summaryrefslogtreecommitdiff
path: root/kernel/rcu/tree_plugin.h
blob: 7487c7930a47f59039a2205bfec4a0513a3f3a2e (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
1759
1760
1761
1762
1763
1764
1765
1766
1767
1768
1769
1770
1771
1772
1773
1774
1775
1776
1777
1778
1779
1780
1781
1782
1783
1784
1785
1786
1787
1788
1789
1790
1791
1792
1793
1794
1795
1796
1797
1798
1799
1800
1801
1802
1803
1804
1805
1806
1807
1808
1809
1810
1811
1812
1813
1814
1815
1816
1817
1818
1819
1820
1821
1822
1823
1824
1825
1826
1827
1828
1829
1830
1831
1832
1833
1834
1835
1836
1837
1838
1839
1840
1841
1842
1843
1844
1845
1846
1847
1848
1849
1850
1851
1852
1853
1854
1855
1856
1857
1858
1859
1860
1861
1862
1863
1864
1865
1866
1867
1868
1869
1870
1871
1872
1873
1874
1875
1876
1877
1878
1879
1880
1881
1882
1883
1884
1885
1886
1887
1888
1889
1890
1891
1892
1893
1894
1895
1896
1897
1898
1899
1900
1901
1902
1903
1904
1905
1906
1907
1908
1909
1910
1911
1912
1913
1914
1915
1916
1917
1918
1919
1920
1921
1922
1923
1924
1925
1926
1927
1928
1929
1930
1931
1932
1933
1934
1935
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
2010
2011
2012
2013
2014
2015
2016
2017
2018
2019
2020
2021
2022
2023
2024
2025
2026
2027
2028
2029
2030
2031
2032
2033
2034
2035
2036
2037
2038
2039
2040
2041
2042
2043
2044
2045
2046
2047
2048
2049
2050
2051
2052
2053
2054
2055
2056
2057
2058
2059
2060
2061
2062
2063
2064
2065
2066
2067
2068
2069
2070
2071
2072
2073
2074
2075
2076
2077
2078
2079
2080
2081
2082
2083
2084
2085
2086
2087
2088
2089
2090
2091
2092
2093
2094
2095
2096
2097
2098
2099
2100
2101
2102
2103
2104
2105
2106
2107
2108
2109
2110
2111
2112
2113
2114
2115
2116
2117
2118
2119
2120
2121
2122
2123
2124
2125
2126
2127
2128
2129
2130
2131
2132
2133
2134
2135
2136
2137
2138
2139
2140
2141
2142
2143
2144
2145
2146
2147
2148
2149
2150
2151
2152
2153
2154
2155
2156
2157
2158
2159
2160
2161
2162
2163
2164
2165
2166
2167
2168
2169
2170
2171
2172
2173
2174
2175
2176
2177
2178
2179
2180
2181
2182
2183
2184
2185
2186
2187
2188
2189
2190
2191
2192
2193
2194
2195
2196
2197
2198
2199
2200
2201
2202
2203
2204
2205
2206
2207
2208
2209
2210
2211
2212
2213
2214
2215
2216
2217
2218
2219
2220
2221
2222
2223
2224
2225
2226
2227
2228
2229
2230
2231
2232
2233
2234
2235
2236
2237
2238
2239
2240
2241
2242
2243
2244
2245
2246
2247
2248
2249
2250
2251
2252
2253
2254
2255
2256
2257
2258
2259
2260
2261
2262
2263
2264
2265
2266
2267
2268
2269
2270
2271
2272
2273
2274
2275
2276
2277
2278
2279
2280
2281
2282
2283
2284
2285
2286
2287
2288
2289
2290
2291
2292
2293
2294
2295
2296
2297
2298
2299
2300
2301
2302
2303
2304
2305
2306
2307
2308
2309
2310
2311
2312
2313
2314
2315
2316
2317
2318
2319
2320
2321
2322
2323
2324
2325
2326
2327
2328
2329
2330
2331
2332
2333
2334
2335
2336
2337
2338
2339
2340
2341
2342
2343
2344
2345
2346
2347
2348
2349
2350
2351
2352
2353
2354
2355
2356
2357
2358
2359
2360
2361
2362
2363
2364
2365
2366
2367
2368
2369
2370
2371
2372
2373
2374
2375
2376
2377
2378
2379
2380
2381
2382
2383
2384
2385
2386
2387
2388
2389
2390
2391
2392
2393
2394
2395
2396
2397
2398
2399
2400
2401
2402
2403
2404
2405
2406
2407
2408
2409
2410
2411
2412
2413
2414
2415
2416
2417
2418
2419
2420
2421
2422
2423
2424
2425
2426
2427
2428
2429
2430
2431
2432
2433
2434
2435
2436
2437
2438
2439
2440
2441
2442
2443
2444
2445
2446
2447
2448
2449
2450
2451
2452
2453
2454
2455
2456
2457
2458
2459
2460
2461
2462
2463
2464
2465
2466
2467
2468
2469
2470
2471
2472
2473
2474
2475
2476
2477
2478
2479
2480
2481
2482
2483
2484
2485
2486
2487
2488
2489
2490
2491
2492
2493
2494
2495
2496
2497
2498
2499
2500
2501
2502
2503
2504
2505
2506
2507
2508
2509
2510
2511
2512
2513
2514
2515
2516
2517
2518
2519
2520
2521
2522
2523
2524
2525
2526
2527
2528
2529
2530
2531
2532
2533
2534
2535
2536
2537
2538
2539
2540
2541
2542
2543
2544
2545
2546
2547
2548
2549
2550
2551
2552
2553
2554
2555
2556
2557
2558
2559
2560
2561
2562
2563
2564
2565
2566
2567
2568
2569
2570
2571
2572
2573
2574
2575
2576
2577
2578
2579
2580
2581
2582
2583
2584
2585
2586
/* 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
 *
 * Author: Ingo Molnar <mingo@elte.hu>
 *	   Paul E. McKenney <paulmck@linux.ibm.com>
 */

#include "../locking/rtmutex_common.h"

#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. */
#endif /* #ifdef CONFIG_RCU_NOCB_CPU */

/*
 * Check the RCU kernel configuration parameters and print informative
 * messages about anything out of the ordinary.
 */
static void __init rcu_bootup_announce_oddness(void)
{
	if (IS_ENABLED(CONFIG_RCU_TRACE))
		pr_info("\tRCU event tracing is enabled.\n");
	if ((IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 64) ||
	    (!IS_ENABLED(CONFIG_64BIT) && RCU_FANOUT != 32))
		pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d.\n",
			RCU_FANOUT);
	if (rcu_fanout_exact)
		pr_info("\tHierarchical RCU autobalancing is disabled.\n");
	if (IS_ENABLED(CONFIG_RCU_FAST_NO_HZ))
		pr_info("\tRCU dyntick-idle grace-period acceleration is enabled.\n");
	if (IS_ENABLED(CONFIG_PROVE_RCU))
		pr_info("\tRCU lockdep checking is enabled.\n");
	if (RCU_NUM_LVLS >= 4)
		pr_info("\tFour(or more)-level hierarchy is enabled.\n");
	if (RCU_FANOUT_LEAF != 16)
		pr_info("\tBuild-time adjustment of leaf fanout to %d.\n",
			RCU_FANOUT_LEAF);
	if (rcu_fanout_leaf != RCU_FANOUT_LEAF)
		pr_info("\tBoot-time adjustment of leaf fanout to %d.\n",
			rcu_fanout_leaf);
	if (nr_cpu_ids != NR_CPUS)
		pr_info("\tRCU restricting CPUs from NR_CPUS=%d to nr_cpu_ids=%u.\n", NR_CPUS, nr_cpu_ids);
#ifdef CONFIG_RCU_BOOST
	pr_info("\tRCU priority boosting: priority %d delay %d ms.\n",
		kthread_prio, CONFIG_RCU_BOOST_DELAY);
#endif
	if (blimit != DEFAULT_RCU_BLIMIT)
		pr_info("\tBoot-time adjustment of callback invocation limit to %ld.\n", blimit);
	if (qhimark != DEFAULT_RCU_QHIMARK)
		pr_info("\tBoot-time adjustment of callback high-water mark to %ld.\n", qhimark);
	if (qlowmark != DEFAULT_RCU_QLOMARK)
		pr_info("\tBoot-time adjustment of callback low-water mark to %ld.\n", qlowmark);
	if (jiffies_till_first_fqs != ULONG_MAX)
		pr_info("\tBoot-time adjustment of first FQS scan delay to %ld jiffies.\n", jiffies_till_first_fqs);
	if (jiffies_till_next_fqs != ULONG_MAX)
		pr_info("\tBoot-time adjustment of subsequent FQS scan delay to %ld jiffies.\n", jiffies_till_next_fqs);
	if (jiffies_till_sched_qs != ULONG_MAX)
		pr_info("\tBoot-time adjustment of scheduler-enlistment delay to %ld jiffies.\n", jiffies_till_sched_qs);
	if (rcu_kick_kthreads)
		pr_info("\tKick kthreads if too-long grace period.\n");
	if (IS_ENABLED(CONFIG_DEBUG_OBJECTS_RCU_HEAD))
		pr_info("\tRCU callback double-/use-after-free debug enabled.\n");
	if (gp_preinit_delay)
		pr_info("\tRCU debug GP pre-init slowdown %d jiffies.\n", gp_preinit_delay);
	if (gp_init_delay)
		pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_init_delay);
	if (gp_cleanup_delay)
		pr_info("\tRCU debug GP init slowdown %d jiffies.\n", gp_cleanup_delay);
	if (!use_softirq)
		pr_info("\tRCU_SOFTIRQ processing moved to rcuc kthreads.\n");
	if (IS_ENABLED(CONFIG_RCU_EQS_DEBUG))
		pr_info("\tRCU debug extended QS entry/exit.\n");
	rcupdate_announce_bootup_oddness();
}

