocb_gp_kthread(void *arg)
{
	struct rcu_data *rdp = arg;

	for (;;) {
		WRITE_ONCE(rdp->nocb_gp_loops, rdp->nocb_gp_loops + 1);
		nocb_gp_wait(rdp);
		cond_resched_tasks_rcu_qs();
	}
	return 0;
}

static inline bool nocb_cb_can_run(struct rcu_data *rdp)
{
	u8 flags = SEGCBLIST_OFFLOADED | SEGCBLIST_KTHREAD_CB;

	return rcu_segcblist_test_flags(&rdp->cblist, flags);
}

static inline bool nocb_cb_wait_cond(struct rcu_data *rdp)
{
	return nocb_cb_can_run(rdp) && !READ_ONCE(rdp->nocb_cb_sleep);
}

/*
 * Invoke any ready callbacks from the corresponding no-CBs CPU,
 * then, if there are no more, wait for more to appear.
 */
static void nocb_cb_wait(struct rcu_data *rdp)
{
	struct rcu_segcblist *cblist = &rdp->cblist;
	unsigned long cur_gp_seq;
	unsigned long flags;
	bool needwake_state = false;
	bool needwake_gp = false;
	bool can_sleep = true;
	struct rcu_node *rnp = rdp->mynode;

	do {
		swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
						    nocb_cb_wait_cond(rdp));

		// VVV Ensure CB invocation follows _sleep test.
		if (smp_load_acquire(&rdp->nocb_cb_sleep)) { // ^^^
			WARN_ON(signal_pending(current));
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
		}
	} while (!nocb_cb_can_run(rdp));


	local_irq_save(flags);
	rcu_momentary_dyntick_idle();
	local_irq_restore(flags);
	/*
	 * Disable BH to provide the expected environment.  Also, when
	 * transitioning to/from NOCB mode, a self-requeuing callback might
	 * be invoked from softirq.  A short grace period could cause both
	 * instances of this callback would execute concurrently.
	 */
	local_bh_disable();
	rcu_do_batch(rdp);
	local_bh_enable();
	lockdep_assert_irqs_enabled();
	rcu_nocb_lock_irqsave(rdp, flags);
	if (rcu_segcblist_nextgp(cblist, &cur_gp_seq) &&
	    rcu_seq_done(&rnp->gp_seq, cur_gp_seq) &&
	    raw_spin_trylock_rcu_node(rnp)) { /* irqs already disabled. */
		needwake_gp = rcu_advance_cbs(rdp->mynode, rdp);
		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
	}

	if (rcu_segcblist_test_flags(cblist, SEGCBLIST_OFFLOADED)) {
		if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB)) {
			rcu_segcblist_set_flags(cblist, SEGCBLIST_KTHREAD_CB);
			if (rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP))
				needwake_state = true;
		}
		if (rcu_segcblist_ready_cbs(cblist))
			can_sleep = false;
	} else {
		/*
		 * De-offloading. Clear our flag and notify the de-offload worker.
		 * We won't touch the callbacks and keep sleeping until we ever
		 * get re-offloaded.
		 */
		WARN_ON_ONCE(!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB));
		rcu_segcblist_clear_flags(cblist, SEGCBLIST_KTHREAD_CB);
		if (!rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP))
			needwake_state = true;
	}

	WRITE_ONCE(rdp->nocb_cb_sleep, can_sleep);

	if (rdp->nocb_cb_sleep)
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));

	rcu_nocb_unlock_irqrestore(rdp, flags);
	if (needwake_gp)
		rcu_gp_kthread_wake();

	if (needwake_state)
		swake_up_one(&rdp->nocb_state_wq);
}

/*
 * Per-rcu_data kthread, but only for no-CBs CPUs.  Repeatedly invoke
 * nocb_cb_wait() to do the dirty work.
 */
static int rcu_nocb_cb_kthread(void *arg)
{
	struct rcu_data *rdp = arg;

	// Each pass through this loop does one callback batch, and,
	// if there are no more ready callbacks, waits for them.
	for (;;) {
		nocb_cb_wait(rdp);
		cond_resched_tasks_rcu_qs();
	}
	return 0;
}

/* Is a deferred wakeup of rcu_nocb_kthread() required? */
static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level)
{
	return READ_ONCE(rdp->nocb_defer_wakeup) >= level;
}

/* Do a deferred wakeup of rcu_nocb_kthread(). */
static bool do_nocb_deferred_wakeup_common(struct rcu_data *rdp_gp,
					   struct rcu_data *rdp, int level,
					   unsigned long flags)
	__releases(rdp_gp->nocb_gp_lock)
{
	int ndw;
	int ret;

	if (!rcu_nocb_need_deferred_wakeup(rdp_gp, level)) {
		raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
		return false;
	}

	ndw = rdp_gp->nocb_defer_wakeup;
	ret = __wake_nocb_gp(rdp_gp, rdp, ndw == RCU_NOCB_WAKE_FORCE, flags);
	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DeferredWake"));

	return ret;
}

/* Do a deferred wakeup of rcu_nocb_kthread() from a timer handler. */
static void do_nocb_deferred_wakeup_timer(struct timer_list *t)
{
	unsigned long flags;
	struct rcu_data *rdp = from_timer(rdp, t, nocb_timer);

	WARN_ON_ONCE(rdp->nocb_gp_rdp != rdp);
	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer"));

	raw_spin_lock_irqsave(&rdp->nocb_gp_lock, flags);
	smp_mb__after_spinlock(); /* Timer expire before wakeup. */
	do_nocb_deferred_wakeup_common(rdp, rdp, RCU_NOCB_WAKE_BYPASS, flags);
}

/*
 * Do a deferred wakeup of rcu_nocb_kthread() from fastpath.
 * This means we do an inexact common-case check.  Note that if
 * we miss, ->nocb_timer will eventually clean things up.
 */
static bool do_nocb_deferred_wakeup(struct rcu_data *rdp)
{
	unsigned long flags;
	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;

	if (!rdp_gp || !rcu_nocb_need_deferred_wakeup(rdp_gp, RCU_NOCB_WAKE))
		return false;

	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
	return do_nocb_deferred_wakeup_common(rdp_gp, rdp, RCU_NOCB_WAKE, flags);
}

void rcu_nocb_flush_deferred_wakeup(void)
{
	do_nocb_deferred_wakeup(this_cpu_ptr(&rcu_data));
}
EXPORT_SYMBOL_GPL(rcu_nocb_flush_deferred_wakeup);

static int rdp_offload_toggle(struct rcu_data *rdp,
			       bool offload, unsigned long flags)
	__releases(rdp->nocb_lock)
{
	struct rcu_segcblist *cblist = &rdp->cblist;
	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;
	bool wake_gp = false;

	rcu_segcblist_offload(cblist, offload);

	if (rdp->nocb_cb_sleep)
		rdp->nocb_cb_sleep = false;
	rcu_nocb_unlock_irqrestore(rdp, flags);

	/*
	 * Ignore former value of nocb_cb_sleep and force wake up as it could
	 * have been spuriously set to false already.
	 */
	swake_up_one(&rdp->nocb_cb_wq);

	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
	// Queue this rdp for add/del to/from the list to iterate on rcuog
	WRITE_ONCE(rdp_gp->nocb_toggling_rdp, rdp);
	if (rdp_gp->nocb_gp_sleep) {
		rdp_gp->nocb_gp_sleep = false;
		wake_gp = true;
	}
	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);

	return wake_gp;
}

static long rcu_nocb_rdp_deoffload(void *arg)
{
	struct rcu_data *rdp = arg;
	struct rcu_segcblist *cblist = &rdp->cblist;
	unsigned long flags;
	int wake_gp;
	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;