#ifdef CONFIG_PREEMPT_RCU

static void rcu_report_exp_rnp(struct rcu_node *rnp, bool wake);
static void rcu_read_unlock_special(struct task_struct *t);

/*
 * Tell them what RCU they are running.
 */
static void __init rcu_bootup_announce(void)
{
	pr_info("Preemptible hierarchical RCU implementation.\n");
	rcu_bootup_announce_oddness();
}

/* Flags for rcu_preempt_ctxt_queue() decision table. */
#define RCU_GP_TASKS	0x8
#define RCU_EXP_TASKS	0x4
#define RCU_GP_BLKD	0x2
#define RCU_EXP_BLKD	0x1

/*
 * Queues a task preempted within an RCU-preempt read-side critical
 * section into the appropriate location within the ->blkd_tasks list,
 * depending on the states of any ongoing normal and expedited grace
 * periods.  The ->gp_tasks pointer indicates which element the normal
 * grace period is waiting on (NULL if none), and the ->exp_tasks pointer
 * indicates which element the expedited grace period is waiting on (again,
 * NULL if none).  If a grace period is waiting on a given element in the
 * ->blkd_tasks list, it also waits on all subsequent elements.  Thus,
 * adding a task to the tail of the list blocks any grace period that is
 * already waiting on one of the elements.  In contrast, adding a task
 * to the head of the list won't block any grace period that is already
 * waiting on one of the elements.
 *
 * This queuing is imprecise, and can sometimes make an ongoing grace
 * period wait for a task that is not strictly speaking blocking it.
 * Given the choice, we needlessly block a normal grace period rather than
 * blocking an expedited grace period.
 *
 * Note that an endless sequence of expedited grace periods still cannot
 * indefinitely postpone a normal grace period.  Eventually, all of the
 * fixed number of preempted tasks blocking the normal grace period that are
 * not also blocking the expedited grace period will resume and complete
 * their RCU read-side critical sections.  At that point, the ->gp_tasks
 * pointer will equal the ->exp_tasks pointer, at which point the end of
 * the corresponding expedited grace period will also be the end of the
 * normal grace period.
 */
static void rcu_preempt_ctxt_queue(struct rcu_node *rnp, struct rcu_data *rdp)
	__releases(rnp->lock) /* But leaves rrupts disabled. */
{
	int blkd_state = (rnp->gp_tasks ? RCU_GP_TASKS : 0) +
			 (rnp->exp_tasks ? RCU_EXP_TASKS : 0) +
			 (rnp->qsmask & rdp->grpmask ? RCU_GP_BLKD : 0) +
			 (rnp->expmask & rdp->grpmask ? RCU_EXP_BLKD : 0);
	struct task_struct *t = current;

	raw_lockdep_assert_held_rcu_node(rnp);
	WARN_ON_ONCE(rdp->mynode != rnp);
	WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
	/* RCU better not be waiting on newly onlined CPUs! */
	WARN_ON_ONCE(rnp->qsmaskinitnext & ~rnp->qsmaskinit & rnp->qsmask &
		     rdp->grpmask);

	/*
	 * Decide where to queue the newly blocked task.  In theory,
	 * this could be an if-statement.  In practice, when I tried
	 * that, it was quite messy.
	 */
	switch (blkd_state) {
	case 0:
	case                RCU_EXP_TASKS:
	case                RCU_EXP_TASKS + RCU_GP_BLKD:
	case RCU_GP_TASKS:
	case RCU_GP_TASKS + RCU_EXP_TASKS:

		/*
		 * Blocking neither GP, or first task blocking the normal
		 * GP but not blocking the already-waiting expedited GP.
		 * Queue at the head of the list to avoid unnecessarily
		 * blocking the already-waiting GPs.
		 */
		list_add(&t->rcu_node_entry, &rnp->blkd_tasks);
		break;

	case                                              RCU_EXP_BLKD:
	case                                RCU_GP_BLKD:
	case                                RCU_GP_BLKD + RCU_EXP_BLKD:
	case RCU_GP_TASKS +                               RCU_EXP_BLKD:
	case RCU_GP_TASKS +                 RCU_GP_BLKD + RCU_EXP_BLKD:
	case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:

		/*
		 * First task arriving that blocks either GP, or first task
		 * arriving that blocks the expedited GP (with the normal
		 * GP already waiting), or a task arriving that blocks
		 * both GPs with both GPs already waiting.  Queue at the
		 * tail of the list to avoid any GP waiting on any of the
		 * already queued tasks that are not blocking it.
		 */
		list_add_tail(&t->rcu_node_entry, &rnp->blkd_tasks);
		break;

	case                RCU_EXP_TASKS +               RCU_EXP_BLKD:
	case                RCU_EXP_TASKS + RCU_GP_BLKD + RCU_EXP_BLKD:
	case RCU_GP_TASKS + RCU_EXP_TASKS +               RCU_EXP_BLKD:

		/*
		 * Second or subsequent task blocking the expedited GP.
		 * The task either does not block the normal GP, or is the
		 * first task blocking the normal GP.  Queue just after
		 * the first task blocking the expedited GP.
		 */
		list_add(&t->rcu_node_entry, rnp->exp_tasks);
		break;

	case RCU_GP_TASKS +                 RCU_GP_BLKD:
	case RCU_GP_TASKS + RCU_EXP_TASKS + RCU_GP_BLKD:

		/*
		 * Second or subsequent task blocking the normal GP.
		 * The task does not block the expedited GP. Queue just
		 * after the first task blocking the normal GP.
		 */
		list_add(&t->rcu_node_entry, rnp->gp_tasks);
		break;

	default:

		/* Yet another exercise in excessive paranoia. */
		WARN_ON_ONCE(1);
		break;
	}

	/*
	 * We have now queued the task.  If it was the first one to
	 * block either grace period, update the ->gp_tasks and/or
	 * ->exp_tasks pointers, respectively, to reference the newly
	 * blocked tasks.
	 */
	if (!rnp->gp_tasks && (blkd_state & RCU_GP_BLKD)) {
		WRITE_ONCE(rnp->gp_tasks, &t->rcu_node_entry);
		WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq);
	}
	if (!rnp->exp_tasks && (blkd_state & RCU_EXP_BLKD))
		rnp->exp_tasks = &t->rcu_node_entry;
	WARN_ON_ONCE(!(blkd_state & RCU_GP_BLKD) !=
		     !(rnp->qsmask & rdp->grpmask));
	WARN_ON_ONCE(!(blkd_state & RCU_EXP_BLKD) !=
		     !(rnp->expmask & rdp->grpmask));
	raw_spin_unlock_rcu_node(rnp); /* interrupts remain disabled. */

	/*
	 * Report the quiescent state for the expedited GP.  This expedited
	 * GP should not be able to end until we report, so there should be
	 * no need to check for a subsequent expedited GP.  (Though we are
	 * still in a quiescent state in any case.)
	 */
	if (blkd_state & RCU_EXP_BLKD && rdp->exp_deferred_qs)
		rcu_report_exp_rdp(rdp);
	else
		WARN_ON_ONCE(rdp->exp_deferred_qs);
}

/*
 * Record a preemptible-RCU quiescent state for the specified CPU.
 * Note that this does not necessarily mean that the task currently running
 * on the CPU is in a quiescent state:  Instead, it means that the current
 * grace period need not wait on any RCU read-side critical section that
 * starts later on this CPU.  It also means that if the current task is
 * in an RCU read-side critical section, it has already added itself to
 * some leaf rcu_node structure's ->blkd_tasks list.  In addition to the
 * current task, there might be any number of other tasks blocked while
 * in an RCU read-side critical section.
 *
 * Callers to this function must disable preemption.
 */
static void rcu_qs(void)
{
	RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!\n");
	if (__this_cpu_read(rcu_data.cpu_no_qs.s)) {
		trace_rcu_grace_period(TPS("rcu_preempt"),
				       __this_cpu_read(rcu_data.gp_seq),
				       TPS("cpuqs"));
		__this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
		barrier(); /* Coordinate with rcu_flavor_sched_clock_irq(). */
		WRITE_ONCE(current->rcu_read_unlock_special.b.need_qs, false);
	}
}

/*
 * We have entered the scheduler, and the current task might soon be
 * context-switched away from.  If this task is in an RCU read-side
 * critical section, we will no longer be able to rely on the CPU to
 * record that fact, so we enqueue the task on the blkd_tasks list.
 * The task will dequeue itself when it exits the outermost enclosing
 * RCU read-side critical section.  Therefore, the current grace period
 * cannot be permitted to complete until the blkd_tasks list entries
 * predating the current grace period drain, in other words, until
 * rnp->gp_tasks becomes NULL.
 *
 * Caller must disable interrupts.
 */
void rcu_note_context_switch(bool preempt)
{
	struct task_struct *t = current;
	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
	struct rcu_node *rnp;

	trace_rcu_utilization(TPS("Start context switch"));
	lockdep_assert_irqs_disabled();
	WARN_ON_ONCE(!preempt && t->rcu_read_lock_nesting > 0);
	if (t->rcu_read_lock_nesting > 0 &&
	    !t->rcu_read_unlock_special.b.blocked) {

		/* Possibly blocking in an RCU read-side critical section. */
		rnp = rdp->mynode;
		raw_spin_lock_rcu_node(rnp);
		t->rcu_read_unlock_special.b.blocked = true;
		t->rcu_blocked_node = rnp;