	/*
	 * rcu_nocb_rdp_deoffload() may be called directly if
	 * rcuog/o[p] spawn failed, because at this time the rdp->cpu
	 * is not online yet.
	 */
	WARN_ON_ONCE((rdp->cpu != raw_smp_processor_id()) && cpu_online(rdp->cpu));

	pr_info("De-offloading %d\n", rdp->cpu);

	rcu_nocb_lock_irqsave(rdp, flags);
	/*
	 * Flush once and for all now. This suffices because we are
	 * running on the target CPU holding ->nocb_lock (thus having
	 * interrupts disabled), and because rdp_offload_toggle()
	 * invokes rcu_segcblist_offload(), which clears SEGCBLIST_OFFLOADED.
	 * Thus future calls to rcu_segcblist_completely_offloaded() will
	 * return false, which means that future calls to rcu_nocb_try_bypass()
	 * will refuse to put anything into the bypass.
	 */
	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false));
	/*
	 * Start with invoking rcu_core() early. This way if the current thread
	 * happens to preempt an ongoing call to rcu_core() in the middle,
	 * leaving some work dismissed because rcu_core() still thinks the rdp is
	 * completely offloaded, we are guaranteed a nearby future instance of
	 * rcu_core() to catch up.
	 */
	rcu_segcblist_set_flags(cblist, SEGCBLIST_RCU_CORE);
	invoke_rcu_core();
	wake_gp = rdp_offload_toggle(rdp, false, flags);

	mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
	if (rdp_gp->nocb_gp_kthread) {
		if (wake_gp)
			wake_up_process(rdp_gp->nocb_gp_kthread);

		/*
		 * If rcuo[p] kthread spawn failed, directly remove SEGCBLIST_KTHREAD_CB.
		 * Just wait SEGCBLIST_KTHREAD_GP to be cleared by rcuog.
		 */
		if (!rdp->nocb_cb_kthread) {
			rcu_nocb_lock_irqsave(rdp, flags);
			rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_KTHREAD_CB);
			rcu_nocb_unlock_irqrestore(rdp, flags);
		}

		swait_event_exclusive(rdp->nocb_state_wq,
					!rcu_segcblist_test_flags(cblist,
					  SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP));
	} else {
		/*
		 * No kthread to clear the flags for us or remove the rdp from the nocb list
		 * to iterate. Do it here instead. Locking doesn't look stricly necessary
		 * but we stick to paranoia in this rare path.
		 */
		rcu_nocb_lock_irqsave(rdp, flags);
		rcu_segcblist_clear_flags(&rdp->cblist,
				SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP);
		rcu_nocb_unlock_irqrestore(rdp, flags);

		list_del(&rdp->nocb_entry_rdp);
	}
	mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);

	/*
	 * Lock one last time to acquire latest callback updates from kthreads
	 * so we can later handle callbacks locally without locking.
	 */
	rcu_nocb_lock_irqsave(rdp, flags);
	/*
	 * Theoretically we could clear SEGCBLIST_LOCKING after the nocb
	 * lock is released but how about being paranoid for once?
	 */
	rcu_segcblist_clear_flags(cblist, SEGCBLIST_LOCKING);
	/*
	 * Without SEGCBLIST_LOCKING, we can't use
	 * rcu_nocb_unlock_irqrestore() anymore.
	 */
	raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);

	/* Sanity check */
	WARN_ON_ONCE(rcu_cblist_n_cbs(&rdp->nocb_bypass));


	return 0;
}

int rcu_nocb_cpu_deoffload(int cpu)
{
	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
	int ret = 0;

	cpus_read_lock();
	mutex_lock(&rcu_state.barrier_mutex);
	if (rcu_rdp_is_offloaded(rdp)) {
		if (cpu_online(cpu)) {
			ret = work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
			if (!ret)
				cpumask_clear_cpu(cpu, rcu_nocb_mask);
		} else {
			pr_info("NOCB: Cannot CB-deoffload offline CPU %d\n", rdp->cpu);
			ret = -EINVAL;
		}
	}
	mutex_unlock(&rcu_state.barrier_mutex);
	cpus_read_unlock();

	return ret;
}
EXPORT_SYMBOL_GPL(rcu_nocb_cpu_deoffload);

static long rcu_nocb_rdp_offload(void *arg)
{
	struct rcu_data *rdp = arg;
	struct rcu_segcblist *cblist = &rdp->cblist;
	unsigned long flags;
	int wake_gp;
	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;

	WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
	/*
	 * For now we only support re-offload, ie: the rdp must have been
	 * offloaded on boot first.
	 */
	if (!rdp->nocb_gp_rdp)
		return -EINVAL;

	if (WARN_ON_ONCE(!rdp_gp->nocb_gp_kthread))
		return -EINVAL;

	pr_info("Offloading %d\n", rdp->cpu);

	/*
	 * Can't use rcu_nocb_lock_irqsave() before SEGCBLIST_LOCKING
	 * is set.
	 */
	raw_spin_lock_irqsave(&rdp->nocb_lock, flags);

	/*
	 * We didn't take the nocb lock while working on the
	 * rdp->cblist with SEGCBLIST_LOCKING cleared (pure softirq/rcuc mode).
	 * Every modifications that have been done previously on
	 * rdp->cblist must be visible remotely by the nocb kthreads
	 * upon wake up after reading the cblist flags.
	 *
	 * The layout against nocb_lock enforces that ordering:
	 *
	 *  __rcu_nocb_rdp_offload()   nocb_cb_wait()/nocb_gp_wait()
	 * -------------------------   ----------------------------
	 *      WRITE callbacks           rcu_nocb_lock()
	 *      rcu_nocb_lock()           READ flags
	 *      WRITE flags               READ callbacks
	 *      rcu_nocb_unlock()         rcu_nocb_unlock()
	 */
	wake_gp = rdp_offload_toggle(rdp, true, flags);
	if (wake_gp)
		wake_up_process(rdp_gp->nocb_gp_kthread);
	swait_event_exclusive(rdp->nocb_state_wq,
			      rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_CB) &&
			      rcu_segcblist_test_flags(cblist, SEGCBLIST_KTHREAD_GP));

	/*
	 * All kthreads are ready to work, we can finally relieve rcu_core() and
	 * enable nocb bypass.
	 */
	rcu_nocb_lock_irqsave(rdp, flags);
	rcu_segcblist_clear_flags(cblist, SEGCBLIST_RCU_CORE);
	rcu_nocb_unlock_irqrestore(rdp, flags);

	return 0;
}

int rcu_nocb_cpu_offload(int cpu)
{
	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
	int ret = 0;

	cpus_read_lock();
	mutex_lock(&rcu_state.barrier_mutex);
	if (!rcu_rdp_is_offloaded(rdp)) {
		if (cpu_online(cpu)) {
			ret = work_on_cpu(cpu, rcu_nocb_rdp_offload, rdp);
			if (!ret)
				cpumask_set_cpu(cpu, rcu_nocb_mask);
		} else {
			pr_info("NOCB: Cannot CB-offload offline CPU %d\n", rdp->cpu);
			ret = -EINVAL;
		}
	}
	mutex_unlock(&rcu_state.barrier_mutex);
	cpus_read_unlock();

	return ret;
}
EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);

static unsigned long
lazy_rcu_shrink_count(struct shrinker *shrink, struct shrink_control *sc)
{
	int cpu;
	unsigned long count = 0;

	/* Snapshot count of all CPUs */
	for_each_possible_cpu(cpu) {
		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);

		count +=  READ_ONCE(rdp->lazy_len);
	}

	return count ? count : SHRINK_EMPTY;
}

static unsigned long
lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc)
{
	int cpu;
	unsigned long flags;
	unsigned long count = 0;