		/*
		 * Verify the CPU's sanity, trace the preemption, and
		 * then queue the task as required based on the states
		 * of any ongoing and expedited grace periods.
		 */
		WARN_ON_ONCE((rdp->grpmask & rcu_rnp_online_cpus(rnp)) == 0);
		WARN_ON_ONCE(!list_empty(&t->rcu_node_entry));
		trace_rcu_preempt_task(rcu_state.name,
				       t->pid,
				       (rnp->qsmask & rdp->grpmask)
				       ? rnp->gp_seq
				       : rcu_seq_snap(&rnp->gp_seq));
		rcu_preempt_ctxt_queue(rnp, rdp);
	} else {
		rcu_preempt_deferred_qs(t);
	}

	/*
	 * Either we were not in an RCU read-side critical section to
	 * begin with, or we have now recorded that critical section
	 * globally.  Either way, we can now note a quiescent state
	 * for this CPU.  Again, if we were in an RCU read-side critical
	 * section, and if that critical section was blocking the current
	 * grace period, then the fact that the task has been enqueued
	 * means that we continue to block the current grace period.
	 */
	rcu_qs();
	if (rdp->exp_deferred_qs)
		rcu_report_exp_rdp(rdp);
	trace_rcu_utilization(TPS("End context switch"));
}
EXPORT_SYMBOL_GPL(rcu_note_context_switch);

/*
 * Check for preempted RCU readers blocking the current grace period
 * for the specified rcu_node structure.  If the caller needs a reliable
 * answer, it must hold the rcu_node's ->lock.
 */
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
	return READ_ONCE(rnp->gp_tasks) != NULL;
}

/* Bias and limit values for ->rcu_read_lock_nesting. */
#define RCU_NEST_BIAS INT_MAX
#define RCU_NEST_NMAX (-INT_MAX / 2)
#define RCU_NEST_PMAX (INT_MAX / 2)

/*
 * Preemptible RCU implementation for rcu_read_lock().
 * Just increment ->rcu_read_lock_nesting, shared state will be updated
 * if we block.
 */
void __rcu_read_lock(void)
{
	current->rcu_read_lock_nesting++;
	if (IS_ENABLED(CONFIG_PROVE_LOCKING))
		WARN_ON_ONCE(current->rcu_read_lock_nesting > RCU_NEST_PMAX);
	barrier();  /* critical section after entry code. */
}
EXPORT_SYMBOL_GPL(__rcu_read_lock);

/*
 * Preemptible RCU implementation for rcu_read_unlock().
 * Decrement ->rcu_read_lock_nesting.  If the result is zero (outermost
 * rcu_read_unlock()) and ->rcu_read_unlock_special is non-zero, then
 * invoke rcu_read_unlock_special() to clean up after a context switch
 * in an RCU read-side critical section and other special cases.
 */
void __rcu_read_unlock(void)
{
	struct task_struct *t = current;

	if (t->rcu_read_lock_nesting != 1) {
		--t->rcu_read_lock_nesting;
	} else {
		barrier();  /* critical section before exit code. */
		t->rcu_read_lock_nesting = -RCU_NEST_BIAS;
		barrier();  /* assign before ->rcu_read_unlock_special load */
		if (unlikely(READ_ONCE(t->rcu_read_unlock_special.s)))
			rcu_read_unlock_special(t);
		barrier();  /* ->rcu_read_unlock_special load before assign */
		t->rcu_read_lock_nesting = 0;
	}
	if (IS_ENABLED(CONFIG_PROVE_LOCKING)) {
		int rrln = t->rcu_read_lock_nesting;

		WARN_ON_ONCE(rrln < 0 && rrln > RCU_NEST_NMAX);
	}
}
EXPORT_SYMBOL_GPL(__rcu_read_unlock);

/*
 * Advance a ->blkd_tasks-list pointer to the next entry, instead
 * returning NULL if at the end of the list.
 */
static struct list_head *rcu_next_node_entry(struct task_struct *t,
					     struct rcu_node *rnp)
{
	struct list_head *np;

	np = t->rcu_node_entry.next;
	if (np == &rnp->blkd_tasks)
		np = NULL;
	return np;
}

/*
 * Return true if the specified rcu_node structure has tasks that were
 * preempted within an RCU read-side critical section.
 */
static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
{
	return !list_empty(&rnp->blkd_tasks);
}

/*
 * Report deferred quiescent states.  The deferral time can
 * be quite short, for example, in the case of the call from
 * rcu_read_unlock_special().
 */
static void
rcu_preempt_deferred_qs_irqrestore(struct task_struct *t, unsigned long flags)
{
	bool empty_exp;
	bool empty_norm;
	bool empty_exp_now;
	struct list_head *np;
	bool drop_boost_mutex = false;
	struct rcu_data *rdp;
	struct rcu_node *rnp;
	union rcu_special special;

	/*
	 * If RCU core is waiting for this CPU to exit its critical section,
	 * report the fact that it has exited.  Because irqs are disabled,
	 * t->rcu_read_unlock_special cannot change.
	 */
	special = t->rcu_read_unlock_special;
	rdp = this_cpu_ptr(&rcu_data);
	if (!special.s && !rdp->exp_deferred_qs) {
		local_irq_restore(flags);
		return;
	}
	t->rcu_read_unlock_special.b.exp_hint = false;
	t->rcu_read_unlock_special.b.deferred_qs = false;
	if (special.b.need_qs) {
		rcu_qs();
		t->rcu_read_unlock_special.b.need_qs = false;
		if (!t->rcu_read_unlock_special.s && !rdp->exp_deferred_qs) {
			local_irq_restore(flags);
			return;
		}
	}

	/*
	 * Respond to a request by an expedited grace period for a
	 * quiescent state from this CPU.  Note that requests from
	 * tasks are handled when removing the task from the
	 * blocked-tasks list below.
	 */
	if (rdp->exp_deferred_qs) {
		rcu_report_exp_rdp(rdp);
		if (!t->rcu_read_unlock_special.s) {
			local_irq_restore(flags);
			return;
		}
	}

	/* Clean up if blocked during RCU read-side critical section. */
	if (special.b.blocked) {
		t->rcu_read_unlock_special.b.blocked = false;

		/*
		 * Remove this task from the list it blocked on.  The task
		 * now remains queued on the rcu_node corresponding to the
		 * CPU it first blocked on, so there is no longer any need
		 * to loop.  Retain a WARN_ON_ONCE() out of sheer paranoia.
		 */
		rnp = t->rcu_blocked_node;
		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
		WARN_ON_ONCE(rnp != t->rcu_blocked_node);
		WARN_ON_ONCE(!rcu_is_leaf_node(rnp));
		empty_norm = !rcu_preempt_blocked_readers_cgp(rnp);
		WARN_ON_ONCE(rnp->completedqs == rnp->gp_seq &&
			     (!empty_norm || rnp->qsmask));
		empty_exp = sync_rcu_preempt_exp_done(rnp);
		smp_mb(); /* ensure expedited fastpath sees end of RCU c-s. */
		np = rcu_next_node_entry(t, rnp);
		list_del_init(&t->rcu_node_entry);
		t->rcu_blocked_node = NULL;
		trace_rcu_unlock_preempted_task(TPS("rcu_preempt"),
						rnp->gp_seq, t->pid);
		if (&t->rcu_node_entry == rnp->gp_tasks)
			WRITE_ONCE(rnp->gp_tasks, np);
		if (&t->rcu_node_entry == rnp->exp_tasks)
			rnp->exp_tasks = np;
		if (IS_ENABLED(CONFIG_RCU_BOOST)) {
			/* Snapshot ->boost_mtx ownership w/rnp->lock held. */
			drop_boost_mutex = rt_mutex_owner(&rnp->boost_mtx) == t;
			if (&t->rcu_node_entry == rnp->boost_tasks)
				rnp->boost_tasks = np;
		}

		/*
		 * If this was the last task on the current list, and if
		 * we aren't waiting on any CPUs, report the quiescent state.
		 * Note that rcu_report_unblock_qs_rnp() releases rnp->lock,
		 * so we must take a snapshot of the expedited state.
		 */
		empty_exp_now = sync_rcu_preempt_exp_done(rnp);
		if (!empty_norm && !rcu_preempt_blocked_readers_cgp(rnp)) {
			trace_rcu_quiescent_state_report(TPS("preempt_rcu"),
							 rnp->gp_seq,
							 0, rnp->qsmask,
							 rnp->level,
							 rnp->grplo,
							 rnp->grphi,
							 !!rnp->gp_tasks);
			rcu_report_unblock_qs_rnp(rnp, flags);
		} else {
			raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		}

		/* Unboost if we were boosted. */
		if (IS_ENABLED(CONFIG_RCU_BOOST) && drop_boost_mutex)
			rt_mutex_futex_unlock(&rnp->boost_mtx);

		/*
		 * If this was the last task on the expedited lists,
		 * then we need to report up the rcu_node hierarchy.
		 */
		if (!empty_exp && empty_exp_now)
			rcu_report_exp_rnp(rnp, true);
	} else {
		local_irq_restore(flags);
	}
}

/*
 * Is a deferred quiescent-state pending, and are we also not in
 * an RCU read-side critical section?  It is the caller's responsibility
 * to ensure it is otherwise safe to report any deferred quiescent
 * states.  The reason for this is that it is safe to report a
 * quiescent state during context switch even though preemption
 * is disabled.  This function cannot be expected to understand these
 * nuances, so the caller must handle them.
 */
static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
	return (__this_cpu_read(rcu_data.exp_deferred_qs) ||
		READ_ONCE(t->rcu_read_unlock_special.s)) &&
	       t->rcu_read_lock_nesting <= 0;
}