	/* Snapshot count of all CPUs */
	for_each_possible_cpu(cpu) {
		struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
		int _count = READ_ONCE(rdp->lazy_len);

		if (_count == 0)
			continue;
		rcu_nocb_lock_irqsave(rdp, flags);
		WRITE_ONCE(rdp->lazy_len, 0);
		rcu_nocb_unlock_irqrestore(rdp, flags);
		wake_nocb_gp(rdp, false);
		sc->nr_to_scan -= _count;
		count += _count;
		if (sc->nr_to_scan <= 0)
			break;
	}
	return count ? count : SHRINK_STOP;
}

static struct shrinker lazy_rcu_shrinker = {
	.count_objects = lazy_rcu_shrink_count,
	.scan_objects = lazy_rcu_shrink_scan,
	.batch = 0,
	.seeks = DEFAULT_SEEKS,
};

void __init rcu_init_nohz(void)
{
	int cpu;
	struct rcu_data *rdp;
	const struct cpumask *cpumask = NULL;

#if defined(CONFIG_NO_HZ_FULL)
	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
		cpumask = tick_nohz_full_mask;
#endif

	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) &&
	    !rcu_state.nocb_is_setup && !cpumask)
		cpumask = cpu_possible_mask;

	if (cpumask) {
		if (!cpumask_available(rcu_nocb_mask)) {
			if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
				pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
				return;
			}
		}

		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
		rcu_state.nocb_is_setup = true;
	}

	if (!rcu_state.nocb_is_setup)
		return;

	if (register_shrinker(&lazy_rcu_shrinker, "rcu-lazy"))
		pr_err("Failed to register lazy_rcu shrinker!\n");

	if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
		pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
		cpumask_and(rcu_nocb_mask, cpu_possible_mask,
			    rcu_nocb_mask);
	}
	if (cpumask_empty(rcu_nocb_mask))
		pr_info("\tOffload RCU callbacks from CPUs: (none).\n");
	else
		pr_info("\tOffload RCU callbacks from CPUs: %*pbl.\n",
			cpumask_pr_args(rcu_nocb_mask));
	if (rcu_nocb_poll)
		pr_info("\tPoll for callbacks from no-CBs CPUs.\n");

	for_each_cpu(cpu, rcu_nocb_mask) {
		rdp = per_cpu_ptr(&rcu_data, cpu);
		if (rcu_segcblist_empty(&rdp->cblist))
			rcu_segcblist_init(&rdp->cblist);
		rcu_segcblist_offload(&rdp->cblist, true);
		rcu_segcblist_set_flags(&rdp->cblist, SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP);
		rcu_segcblist_clear_flags(&rdp->cblist, SEGCBLIST_RCU_CORE);
	}
	rcu_organize_nocb_kthreads();
}

/* Initialize per-rcu_data variables for no-CBs CPUs. */
static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
{
	init_swait_queue_head(&rdp->nocb_cb_wq);
	init_swait_queue_head(&rdp->nocb_gp_wq);
	init_swait_queue_head(&rdp->nocb_state_wq);
	raw_spin_lock_init(&rdp->nocb_lock);
	raw_spin_lock_init(&rdp->nocb_bypass_lock);
	raw_spin_lock_init(&rdp->nocb_gp_lock);
	timer_setup(&rdp->nocb_timer, do_nocb_deferred_wakeup_timer, 0);
	rcu_cblist_init(&rdp->nocb_bypass);
	WRITE_ONCE(rdp->lazy_len, 0);
	mutex_init(&rdp->nocb_gp_kthread_mutex);
}

/*
 * If the specified CPU is a no-CBs CPU that does not already have its
 * rcuo CB kthread, spawn it.  Additionally, if the rcuo GP kthread
 * for this CPU's group has not yet been created, spawn it as well.
 */
static void rcu_spawn_cpu_nocb_kthread(int cpu)
{
	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
	struct rcu_data *rdp_gp;
	struct task_struct *t;
	struct sched_param sp;

	if (!rcu_scheduler_fully_active || !rcu_state.nocb_is_setup)
		return;

	/* If there already is an rcuo kthread, then nothing to do. */
	if (rdp->nocb_cb_kthread)
		return;