/*
 * Report a deferred quiescent state if needed and safe to do so.
 * As with rcu_preempt_need_deferred_qs(), "safe" involves only
 * not being in an RCU read-side critical section.  The caller must
 * evaluate safety in terms of interrupt, softirq, and preemption
 * disabling.
 */
static void rcu_preempt_deferred_qs(struct task_struct *t)
{
	unsigned long flags;
	bool couldrecurse = t->rcu_read_lock_nesting >= 0;

	if (!rcu_preempt_need_deferred_qs(t))
		return;
	if (couldrecurse)
		t->rcu_read_lock_nesting -= RCU_NEST_BIAS;
	local_irq_save(flags);
	rcu_preempt_deferred_qs_irqrestore(t, flags);
	if (couldrecurse)
		t->rcu_read_lock_nesting += RCU_NEST_BIAS;
}

/*
 * Minimal handler to give the scheduler a chance to re-evaluate.
 */
static void rcu_preempt_deferred_qs_handler(struct irq_work *iwp)
{
	struct rcu_data *rdp;

	rdp = container_of(iwp, struct rcu_data, defer_qs_iw);
	rdp->defer_qs_iw_pending = false;
}

/*
 * Handle special cases during rcu_read_unlock(), such as needing to
 * notify RCU core processing or task having blocked during the RCU
 * read-side critical section.
 */
static void rcu_read_unlock_special(struct task_struct *t)
{
	unsigned long flags;
	bool preempt_bh_were_disabled =
			!!(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK));
	bool irqs_were_disabled;

	/* NMI handlers cannot block and cannot safely manipulate state. */
	if (in_nmi())
		return;

	local_irq_save(flags);
	irqs_were_disabled = irqs_disabled_flags(flags);
	if (preempt_bh_were_disabled || irqs_were_disabled) {
		bool exp;
		struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
		struct rcu_node *rnp = rdp->mynode;

		exp = (t->rcu_blocked_node && t->rcu_blocked_node->exp_tasks) ||
		      (rdp->grpmask & rnp->expmask) ||
		      tick_nohz_full_cpu(rdp->cpu);
		// Need to defer quiescent state until everything is enabled.
		if (irqs_were_disabled && use_softirq &&
		    (in_interrupt() ||
		     (exp && !t->rcu_read_unlock_special.b.deferred_qs))) {
			// Using softirq, safe to awaken, and we get
			// no help from enabling irqs, unlike bh/preempt.
			raise_softirq_irqoff(RCU_SOFTIRQ);
		} else {
			// Enabling BH or preempt does reschedule, so...
			// Also if no expediting or NO_HZ_FULL, slow is OK.
			set_tsk_need_resched(current);
			set_preempt_need_resched();
			if (IS_ENABLED(CONFIG_IRQ_WORK) && irqs_were_disabled &&
			    !rdp->defer_qs_iw_pending && exp) {
				// Get scheduler to re-evaluate and call hooks.
				// If !IRQ_WORK, FQS scan will eventually IPI.
				init_irq_work(&rdp->defer_qs_iw,
					      rcu_preempt_deferred_qs_handler);
				rdp->defer_qs_iw_pending = true;
				irq_work_queue_on(&rdp->defer_qs_iw, rdp->cpu);
			}
		}
		t->rcu_read_unlock_special.b.deferred_qs = true;
		local_irq_restore(flags);
		return;
	}
	rcu_preempt_deferred_qs_irqrestore(t, flags);
}

/*
 * Check that the list of blocked tasks for the newly completed grace
 * period is in fact empty.  It is a serious bug to complete a grace
 * period that still has RCU readers blocked!  This function must be
 * invoked -before- updating this rnp's ->gp_seq.
 *
 * Also, if there are blocked tasks on the list, they automatically
 * block the newly created grace period, so set up ->gp_tasks accordingly.
 */
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
	struct task_struct *t;

	RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
	raw_lockdep_assert_held_rcu_node(rnp);
	if (WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp)))
		dump_blkd_tasks(rnp, 10);
	if (rcu_preempt_has_tasks(rnp) &&
	    (rnp->qsmaskinit || rnp->wait_blkd_tasks)) {
		WRITE_ONCE(rnp->gp_tasks, rnp->blkd_tasks.next);
		t = container_of(rnp->gp_tasks, struct task_struct,
				 rcu_node_entry);
		trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
						rnp->gp_seq, t->pid);
	}
	WARN_ON_ONCE(rnp->qsmask);
}

/*
 * Check for a quiescent state from the current CPU, including voluntary
 * context switches for Tasks RCU.  When a task blocks, the task is
 * recorded in the corresponding CPU's rcu_node structure, which is checked
 * elsewhere, hence this function need only check for quiescent states
 * related to the current CPU, not to those related to tasks.
 */
static void rcu_flavor_sched_clock_irq(int user)
{
	struct task_struct *t = current;

	if (user || rcu_is_cpu_rrupt_from_idle()) {
		rcu_note_voluntary_context_switch(current);
	}
	if (t->rcu_read_lock_nesting > 0 ||
	    (preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK))) {
		/* No QS, force context switch if deferred. */
		if (rcu_preempt_need_deferred_qs(t)) {
			set_tsk_need_resched(t);
			set_preempt_need_resched();
		}
	} else if (rcu_preempt_need_deferred_qs(t)) {
		rcu_preempt_deferred_qs(t); /* Report deferred QS. */
		return;
	} else if (!t->rcu_read_lock_nesting) {
		rcu_qs(); /* Report immediate QS. */
		return;
	}

	/* If GP is oldish, ask for help from rcu_read_unlock_special(). */
	if (t->rcu_read_lock_nesting > 0 &&
	    __this_cpu_read(rcu_data.core_needs_qs) &&
	    __this_cpu_read(rcu_data.cpu_no_qs.b.norm) &&
	    !t->rcu_read_unlock_special.b.need_qs &&
	    time_after(jiffies, rcu_state.gp_start + HZ))
		t->rcu_read_unlock_special.b.need_qs = true;
}

/*
 * Check for a task exiting while in a preemptible-RCU read-side
 * critical section, clean up if so.  No need to issue warnings, as
 * debug_check_no_locks_held() already does this if lockdep is enabled.
 * Besides, if this function does anything other than just immediately
 * return, there was a bug of some sort.  Spewing warnings from this
 * function is like as not to simply obscure important prior warnings.
 */
void exit_rcu(void)
{
	struct task_struct *t = current;

	if (unlikely(!list_empty(&current->rcu_node_entry))) {
		t->rcu_read_lock_nesting = 1;
		barrier();
		WRITE_ONCE(t->rcu_read_unlock_special.b.blocked, true);
	} else if (unlikely(t->rcu_read_lock_nesting)) {
		t->rcu_read_lock_nesting = 1;
	} else {
		return;
	}
	__rcu_read_unlock();
	rcu_preempt_deferred_qs(current);
}

/*
 * Dump the blocked-tasks state, but limit the list dump to the
 * specified number of elements.
 */
static void
dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
{
	int cpu;
	int i;
	struct list_head *lhp;
	bool onl;
	struct rcu_data *rdp;
	struct rcu_node *rnp1;

	raw_lockdep_assert_held_rcu_node(rnp);
	pr_info("%s: grp: %d-%d level: %d ->gp_seq %ld ->completedqs %ld\n",
		__func__, rnp->grplo, rnp->grphi, rnp->level,
		(long)rnp->gp_seq, (long)rnp->completedqs);
	for (rnp1 = rnp; rnp1; rnp1 = rnp1->parent)
		pr_info("%s: %d:%d ->qsmask %#lx ->qsmaskinit %#lx ->qsmaskinitnext %#lx\n",
			__func__, rnp1->grplo, rnp1->grphi, rnp1->qsmask, rnp1->qsmaskinit, rnp1->qsmaskinitnext);
	pr_info("%s: ->gp_tasks %p ->boost_tasks %p ->exp_tasks %p\n",
		__func__, READ_ONCE(rnp->gp_tasks), rnp->boost_tasks,
		rnp->exp_tasks);
	pr_info("%s: ->blkd_tasks", __func__);
	i = 0;
	list_for_each(lhp, &rnp->blkd_tasks) {
		pr_cont(" %p", lhp);
		if (++i >= ncheck)
			break;
	}
	pr_cont("\n");
	for (cpu = rnp->grplo; cpu <= rnp->grphi; cpu++) {
		rdp = per_cpu_ptr(&rcu_data, cpu);
		onl = !!(rdp->grpmask & rcu_rnp_online_cpus(rnp));
		pr_info("\t%d: %c online: %ld(%d) offline: %ld(%d)\n",
			cpu, ".o"[onl],
			(long)rdp->rcu_onl_gp_seq, rdp->rcu_onl_gp_flags,
			(long)rdp->rcu_ofl_gp_seq, rdp->rcu_ofl_gp_flags);
	}
}

#else /* #ifdef CONFIG_PREEMPT_RCU */

/*
 * Tell them what RCU they are running.
 */
static void __init rcu_bootup_announce(void)
{
	pr_info("Hierarchical RCU implementation.\n");
	rcu_bootup_announce_oddness();
}

/*
 * Note a quiescent state for PREEMPTION=n.  Because we do not need to know
 * how many quiescent states passed, just if there was at least one since
 * the start of the grace period, this just sets a flag.  The caller must
 * have disabled preemption.
 */
static void rcu_qs(void)
{
	RCU_LOCKDEP_WARN(preemptible(), "rcu_qs() invoked with preemption enabled!!!");
	if (!__this_cpu_read(rcu_data.cpu_no_qs.s))
		return;
	trace_rcu_grace_period(TPS("rcu_sched"),
			       __this_cpu_read(rcu_data.gp_seq), TPS("cpuqs"));
	__this_cpu_write(rcu_data.cpu_no_qs.b.norm, false);
	if (!__this_cpu_read(rcu_data.cpu_no_qs.b.exp))
		return;
	__this_cpu_write(rcu_data.cpu_no_qs.b.exp, false);
	rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
}