	/* If we didn't spawn the GP kthread first, reorganize! */
	sp.sched_priority = kthread_prio;
	rdp_gp = rdp->nocb_gp_rdp;
	mutex_lock(&rdp_gp->nocb_gp_kthread_mutex);
	if (!rdp_gp->nocb_gp_kthread) {
		t = kthread_run(rcu_nocb_gp_kthread, rdp_gp,
				"rcuog/%d", rdp_gp->cpu);
		if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo GP kthread, OOM is now expected behavior\n", __func__)) {
			mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);
			goto end;
		}
		WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
		if (kthread_prio)
			sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
	}
	mutex_unlock(&rdp_gp->nocb_gp_kthread_mutex);

	/* Spawn the kthread for this CPU. */
	t = kthread_run(rcu_nocb_cb_kthread, rdp,
			"rcuo%c/%d", rcu_state.abbr, cpu);
	if (WARN_ONCE(IS_ERR(t), "%s: Could not start rcuo CB kthread, OOM is now expected behavior\n", __func__))
		goto end;

	if (IS_ENABLED(CONFIG_RCU_NOCB_CPU_CB_BOOST) && kthread_prio)
		sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);

	WRITE_ONCE(rdp->nocb_cb_kthread, t);
	WRITE_ONCE(rdp->nocb_gp_kthread, rdp_gp->nocb_gp_kthread);
	return;
end:
	mutex_lock(&rcu_state.barrier_mutex);
	if (rcu_rdp_is_offloaded(rdp)) {
		rcu_nocb_rdp_deoffload(rdp);
		cpumask_clear_cpu(cpu, rcu_nocb_mask);
	}
	mutex_unlock(&rcu_state.barrier_mutex);
}

/* How many CB CPU IDs per GP kthread?  Default of -1 for sqrt(nr_cpu_ids). */
static int rcu_nocb_gp_stride = -1;
module_param(rcu_nocb_gp_stride, int, 0444);

/*
 * Initialize GP-CB relationships for all no-CBs CPU.
 */
static void __init rcu_organize_nocb_kthreads(void)
{
	int cpu;
	bool firsttime = true;
	bool gotnocbs = false;
	bool gotnocbscbs = true;
	int ls = rcu_nocb_gp_stride;
	int nl = 0;  /* Next GP kthread. */
	struct rcu_data *rdp;
	struct rcu_data *rdp_gp = NULL;  /* Suppress misguided gcc warn. */

	if (!cpumask_available(rcu_nocb_mask))
		return;
	if (ls == -1) {
		ls = nr_cpu_ids / int_sqrt(nr_cpu_ids);
		rcu_nocb_gp_stride = ls;
	}

	/*
	 * Each pass through this loop sets up one rcu_data structure.
	 * Should the corresponding CPU come online in the future, then
	 * we will spawn the needed set of rcu_nocb_kthread() kthreads.
	 */
	for_each_possible_cpu(cpu) {
		rdp = per_cpu_ptr(&rcu_data, cpu);
		if (rdp->cpu >= nl) {
			/* New GP kthread, set up for CBs & next GP. */
			gotnocbs = true;
			nl = DIV_ROUND_UP(rdp->cpu + 1, ls) * ls;
			rdp_gp = rdp;
			INIT_LIST_HEAD(&rdp->nocb_head_rdp);
			if (dump_tree) {
				if (!firsttime)
					pr_cont("%s\n", gotnocbscbs
							? "" : " (self only)");
				gotnocbscbs = false;
				firsttime = false;
				pr_alert("%s: No-CB GP kthread CPU %d:",
					 __func__, cpu);
			}
		} else {
			/* Another CB kthread, link to previous GP kthread. */
			gotnocbscbs = true;
			if (dump_tree)
				pr_cont(" %d", cpu);
		}
		rdp->nocb_gp_rdp = rdp_gp;
		if (cpumask_test_cpu(cpu, rcu_nocb_mask))
			list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
	}
	if (gotnocbs && dump_tree)
		pr_cont("%s\n", gotnocbscbs ? "" : " (self only)");
}