/*
 * Register an urgently needed quiescent state.  If there is an
 * emergency, invoke rcu_momentary_dyntick_idle() to do a heavy-weight
 * dyntick-idle quiescent state visible to other CPUs, which will in
 * some cases serve for expedited as well as normal grace periods.
 * Either way, register a lightweight quiescent state.
 */
void rcu_all_qs(void)
{
	unsigned long flags;

	if (!raw_cpu_read(rcu_data.rcu_urgent_qs))
		return;
	preempt_disable();
	/* Load rcu_urgent_qs before other flags. */
	if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs))) {
		preempt_enable();
		return;
	}
	this_cpu_write(rcu_data.rcu_urgent_qs, false);
	if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs))) {
		local_irq_save(flags);
		rcu_momentary_dyntick_idle();
		local_irq_restore(flags);
	}
	rcu_qs();
	preempt_enable();
}
EXPORT_SYMBOL_GPL(rcu_all_qs);

/*
 * Note a PREEMPTION=n context switch. The caller must have disabled interrupts.
 */
void rcu_note_context_switch(bool preempt)
{
	trace_rcu_utilization(TPS("Start context switch"));
	rcu_qs();
	/* Load rcu_urgent_qs before other flags. */
	if (!smp_load_acquire(this_cpu_ptr(&rcu_data.rcu_urgent_qs)))
		goto out;
	this_cpu_write(rcu_data.rcu_urgent_qs, false);
	if (unlikely(raw_cpu_read(rcu_data.rcu_need_heavy_qs)))
		rcu_momentary_dyntick_idle();
	if (!preempt)
		rcu_tasks_qs(current);
out:
	trace_rcu_utilization(TPS("End context switch"));
}
EXPORT_SYMBOL_GPL(rcu_note_context_switch);

/*
 * Because preemptible RCU does not exist, there are never any preempted
 * RCU readers.
 */
static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp)
{
	return 0;
}

/*
 * Because there is no preemptible RCU, there can be no readers blocked.
 */
static bool rcu_preempt_has_tasks(struct rcu_node *rnp)
{
	return false;
}

/*
 * Because there is no preemptible RCU, there can be no deferred quiescent
 * states.
 */
static bool rcu_preempt_need_deferred_qs(struct task_struct *t)
{
	return false;
}
static void rcu_preempt_deferred_qs(struct task_struct *t) { }

/*
 * Because there is no preemptible RCU, there can be no readers blocked,
 * so there is no need to check for blocked tasks.  So check only for
 * bogus qsmask values.
 */
static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
{
	WARN_ON_ONCE(rnp->qsmask);
}

/*
 * Check to see if this CPU is in a non-context-switch quiescent state,
 * namely user mode and idle loop.
 */
static void rcu_flavor_sched_clock_irq(int user)
{
	if (user || rcu_is_cpu_rrupt_from_idle()) {

		/*
		 * Get here if this CPU took its interrupt from user
		 * mode or from the idle loop, and if this is not a
		 * nested interrupt.  In this case, the CPU is in
		 * a quiescent state, so note it.
		 *
		 * No memory barrier is required here because rcu_qs()
		 * references only CPU-local variables that other CPUs
		 * neither access nor modify, at least not while the
		 * corresponding CPU is online.
		 */

		rcu_qs();
	}
}

/*
 * Because preemptible RCU does not exist, tasks cannot possibly exit
 * while in preemptible RCU read-side critical sections.
 */
void exit_rcu(void)
{
}

/*
 * Dump the guaranteed-empty blocked-tasks state.  Trust but verify.
 */
static void
dump_blkd_tasks(struct rcu_node *rnp, int ncheck)
{
	WARN_ON_ONCE(!list_empty(&rnp->blkd_tasks));
}

#endif /* #else #ifdef CONFIG_PREEMPT_RCU */

/*
 * If boosting, set rcuc kthreads to realtime priority.
 */
static void rcu_cpu_kthread_setup(unsigned int cpu)
{
#ifdef CONFIG_RCU_BOOST
	struct sched_param sp;

	sp.sched_priority = kthread_prio;
	sched_setscheduler_nocheck(current, SCHED_FIFO, &sp);
#endif /* #ifdef CONFIG_RCU_BOOST */
}

#ifdef CONFIG_RCU_BOOST

/*
 * Carry out RCU priority boosting on the task indicated by ->exp_tasks
 * or ->boost_tasks, advancing the pointer to the next task in the
 * ->blkd_tasks list.
 *
 * Note that irqs must be enabled: boosting the task can block.
 * Returns 1 if there are more tasks needing to be boosted.
 */
static int rcu_boost(struct rcu_node *rnp)
{
	unsigned long flags;
	struct task_struct *t;
	struct list_head *tb;

	if (READ_ONCE(rnp->exp_tasks) == NULL &&
	    READ_ONCE(rnp->boost_tasks) == NULL)
		return 0;  /* Nothing left to boost. */

	raw_spin_lock_irqsave_rcu_node(rnp, flags);

	/*
	 * Recheck under the lock: all tasks in need of boosting
	 * might exit their RCU read-side critical sections on their own.
	 */
	if (rnp->exp_tasks == NULL && rnp->boost_tasks == NULL) {
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return 0;
	}

	/*
	 * Preferentially boost tasks blocking expedited grace periods.
	 * This cannot starve the normal grace periods because a second
	 * expedited grace period must boost all blocked tasks, including
	 * those blocking the pre-existing normal grace period.
	 */
	if (rnp->exp_tasks != NULL)
		tb = rnp->exp_tasks;
	else
		tb = rnp->boost_tasks;

	/*
	 * We boost task t by manufacturing an rt_mutex that appears to
	 * be held by task t.  We leave a pointer to that rt_mutex where
	 * task t can find it, and task t will release the mutex when it
	 * exits its outermost RCU read-side critical section.  Then
	 * simply acquiring this artificial rt_mutex will boost task
	 * t's priority.  (Thanks to tglx for suggesting this approach!)
	 *
	 * Note that task t must acquire rnp->lock to remove itself from
	 * the ->blkd_tasks list, which it will do from exit() if from
	 * nowhere else.  We therefore are guaranteed that task t will
	 * stay around at least until we drop rnp->lock.  Note that
	 * rnp->lock also resolves races between our priority boosting
	 * and task t's exiting its outermost RCU read-side critical
	 * section.
	 */
	t = container_of(tb, struct task_struct, rcu_node_entry);
	rt_mutex_init_proxy_locked(&rnp->boost_mtx, t);
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	/* Lock only for side effect: boosts task t's priority. */
	rt_mutex_lock(&rnp->boost_mtx);
	rt_mutex_unlock(&rnp->boost_mtx);  /* Then keep lockdep happy. */

	return READ_ONCE(rnp->exp_tasks) != NULL ||
	       READ_ONCE(rnp->boost_tasks) != NULL;
}

/*
 * Priority-boosting kthread, one per leaf rcu_node.
 */
static int rcu_boost_kthread(void *arg)
{
	struct rcu_node *rnp = (struct rcu_node *)arg;
	int spincnt = 0;
	int more2boost;

	trace_rcu_utilization(TPS("Start boost kthread@init"));
	for (;;) {
		rnp->boost_kthread_status = RCU_KTHREAD_WAITING;
		trace_rcu_utilization(TPS("End boost kthread@rcu_wait"));
		rcu_wait(rnp->boost_tasks || rnp->exp_tasks);
		trace_rcu_utilization(TPS("Start boost kthread@rcu_wait"));
		rnp->boost_kthread_status = RCU_KTHREAD_RUNNING;
		more2boost = rcu_boost(rnp);
		if (more2boost)
			spincnt++;
		else
			spincnt = 0;
		if (spincnt > 10) {
			rnp->boost_kthread_status = RCU_KTHREAD_YIELDING;
			trace_rcu_utilization(TPS("End boost kthread@rcu_yield"));
			schedule_timeout_interruptible(2);
			trace_rcu_utilization(TPS("Start boost kthread@rcu_yield"));
			spincnt = 0;
		}
	}
	/* NOTREACHED */
	trace_rcu_utilization(TPS("End boost kthread@notreached"));
	return 0;
}

/*
 * Check to see if it is time to start boosting RCU readers that are
 * blocking the current grace period, and, if so, tell the per-rcu_node
 * kthread to start boosting them.  If there is an expedited grace
 * period in progress, it is always time to boost.
 *
 * The caller must hold rnp->lock, which this function releases.
 * The ->boost_kthread_task is immortal, so we don't need to worry
 * about it going away.
 */
static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
	__releases(rnp->lock)
{
	raw_lockdep_assert_held_rcu_node(rnp);
	if (!rcu_preempt_blocked_readers_cgp(rnp) && rnp->exp_tasks == NULL) {
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		return;
	}
	if (rnp->exp_tasks != NULL ||
	    (rnp->gp_tasks != NULL &&
	     rnp->boost_tasks == NULL &&
	     rnp->qsmask == 0 &&
	     ULONG_CMP_GE(jiffies, rnp->boost_time))) {
		if (rnp->exp_tasks == NULL)
			rnp->boost_tasks = rnp->gp_tasks;
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
		rcu_wake_cond(rnp->boost_kthread_task,
			      rnp->boost_kthread_status);
	} else {
		raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	}
}

/*
 * Is the current CPU running the RCU-callbacks kthread?
 * Caller must have preemption disabled.
 */
static bool rcu_is_callbacks_kthread(void)
{
	return __this_cpu_read(rcu_data.rcu_cpu_kthread_task) == current;
}

#define RCU_BOOST_DELAY_JIFFIES DIV_ROUND_UP(CONFIG_RCU_BOOST_DELAY * HZ, 1000)

/*
 * Do priority-boost accounting for the start of a new grace period.
 */
static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
{
	rnp->boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
}

/*
 * Create an RCU-boost kthread for the specified node if one does not
 * already exist.  We only create this kthread for preemptible RCU.
 * Returns zero if all is well, a negated errno otherwise.
 */
static void rcu_spawn_one_boost_kthread(struct rcu_node *rnp)
{
	int rnp_index = rnp - rcu_get_root();
	unsigned long flags;
	struct sched_param sp;
	struct task_struct *t;

	if (!IS_ENABLED(CONFIG_PREEMPT_RCU))
		return;

	if (!rcu_scheduler_fully_active || rcu_rnp_online_cpus(rnp) == 0)
		return;

	rcu_state.boost = 1;

	if (rnp->boost_kthread_task != NULL)
		return;

	t = kthread_create(rcu_boost_kthread, (void *)rnp,
			   "rcub/%d", rnp_index);
	if (WARN_ON_ONCE(IS_ERR(t)))
		return;

	raw_spin_lock_irqsave_rcu_node(rnp, flags);
	rnp->boost_kthread_task = t;
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
	sp.sched_priority = kthread_prio;
	sched_setscheduler_nocheck(t, SCHED_FIFO, &sp);
	wake_up_process(t); /* get to TASK_INTERRUPTIBLE quickly. */
}

/*
 * Set the per-rcu_node kthread's affinity to cover all CPUs that are
 * served by the rcu_node in question.  The CPU hotplug lock is still
 * held, so the value of rnp->qsmaskinit will be stable.
 *
 * We don't include outgoingcpu in the affinity set, use -1 if there is
 * no outgoing CPU.  If there are no CPUs left in the affinity set,
 * this function allows the kthread to execute on any CPU.
 */
static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
{
	struct task_struct *t = rnp->boost_kthread_task;
	unsigned long mask = rcu_rnp_online_cpus(rnp);
	cpumask_var_t cm;
	int cpu;

	if (!t)
		return;
	if (!zalloc_cpumask_var(&cm, GFP_KERNEL))
		return;
	for_each_leaf_node_possible_cpu(rnp, cpu)
		if ((mask & leaf_node_cpu_bit(rnp, cpu)) &&
		    cpu != outgoingcpu)
			cpumask_set_cpu(cpu, cm);
	if (cpumask_weight(cm) == 0)
		cpumask_setall(cm);
	set_cpus_allowed_ptr(t, cm);
	free_cpumask_var(cm);
}

/*
 * Spawn boost kthreads -- called as soon as the scheduler is running.
 */
static void __init rcu_spawn_boost_kthreads(void)
{
	struct rcu_node *rnp;

	rcu_for_each_leaf_node(rnp)
		rcu_spawn_one_boost_kthread(rnp);
}

static void rcu_prepare_kthreads(int cpu)
{
	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
	struct rcu_node *rnp = rdp->mynode;

	/* Fire up the incoming CPU's kthread and leaf rcu_node kthread. */
	if (rcu_scheduler_fully_active)
		rcu_spawn_one_boost_kthread(rnp);
}

#else /* #ifdef CONFIG_RCU_BOOST */

static void rcu_initiate_boost(struct rcu_node *rnp, unsigned long flags)
	__releases(rnp->lock)
{
	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}

static bool rcu_is_callbacks_kthread(void)
{
	return false;
}

static void rcu_preempt_boost_start_gp(struct rcu_node *rnp)
{
}

static void rcu_boost_kthread_setaffinity(struct rcu_node *rnp, int outgoingcpu)
{
}

static void __init rcu_spawn_boost_kthreads(void)
{
}

static void rcu_prepare_kthreads(int cpu)
{
}

#endif /* #else #ifdef CONFIG_RCU_BOOST */

#if !defined(CONFIG_RCU_FAST_NO_HZ)

/*
 * Check to see if any future non-offloaded RCU-related work will need
 * to be done by the current CPU, even if none need be done immediately,
 * returning 1 if so.  This function is part of the RCU implementation;
 * it is -not- an exported member of the RCU API.
 *
 * Because we not have RCU_FAST_NO_HZ, just check whether or not this
 * CPU has RCU callbacks queued.
 */
int rcu_needs_cpu(u64 basemono, u64 *nextevt)
{
	*nextevt = KTIME_MAX;
	return !rcu_segcblist_empty(&this_cpu_ptr(&rcu_data)->cblist) &&
	       !rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist);
}

/*
 * Because we do not have RCU_FAST_NO_HZ, don't bother cleaning up
 * after it.
 */
static void rcu_cleanup_after_idle(void)
{
}

/*
 * Do the idle-entry grace-period work, which, because CONFIG_RCU_FAST_NO_HZ=n,
 * is nothing.
 */
static void rcu_prepare_for_idle(void)
{
}

#else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */

/*
 * This code is invoked when a CPU goes idle, at which point we want
 * to have the CPU do everything required for RCU so that it can enter
 * the energy-efficient dyntick-idle mode.  This is handled by a
 * state machine implemented by rcu_prepare_for_idle() below.
 *
 * The following three proprocessor symbols control this state machine:
 *
 * RCU_IDLE_GP_DELAY gives the number of jiffies that a CPU is permitted
 *	to sleep in dyntick-idle mode with RCU callbacks pending.  This
 *	is sized to be roughly one RCU grace period.  Those energy-efficiency
 *	benchmarkers who might otherwise be tempted to set this to a large
 *	number, be warned: Setting RCU_IDLE_GP_DELAY too high can hang your
 *	system.  And if you are -that- concerned about energy efficiency,
 *	just power the system down and be done with it!
 * RCU_IDLE_LAZY_GP_DELAY gives the number of jiffies that a CPU is
 *	permitted to sleep in dyntick-idle mode with only lazy RCU
 *	callbacks pending.  Setting this too high can OOM your system.
 *
 * The values below work well in practice.  If future workloads require
 * adjustment, they can be converted into kernel config parameters, though
 * making the state machine smarter might be a better option.
 */
#define RCU_IDLE_GP_DELAY 4		/* Roughly one grace period. */
#define RCU_IDLE_LAZY_GP_DELAY (6 * HZ)	/* Roughly six seconds. */

static int rcu_idle_gp_delay = RCU_IDLE_GP_DELAY;
module_param(rcu_idle_gp_delay, int, 0644);
static int rcu_idle_lazy_gp_delay = RCU_IDLE_LAZY_GP_DELAY;
module_param(rcu_idle_lazy_gp_delay, int, 0644);

/*
 * Try to advance callbacks on the current CPU, but only if it has been
 * awhile since the last time we did so.  Afterwards, if there are any
 * callbacks ready for immediate invocation, return true.
 */
static bool __maybe_unused rcu_try_advance_all_cbs(void)
{
	bool cbs_ready = false;
	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
	struct rcu_node *rnp;

	/* Exit early if we advanced recently. */
	if (jiffies == rdp->last_advance_all)
		return false;
	rdp->last_advance_all = jiffies;

	rnp = rdp->mynode;

	/*
	 * Don't bother checking unless a grace period has
	 * completed since we last checked and there are
	 * callbacks not yet ready to invoke.
	 */
	if ((rcu_seq_completed_gp(rdp->gp_seq,
				  rcu_seq_current(&rnp->gp_seq)) ||
	     unlikely(READ_ONCE(rdp->gpwrap))) &&
	    rcu_segcblist_pend_cbs(&rdp->cblist))
		note_gp_changes(rdp);

	if (rcu_segcblist_ready_cbs(&rdp->cblist))
		cbs_ready = true;
	return cbs_ready;
}

/*
 * Allow the CPU to enter dyntick-idle mode unless it has callbacks ready
 * to invoke.  If the CPU has callbacks, try to advance them.  Tell the
 * caller to set the timeout based on whether or not there are non-lazy
 * callbacks.
 *
 * The caller must have disabled interrupts.
 */
int rcu_needs_cpu(u64 basemono, u64 *nextevt)
{
	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
	unsigned long dj;

	lockdep_assert_irqs_disabled();

	/* If no non-offloaded callbacks, RCU doesn't need the CPU. */
	if (rcu_segcblist_empty(&rdp->cblist) ||
	    rcu_segcblist_is_offloaded(&this_cpu_ptr(&rcu_data)->cblist)) {
		*nextevt = KTIME_MAX;
		return 0;
	}

	/* Attempt to advance callbacks. */
	if (rcu_try_advance_all_cbs()) {
		/* Some ready to invoke, so initiate later invocation. */
		invoke_rcu_core();
		return 1;
	}
	rdp->last_accelerate = jiffies;

	/* Request timer delay depending on laziness, and round. */
	rdp->all_lazy = !rcu_segcblist_n_nonlazy_cbs(&rdp->cblist);
	if (rdp->all_lazy) {
		dj = round_jiffies(rcu_idle_lazy_gp_delay + jiffies) - jiffies;
	} else {
		dj = round_up(rcu_idle_gp_delay + jiffies,
			       rcu_idle_gp_delay) - jiffies;
	}
	*nextevt = basemono + dj * TICK_NSEC;
	return 0;
}

/*
 * Prepare a CPU for idle from an RCU perspective.  The first major task
 * is to sense whether nohz mode has been enabled or disabled via sysfs.
 * The second major task is to check to see if a non-lazy callback has
 * arrived at a CPU that previously had only lazy callbacks.  The third
 * major task is to accelerate (that is, assign grace-period numbers to)
 * any recently arrived callbacks.
 *
 * The caller must have disabled interrupts.
 */
static void rcu_prepare_for_idle(void)
{
	bool needwake;
	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
	struct rcu_node *rnp;
	int tne;

	lockdep_assert_irqs_disabled();
	if (rcu_segcblist_is_offloaded(&rdp->cblist))
		return;

	/* Handle nohz enablement switches conservatively. */
	tne = READ_ONCE(tick_nohz_active);
	if (tne != rdp->tick_nohz_enabled_snap) {
		if (!rcu_segcblist_empty(&rdp->cblist))
			invoke_rcu_core(); /* force nohz to see update. */
		rdp->tick_nohz_enabled_snap = tne;
		return;
	}
	if (!tne)
		return;

	/*
	 * If a non-lazy callback arrived at a CPU having only lazy
	 * callbacks, invoke RCU core for the side-effect of recalculating
	 * idle duration on re-entry to idle.
	 */
	if (rdp->all_lazy && rcu_segcblist_n_nonlazy_cbs(&rdp->cblist)) {
		rdp->all_lazy = false;
		invoke_rcu_core();
		return;
	}

	/*
	 * If we have not yet accelerated this jiffy, accelerate all
	 * callbacks on this CPU.
	 */
	if (rdp->last_accelerate == jiffies)
		return;
	rdp->last_accelerate = jiffies;
	if (rcu_segcblist_pend_cbs(&rdp->cblist)) {
		rnp = rdp->mynode;
		raw_spin_lock_rcu_node(rnp); /* irqs already disabled. */
		needwake = rcu_accelerate_cbs(rnp, rdp);
		raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
		if (needwake)
			rcu_gp_kthread_wake();
	}
}

/*
 * Clean up for exit from idle.  Attempt to advance callbacks based on
 * any grace periods that elapsed while the CPU was idle, and if any
 * callbacks are now ready to invoke, initiate invocation.
 */
static void rcu_cleanup_after_idle(void)
{
	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);

	lockdep_assert_irqs_disabled();
	if (rcu_segcblist_is_offloaded(&rdp->cblist))
		return;
	if (rcu_try_advance_all_cbs())
		invoke_rcu_core();
}

#endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */

#ifdef CONFIG_RCU_NOCB_CPU

/*
 * 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.
 * The string after the "rcu_nocbs=" is either "all" for all CPUs, or a
 * comma-separated list of CPUs and/or CPU ranges.  If an invalid list is
 * given, 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 (!strcasecmp(str, "all"))
		cpumask_setall(rcu_nocb_mask);
	else
		if (cpulist_parse(str, rcu_nocb_mask)) {
			pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
			cpumask_setall(rcu_nocb_mask);
		}
	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.
 */
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)
{
	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)
{
	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_segcblist_is_offloaded(&rdp->cblist))
		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_segcblist_is_offloaded(&rdp->cblist)) {
		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_segcblist_is_offloaded(&rdp->cblist)) {
		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_segcblist_is_offloaded(&rdp->cblist) &&
	    cpu_online(rdp->cpu))
		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]);
}

/* Is the specified CPU a no-CBs CPU? */
bool rcu_is_nocb_cpu(int cpu)
{
	if (cpumask_available(rcu_nocb_mask))
		return cpumask_test_cpu(cpu, rcu_nocb_mask);
	return false;
}

/*
 * Kick the GP kthread for this NOCB group.  Caller holds ->nocb_lock
 * and this function releases it.
 */
static void wake_nocb_gp(struct rcu_data *rdp, bool force,
			   unsigned long flags)
	__releases(rdp->nocb_lock)
{
	bool needwake = false;
	struct rcu_data *rdp_gp = rdp->nocb_gp_rdp;

	lockdep_assert_held(&rdp->nocb_lock);
	if (!READ_ONCE(rdp_gp->nocb_gp_kthread)) {
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
				    TPS("AlreadyAwake"));
		rcu_nocb_unlock_irqrestore(rdp, flags);
		return;
	}
	del_timer(&rdp->nocb_timer);
	rcu_nocb_unlock_irqrestore(rdp, flags);
	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
	if (force || READ_ONCE(rdp_gp->nocb_gp_sleep)) {
		WRITE_ONCE(rdp_gp->nocb_gp_sleep, false);
		needwake = true;
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DoWake"));
	}
	raw_spin_unlock_irqrestore(&rdp_gp->nocb_gp_lock, flags);
	if (needwake)
		wake_up_process(rdp_gp->nocb_gp_kthread);
}

/*
 * 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)
{
	if (rdp->nocb_defer_wakeup == RCU_NOCB_WAKE_NOT)
		mod_timer(&rdp->nocb_timer, jiffies + 1);
	if (rdp->nocb_defer_wakeup < waketype)
		WRITE_ONCE(rdp->nocb_defer_wakeup, waketype);
	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.
 *
 * 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,
				     unsigned long j)
{
	struct rcu_cblist rcl;

	WARN_ON_ONCE(!rcu_segcblist_is_offloaded(&rdp->cblist));
	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. */
	rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp);
	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)
{
	if (!rcu_segcblist_is_offloaded(&rdp->cblist))
		return true;
	rcu_lockdep_assert_cblist_protected(rdp);
	rcu_nocb_bypass_lock(rdp);
	return rcu_nocb_do_flush_bypass(rdp, rhp, j);
}

/*
 * 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_segcblist_is_offloaded(&rdp->cblist) ||
	    !rcu_nocb_bypass_trylock(rdp))
		return;
	WARN_ON_ONCE(!rcu_nocb_do_flush_bypass(rdp, NULL, j));
}

/*
 * 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)
{
	unsigned long c;
	unsigned long cur_gp_seq;
	unsigned long j = jiffies;
	long ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);

	if (!rcu_segcblist_is_offloaded(&rdp->cblist)) {
		*was_alldone = !rcu_segcblist_pend_cbs(&rdp->cblist);
		return false; /* Not offloaded, no bypassing. */
	}
	lockdep_assert_irqs_disabled();

	// 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.
	if (rdp->nocb_nobypass_count < nocb_nobypass_lim_per_jiffy) {
		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));
		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 && j != READ_ONCE(rdp->nocb_bypass_first)) ||
	    ncbs >= qhimark) {
		rcu_nocb_lock(rdp);
		if (!rcu_nocb_flush_bypass(rdp, rhp, j)) {
			*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_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;
		}
		rcu_nocb_unlock_irqrestore(rdp, 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 (!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. */
	if (ncbs) {
		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 kthead 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)
{
	unsigned long cur_gp_seq;
	unsigned long j;
	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) {
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
				    TPS("WakeNotPoll"));
		rcu_nocb_unlock_irqrestore(rdp, flags);
		return;
	}
	// Need to actually to a wakeup.
	len = rcu_segcblist_n_cbs(&rdp->cblist);
	if (was_alldone) {
		rdp->qlen_last_fqs_check = len;
		if (!irqs_disabled_flags(flags)) {
			/* ... if queue was empty ... */
			wake_nocb_gp(rdp, false, flags);
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
					    TPS("WakeEmpty"));
		} else {
			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE,
					   TPS("WakeEmptyIsDeferred"));
			rcu_nocb_unlock_irqrestore(rdp, flags);
		}
	} 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_bypass_timer))
			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_FORCE,
					   TPS("WakeOvfIsDeferred"));
		rcu_nocb_unlock_irqrestore(rdp, flags);
	} else {
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WakeNot"));
		rcu_nocb_unlock_irqrestore(rdp, flags);
	}
	return;
}

/* Wake up the no-CBs GP kthread to flush ->nocb_bypass. */
static void do_nocb_bypass_wakeup_timer(struct timer_list *t)
{
	unsigned long flags;
	struct rcu_data *rdp = from_timer(rdp, t, nocb_bypass_timer);

	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Timer"));
	rcu_nocb_lock_irqsave(rdp, flags);
	smp_mb__after_spinlock(); /* Timer expire before wakeup. */
	__call_rcu_nocb_wake(rdp, true, flags);
}

/*
 * 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;
	long bypass_ncbs;
	int __maybe_unused cpu = my_rdp->cpu;
	unsigned long cur_gp_seq;
	unsigned long flags;
	bool gotcbs = false;
	unsigned long j = jiffies;
	bool needwait_gp = false; // This prevents actual uninitialized use.
	bool needwake;
	bool needwake_gp;
	struct rcu_data *rdp;
	struct rcu_node *rnp;
	unsigned long wait_gp_seq = 0; // Suppress "use uninitialized" warning.

	/*
	 * 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.
	 */
	for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_cb_rdp) {
		trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check"));
		rcu_nocb_lock_irqsave(rdp, flags);
		bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
		if (bypass_ncbs &&
		    (time_after(j, READ_ONCE(rdp->nocb_bypass_first) + 1) ||
		     bypass_ncbs > 2 * qhimark)) {
			// Bypass full or old, so flush it.
			(void)rcu_nocb_try_flush_bypass(rdp, j);
			bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
		} else if (!bypass_ncbs && rcu_segcblist_empty(&rdp->cblist)) {
			rcu_nocb_unlock_irqrestore(rdp, flags);
			continue; /* No callbacks here, try next. */
		}
		if (bypass_ncbs) {
			trace_rcu_nocb_wake(rcu_state.name, rdp->cpu,
					    TPS("Bypass"));
			bypass = true;
		}
		rnp = rdp->mynode;
		if (bypass) {  // Avoid race with first bypass CB.
			WRITE_ONCE(my_rdp->nocb_defer_wakeup,
				   RCU_NOCB_WAKE_NOT);
			del_timer(&my_rdp->nocb_timer);
		}
		// 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);
			raw_spin_unlock_rcu_node(rnp); /* irqs disabled. */
		}
		// Need to wait on some grace period?
		WARN_ON_ONCE(!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;
	if (bypass && !rcu_nocb_poll) {
		// At least one child with non-empty ->nocb_bypass, so set
		// timer in order to avoid stranding its callbacks.
		raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);
		mod_timer(&my_rdp->nocb_bypass_timer, j + 2);
		raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
	}
	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"));
		schedule_timeout_interruptible(1);
	} else if (!needwait_gp) {
		/* Wait for callbacks to appear. */
		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"));
	} 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);
		if (bypass)
			del_timer(&my_rdp->nocb_bypass_timer);
		WRITE_ONCE(my_rdp->nocb_gp_sleep, true);
		raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
	}
	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;
}

/*
 * 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)
{
	unsigned long cur_gp_seq;
	unsigned long flags;
	bool needwake_gp = false;
	struct rcu_node *rnp = rdp->mynode;

	local_irq_save(flags);
	rcu_momentary_dyntick_idle();
	local_irq_restore(flags);
	local_bh_disable();
	rcu_do_batch(rdp);
	local_bh_enable();
	lockdep_assert_irqs_enabled();
	rcu_nocb_lock_irqsave(rdp, flags);
	if (rcu_segcblist_nextgp(&rdp->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_ready_cbs(&rdp->cblist)) {
		rcu_nocb_unlock_irqrestore(rdp, flags);
		if (needwake_gp)
			rcu_gp_kthread_wake();
		return;
	}

	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("CBSleep"));
	WRITE_ONCE(rdp->nocb_cb_sleep, true);
	rcu_nocb_unlock_irqrestore(rdp, flags);
	if (needwake_gp)
		rcu_gp_kthread_wake();
	swait_event_interruptible_exclusive(rdp->nocb_cb_wq,
				 !READ_ONCE(rdp->nocb_cb_sleep));
	if (!smp_load_acquire(&rdp->nocb_cb_sleep)) { /* VVV */
		/* ^^^ Ensure CB invocation follows _sleep test. */
		return;
	}
	WARN_ON(signal_pending(current));
	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("WokeEmpty"));
}

/*
 * 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)
{
	return READ_ONCE(rdp->nocb_defer_wakeup);
}

/* Do a deferred wakeup of rcu_nocb_kthread(). */
static void do_nocb_deferred_wakeup_common(struct rcu_data *rdp)
{
	unsigned long flags;
	int ndw;

	rcu_nocb_lock_irqsave(rdp, flags);
	if (!rcu_nocb_need_deferred_wakeup(rdp)) {
		rcu_nocb_unlock_irqrestore(rdp, flags);
		return;
	}
	ndw = READ_ONCE(rdp->nocb_defer_wakeup);
	WRITE_ONCE(rdp->nocb_defer_wakeup, RCU_NOCB_WAKE_NOT);
	wake_nocb_gp(rdp, ndw == RCU_NOCB_WAKE_FORCE, flags);
	trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("DeferredWake"));
}

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

	do_nocb_deferred_wakeup_common(rdp);
}

/*
 * 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 void do_nocb_deferred_wakeup(struct rcu_data *rdp)
{
	if (rcu_nocb_need_deferred_wakeup(rdp))
		do_nocb_deferred_wakeup_common(rdp);
}

void __init rcu_init_nohz(void)
{
	int cpu;
	bool need_rcu_nocb_mask = false;
	struct rcu_data *rdp;

#if defined(CONFIG_NO_HZ_FULL)
	if (tick_nohz_full_running && cpumask_weight(tick_nohz_full_mask))
		need_rcu_nocb_mask = true;
#endif /* #if defined(CONFIG_NO_HZ_FULL) */

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

#if defined(CONFIG_NO_HZ_FULL)
	if (tick_nohz_full_running)
		cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
#endif /* #if defined(CONFIG_NO_HZ_FULL) */

	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);
	}
	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);
	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);
	timer_setup(&rdp->nocb_bypass_timer, do_nocb_bypass_wakeup_timer, 0);
	rcu_cblist_init(&rdp->nocb_bypass);
}

/*
 * 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_one_nocb_kthread(int cpu)
{
	struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
	struct rcu_data *rdp_gp;
	struct task_struct *t;

	/*
	 * If this isn't a no-CBs CPU or if it already has an rcuo kthread,
	 * then nothing to do.
	 */
	if (!rcu_is_nocb_cpu(cpu) || rdp->nocb_cb_kthread)
		return;

	/* If we didn't spawn the GP kthread first, reorganize! */
	rdp_gp = rdp->nocb_gp_rdp;
	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__))
			return;
		WRITE_ONCE(rdp_gp->nocb_gp_kthread, t);
	}

	/* 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__))
		return;
	WRITE_ONCE(rdp->nocb_cb_kthread, t);
	WRITE_ONCE(rdp->nocb_gp_kthread, rdp_gp->nocb_gp_kthread);
}

/*
 * If the specified CPU is a no-CBs CPU that does not already have its
 * rcuo kthread, spawn it.
 */
static void rcu_spawn_cpu_nocb_kthread(int cpu)
{
	if (rcu_scheduler_fully_active)
		rcu_spawn_one_nocb_kthread(cpu);
}

/*
 * Once the scheduler is running, spawn rcuo kthreads for all online
 * no-CBs CPUs.  This assumes that the early_initcall()s happen before
 * non-boot CPUs come online -- if this changes, we will need to add
 * some mutual exclusion.
 */
static void __init rcu_spawn_nocb_kthreads(void)
{
	int cpu;

	for_each_online_cpu(cpu)
		rcu_spawn_cpu_nocb_kthread(cpu);
}

/* 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. */
	struct rcu_data *rdp_prev = NULL;

	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_cpu(cpu, rcu_nocb_mask) {
		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->nocb_gp_rdp = rdp;
			rdp_gp = 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;
			rdp->nocb_gp_rdp = rdp_gp;
			rdp_prev->nocb_next_cb_rdp = rdp;
			if (dump_tree)
				pr_cont(" %d", cpu);
		}
		rdp_prev = 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_weight(rcu_nocb_mask))
		WARN_ON(sched_setaffinity(current->pid, rcu_nocb_mask));
}
EXPORT_SYMBOL_GPL(rcu_bind_current_to_nocb);

/*
 * 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%c:%ld rnp %d:%d %lu\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)],
		"bB"[timer_pending(&rdp->nocb_bypass_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));
}

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

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

	pr_info("   CB %d->%d %c%c%c%c%c%c F%ld L%ld C%d %c%c%c%c%c q%ld\n",
		rdp->cpu, rdp->nocb_gp_rdp->cpu,
		"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_restempty(rsclp, RCU_DONE_TAIL)],
		".R"[!rcu_segcblist_restempty(rsclp, RCU_WAIT_TAIL)],
		".N"[!rcu_segcblist_restempty(rsclp, RCU_NEXT_READY_TAIL)],
		".B"[!!rcu_cblist_n_cbs(&rdp->nocb_bypass)],
		rcu_segcblist_n_cbs(&rdp->cblist));

	/* 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);
	wastimer = timer_pending(&rdp->nocb_timer);
	wassleep = swait_active(&rdp->nocb_gp_wq);
	if (!rdp->nocb_defer_wakeup && !rdp->nocb_gp_sleep &&
	    !waslocked && !wastimer && !wassleep)
		return;  /* Nothing untowards. */

	pr_info("   !!! %c%c%c%c %c\n",
		"lL"[waslocked],
		"dD"[!!rdp->nocb_defer_wakeup],
		"tT"[wastimer],
		"sS"[!!rdp->nocb_gp_sleep],
		".W"[wassleep]);
}

#else /* #ifdef CONFIG_RCU_NOCB_CPU */

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

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

static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
{
}

static void rcu_spawn_cpu_nocb_kthread(int cpu)
{
}

static void __init rcu_spawn_nocb_kthreads(void)
{
}

static void show_rcu_nocb_state(struct rcu_data *rdp)
{
}

#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */

/*
 * Is this CPU a NO_HZ_FULL CPU that should ignore RCU so that the
 * grace-period kthread will do force_quiescent_state() processing?
 * The idea is to avoid waking up RCU core processing on such a
 * CPU unless the grace period has extended for too long.
 *
 * This code relies on the fact that all NO_HZ_FULL CPUs are also
 * CONFIG_RCU_NOCB_CPU CPUs.
 */
static bool rcu_nohz_full_cpu(void)
{
#ifdef CONFIG_NO_HZ_FULL
	if (tick_nohz_full_cpu(smp_processor_id()) &&
	    (!rcu_gp_in_progress() ||
	     ULONG_CMP_LT(jiffies, READ_ONCE(rcu_state.gp_start) + HZ)))
		return true;
#endif /* #ifdef CONFIG_NO_HZ_FULL */
	return false;
}

/*
 * Bind the RCU grace-period kthreads to the housekeeping CPU.
 */
static void rcu_bind_gp_kthread(void)
{
	if (!tick_nohz_full_enabled())
		return;
	housekeeping_affine(current, HK_FLAG_RCU);
}

/* Record the current task on dyntick-idle entry. */
static void rcu_dynticks_task_enter(void)
{
#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
	WRITE_ONCE(current->rcu_tasks_idle_cpu, smp_processor_id());
#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
}

/* Record no current task on dyntick-idle exit. */
static void rcu_dynticks_task_exit(void)
{
#if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL)
	WRITE_ONCE(current->rcu_tasks_idle_cpu, -1);
#endif /* #if defined(CONFIG_TASKS_RCU) && defined(CONFIG_NO_HZ_FULL) */
}