/*
 * Bind the current task to the offloaded CPUs.  If there are no offloaded
 * CPUs, leave the task unbound.  Splat if the bind attempt fails.
 */
void rcu_bind_current_to_nocb(void)
{
	if (cpumask_available(rcu_nocb_mask) && !cpumask_empty(rcu_nocb_mask))
		WARN_ON(sched_setaffinity(current->pid, rcu_nocb_mask));
}
EXPORT_SYMBOL_GPL(rcu_bind_current_to_nocb);

// The ->on_cpu field is available only in CONFIG_SMP=y, so...
#ifdef CONFIG_SMP
static char *show_rcu_should_be_on_cpu(struct task_struct *tsp)
{
	return tsp && task_is_running(tsp) && !tsp->on_cpu ? "!" : "";
}
#else // #ifdef CONFIG_SMP
static char *show_rcu_should_be_on_cpu(struct task_struct *tsp)
{
	return "";
}
#endif // #else #ifdef CONFIG_SMP

/*
 * Dump out nocb grace-period kthread state for the specified rcu_data
 * structure.
 */
static void show_rcu_nocb_gp_state(struct rcu_data *rdp)
{
	struct rcu_node *rnp = rdp->mynode;

	pr_info("nocb GP %d %c%c%c%c%c %c[%c%c] %c%c:%ld rnp %d:%d %lu %c CPU %d%s\n",
		rdp->cpu,
		"kK"[!!rdp->nocb_gp_kthread],
		"lL"[raw_spin_is_locked(&rdp->nocb_gp_lock)],
		"dD"[!!rdp->nocb_defer_wakeup],
		"tT"[timer_pending(&rdp->nocb_timer)],
		"sS"[!!rdp->nocb_gp_sleep],
		".W"[swait_active(&rdp->nocb_gp_wq)],
		".W"[swait_active(&rnp->nocb_gp_wq[0])],
		".W"[swait_active(&rnp->nocb_gp_wq[1])],
		".B"[!!rdp->nocb_gp_bypass],
		".G"[!!rdp->nocb_gp_gp],
		(long)rdp->nocb_gp_seq,
		rnp->grplo, rnp->grphi, READ_ONCE(rdp->nocb_gp_loops),
		rdp->nocb_gp_kthread ? task_state_to_char(rdp->nocb_gp_kthread) : '.',
		rdp->nocb_gp_kthread ? (int)task_cpu(rdp->nocb_gp_kthread) : -1,
		show_rcu_should_be_on_cpu(rdp->nocb_gp_kthread));
}

/* Dump out nocb kthread state for the specified rcu_data structure. */
static void show_rcu_nocb_state(struct rcu_data *rdp)
{
	char bufw[20];
	char bufr[20];
	struct rcu_data *nocb_next_rdp;
	struct rcu_segcblist *rsclp = &rdp->cblist;
	bool waslocked;
	bool wassleep;

	if (rdp->nocb_gp_rdp == rdp)
		show_rcu_nocb_gp_state(rdp);

	nocb_next_rdp = list_next_or_null_rcu(&rdp->nocb_gp_rdp->nocb_head_rdp,
					      &rdp->nocb_entry_rdp,
					      typeof(*rdp),
					      nocb_entry_rdp);

	sprintf(bufw, "%ld", rsclp->gp_seq[RCU_WAIT_TAIL]);
	sprintf(bufr, "%ld", rsclp->gp_seq[RCU_NEXT_READY_TAIL]);
	pr_info("   CB %d^%d->%d %c%c%c%c%c%c F%ld L%ld C%d %c%c%s%c%s%c%c q%ld %c CPU %d%s\n",
		rdp->cpu, rdp->nocb_gp_rdp->cpu,
		nocb_next_rdp ? nocb_next_rdp->cpu : -1,
		"kK"[!!rdp->nocb_cb_kthread],
		"bB"[raw_spin_is_locked(&rdp->nocb_bypass_lock)],
		"cC"[!!atomic_read(&rdp->nocb_lock_contended)],
		"lL"[raw_spin_is_locked(&rdp->nocb_lock)],
		"sS"[!!rdp->nocb_cb_sleep],
		".W"[swait_active(&rdp->nocb_cb_wq)],
		jiffies - rdp->nocb_bypass_first,
		jiffies - rdp->nocb_nobypass_last,
		rdp->nocb_nobypass_count,
		".D"[rcu_segcblist_ready_cbs(rsclp)],
		".W"[!rcu_segcblist_segempty(rsclp, RCU_WAIT_TAIL)],
		rcu_segcblist_segempty(rsclp, RCU_WAIT_TAIL) ? "" : bufw,
		".R"[!rcu_segcblist_segempty(rsclp, RCU_NEXT_READY_TAIL)],
		rcu_segcblist_segempty(rsclp, RCU_NEXT_READY_TAIL) ? "" : bufr,
		".N"[!rcu_segcblist_segempty(rsclp, RCU_NEXT_TAIL)],
		".B"[!!rcu_cblist_n_cbs(&rdp->nocb_bypass)],
		rcu_segcblist_n_cbs(&rdp->cblist),
		rdp->nocb_cb_kthread ? task_state_to_char(rdp->nocb_cb_kthread) : '.',
		rdp->nocb_cb_kthread ? (int)task_cpu(rdp->nocb_cb_kthread) : -1,
		show_rcu_should_be_on_cpu(rdp->nocb_cb_kthread));

	/* It is OK for GP kthreads to have GP state. */
	if (rdp->nocb_gp_rdp == rdp)
		return;

	waslocked = raw_spin_is_locked(&rdp->nocb_gp_lock);
	wassleep = swait_active(&rdp->nocb_gp_wq);
	if (!rdp->nocb_gp_sleep && !waslocked && !wassleep)
		return;  /* Nothing untoward. */

	pr_info("   nocb GP activity on CB-only CPU!!! %c%c%c %c\n",
		"lL"[waslocked],
		"dD"[!!rdp->nocb_defer_wakeup],
		"sS"[!!rdp->nocb_gp_sleep],
		".W"[wassleep]);
}

#else /* #ifdef CONFIG_RCU_NOCB_CPU */

static inline int rcu_lockdep_is_held_nocb(struct rcu_data *rdp)
{
	return 0;
}

static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
{
	return false;
}

/* No ->nocb_lock to acquire.  */
static void rcu_nocb_lock(struct rcu_data *rdp)
{
}

/* No ->nocb_lock to release.  */
static void rcu_nocb_unlock(struct rcu_data *rdp)
{
}

/* No ->nocb_lock to release.  */
static void rcu_nocb_unlock_irqrestore(struct rcu_data *rdp,
				       unsigned long flags)
{
	local_irq_restore(flags);
}

/* Lockdep check that ->cblist may be safely accessed. */
static void rcu_lockdep_assert_cblist_protected(struct rcu_data *rdp)
{
	lockdep_assert_irqs_disabled();
}

static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq)
{
}

static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp)
{
	return NULL;
}

static void rcu_init_one_nocb(struct rcu_node *rnp)
{
}

static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
{
	return false;
}

static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
				  unsigned long j, bool lazy)
{
	return true;
}

static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
				bool *was_alldone, unsigned long flags, bool lazy)
{
	return false;
}

static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
				 unsigned long flags)
{
	WARN_ON_ONCE(1);  /* Should be dead code! */
}

static void __init rcu_boot_init_nocb_percpu_data(struct rcu_data *rdp)
{
}

static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level)
{
	return false;
}

static bool do_nocb_deferred_wakeup(struct rcu_data *rdp)
{
	return false;
}

static void rcu_spawn_cpu_nocb_kthread(int cpu)
{
}

static void show_rcu_nocb_state(struct rcu_data *rdp)
{
}

#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */