summaryrefslogtreecommitdiff
path: root/kernel/rcutiny_plugin.h
blob: 116725b5edfb9cbd5d618f15c445ca72db7e61f6 (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
/*
 * Read-Copy Update mechanism for mutual exclusion, the Bloatwatch edition
 * Internal non-public definitions that provide either classic
 * or preemptible semantics.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 *
 * Copyright (c) 2010 Linaro
 *
 * Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
 */

#include <linux/kthread.h>
#include <linux/module.h>
#include <linux/debugfs.h>
#include <linux/seq_file.h>

/* Global control variables for rcupdate callback mechanism. */
struct rcu_ctrlblk {
	struct rcu_head *rcucblist;	/* List of pending callbacks (CBs). */
	struct rcu_head **donetail;	/* ->next pointer of last "done" CB. */
	struct rcu_head **curtail;	/* ->next pointer of last CB. */
	RCU_TRACE(long qlen);		/* Number of pending CBs. */
	RCU_TRACE(char *name);		/* Name of RCU type. */
};

/* Definition for rcupdate control block. */
static struct rcu_ctrlblk rcu_sched_ctrlblk = {
	.donetail	= &rcu_sched_ctrlblk.rcucblist,
	.curtail	= &rcu_sched_ctrlblk.rcucblist,
	RCU_TRACE(.name = "rcu_sched")
};

static struct rcu_ctrlblk rcu_bh_ctrlblk = {
	.donetail	= &rcu_bh_ctrlblk.rcucblist,
	.curtail	= &rcu_bh_ctrlblk.rcucblist,
	RCU_TRACE(.name = "rcu_bh")
};

#ifdef CONFIG_DEBUG_LOCK_ALLOC
int rcu_scheduler_active __read_mostly;
EXPORT_SYMBOL_GPL(rcu_scheduler_active);
#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

#ifdef CONFIG_TINY_PREEMPT_RCU

#include <linux/delay.h>

/* Global control variables for preemptible RCU. */
struct rcu_preempt_ctrlblk {
	struct rcu_ctrlblk rcb;	/* curtail: ->next ptr of last CB for GP. */
	struct rcu_head **nexttail;
				/* Tasks blocked in a preemptible RCU */
				/*  read-side critical section while an */
				/*  preemptible-RCU grace period is in */
				/*  progress must wait for a later grace */
				/*  period.  This pointer points to the */
				/*  ->next pointer of the last task that */
				/*  must wait for a later grace period, or */
				/*  to &->rcb.rcucblist if there is no */
				/*  such task. */
	struct list_head blkd_tasks;
				/* Tasks blocked in RCU read-side critical */
				/*  section.  Tasks are placed at the head */
				/*  of this list and age towards the tail. */
	struct list_head *gp_tasks;
				/* Pointer to the first task blocking the */
				/*  current grace period, or NULL if there */
				/*  is no such task. */
	struct list_head *exp_tasks;
				/* Pointer to first task blocking the */
				/*  current expedited grace period, or NULL */
				/*  if there is no such task.  If there */
				/*  is no current expedited grace period, */
				/*  then there cannot be any such task. */
#ifdef CONFIG_RCU_BOOST
	struct list_head *boost_tasks;
				/* Pointer to first task that needs to be */
				/*  priority-boosted, or NULL if no priority */
				/*  boosting is needed.  If there is no */
				/*  current or expedited grace period, there */
				/*  can be no such task. */
#endif /* #ifdef CONFIG_RCU_BOOST */
	u8 gpnum;		/* Current grace period. */
	u8 gpcpu;		/* Last grace period blocked by the CPU. */
	u8 completed;		/* Last grace period completed. */
				/*  If all three are equal, RCU is idle. */
#ifdef CONFIG_RCU_BOOST
	unsigned long boost_time; /* When to start boosting (jiffies) */
#endif /* #ifdef CONFIG_RCU_BOOST */
#ifdef CONFIG_RCU_TRACE
	unsigned long n_grace_periods;
#ifdef CONFIG_RCU_BOOST
	unsigned long n_tasks_boosted;
				/* Total number of tasks boosted. */
	unsigned long n_exp_boosts;
				/* Number of tasks boosted for expedited GP. */
	unsigned long n_normal_boosts;
				/* Number of tasks boosted for normal GP. */
	unsigned long n_balk_blkd_tasks;
				/* Refused to boost: no blocked tasks. */
	unsigned long n_balk_exp_gp_tasks;
				/* Refused to boost: nothing blocking GP. */
	unsigned long n_balk_boost_tasks;
				/* Refused to boost: already boosting. */
	unsigned long n_balk_notyet;
				/* Refused to boost: not yet time. */
	unsigned long n_balk_nos;
				/* Refused to boost: not sure why, though. */
				/*  This can happen due to race conditions. */
#endif /* #ifdef CONFIG_RCU_BOOST */
#endif /* #ifdef CONFIG_RCU_TRACE */
};

static struct rcu_preempt_ctrlblk rcu_preempt_ctrlblk = {
	.rcb.donetail = &rcu_preempt_ctrlblk.rcb.rcucblist,
	.rcb.curtail = &rcu_preempt_ctrlblk.rcb.rcucblist,
	.nexttail = &rcu_preempt_ctrlblk.rcb.rcucblist,
	.blkd_tasks = LIST_HEAD_INIT(rcu_preempt_ctrlblk.blkd_tasks),
	RCU_TRACE(.rcb.name = "rcu_preempt")
};

static int rcu_preempted_readers_exp(void);
static void rcu_report_exp_done(void);

/*
 * Return true if the CPU has not yet responded to the current grace period.
 */
static int rcu_cpu_blocking_cur_gp(void)
{
	return rcu_preempt_ctrlblk.gpcpu != rcu_preempt_ctrlblk.gpnum;
}

/*
 * Check for a running RCU reader.  Because there is only one CPU,
 * there can be but one running RCU reader at a time.  ;-)
 *
 * Returns zero if there are no running readers.  Returns a positive
 * number if there is at least one reader within its RCU read-side
 * critical section.  Returns a negative number if an outermost reader
 * is in the midst of exiting from its RCU read-side critical section
 *
 * Returns zero if there are no running readers.  Returns a positive
 * number if there is at least one reader within its RCU read-side
 * critical section.  Returns a negative number if an outermost reader
 * is in the midst of exiting from its RCU read-side critical section.
 */
static int rcu_preempt_running_reader(void)
{
	return current->rcu_read_lock_nesting;
}

/*
 * Check for preempted RCU readers blocking any grace period.
 * If the caller needs a reliable answer, it must disable hard irqs.
 */
static int rcu_preempt_blocked_readers_any(void)
{
	return !list_empty(&rcu_preempt_ctrlblk.blkd_tasks);
}

/*
 * Check for preempted RCU readers blocking the current grace period.
 * If the caller needs a reliable answer, it must disable hard irqs.
 */
static int rcu_preempt_blocked_readers_cgp(void)
{
	return rcu_preempt_ctrlblk.gp_tasks != NULL;
}

/*
 * Return true if another preemptible-RCU grace period is needed.
 */
static int rcu_preempt_needs_another_gp(void)
{
	return *rcu_preempt_ctrlblk.rcb.curtail != NULL;
}

/*
 * Return true if a preemptible-RCU grace period is in progress.
 * The caller must disable hardirqs.
 */
static int rcu_preempt_gp_in_progress(void)
{
	return rcu_preempt_ctrlblk.completed != rcu_preempt_ctrlblk.gpnum;
}

/*
 * 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 list_head *np;

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

#ifdef CONFIG_RCU_TRACE

#ifdef CONFIG_RCU_BOOST
static void rcu_initiate_boost_trace(void);
#endif /* #ifdef CONFIG_RCU_BOOST */

/*
 * Dump additional statistice for TINY_PREEMPT_RCU.
 */
static void show_tiny_preempt_stats(struct seq_file *m)
{
	seq_printf(m, "rcu_preempt: qlen=%ld gp=%lu g%u/p%u/c%u tasks=%c%c%c\n",
		   rcu_preempt_ctrlblk.rcb.qlen,
		   rcu_preempt_ctrlblk.n_grace_periods,
		   rcu_preempt_ctrlblk.gpnum,
		   rcu_preempt_ctrlblk.gpcpu,
		   rcu_preempt_ctrlblk.completed,
		   "T."[list_empty(&rcu_preempt_ctrlblk.blkd_tasks)],
		   "N."[!rcu_preempt_ctrlblk.gp_tasks],
		   "E."[!rcu_preempt_ctrlblk.exp_tasks]);
#ifdef CONFIG_RCU_BOOST
	seq_printf(m, "%sttb=%c ntb=%lu neb=%lu nnb=%lu j=%04x bt=%04x\n",
		   "             ",
		   "B."[!rcu_preempt_ctrlblk.boost_tasks],
		   rcu_preempt_ctrlblk.n_tasks_boosted,
		   rcu_preempt_ctrlblk.n_exp_boosts,
		   rcu_preempt_ctrlblk.n_normal_boosts,
		   (int)(jiffies & 0xffff),
		   (int)(rcu_preempt_ctrlblk.boost_time & 0xffff));
	seq_printf(m, "%s: nt=%lu egt=%lu bt=%lu ny=%lu nos=%lu\n",
		   "             balk",
		   rcu_preempt_ctrlblk.n_balk_blkd_tasks,
		   rcu_preempt_ctrlblk.n_balk_exp_gp_tasks,
		   rcu_preempt_ctrlblk.n_balk_boost_tasks,
		   rcu_preempt_ctrlblk.n_balk_notyet,
		   rcu_preempt_ctrlblk.n_balk_nos);
#endif /* #ifdef CONFIG_RCU_BOOST */
}

#endif /* #ifdef CONFIG_RCU_TRACE */

#ifdef CONFIG_RCU_BOOST

#include "rtmutex_common.h"

#define RCU_BOOST_PRIO CONFIG_RCU_BOOST_PRIO

/* Controls for rcu_kthread() kthread. */
static struct task_struct *rcu_kthread_task;
static DECLARE_WAIT_QUEUE_HEAD(rcu_kthread_wq);
static unsigned long have_rcu_kthread_work;

/*
 * Carry out RCU priority boosting on the task indicated by ->boost_tasks,
 * and advance ->boost_tasks to the next task in the ->blkd_tasks list.
 */
static int rcu_boost(void)
{
	unsigned long flags;
	struct rt_mutex mtx;
	struct task_struct *t;
	struct list_head *tb;

	if (rcu_preempt_ctrlblk.boost_tasks == NULL &&
	    rcu_preempt_ctrlblk.exp_tasks == NULL)
		return 0;  /* Nothing to boost. */

	raw_local_irq_save(flags);

	/*
	 * Recheck with irqs disabled: all tasks in need of boosting
	 * might exit their RCU read-side critical sections on their own
	 * if we are preempted just before disabling irqs.
	 */
	if (rcu_preempt_ctrlblk.boost_tasks == NULL &&
	    rcu_preempt_ctrlblk.exp_tasks == NULL) {
		raw_local_irq_restore(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 (rcu_preempt_ctrlblk.exp_tasks != NULL) {
		tb = rcu_preempt_ctrlblk.exp_tasks;
		RCU_TRACE(rcu_preempt_ctrlblk.n_exp_boosts++);
	} else {
		tb = rcu_preempt_ctrlblk.boost_tasks;
		RCU_TRACE(rcu_preempt_ctrlblk.n_normal_boosts++);
	}
	RCU_TRACE(rcu_preempt_ctrlblk.n_tasks_boosted++);

	/*
	 * 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!)
	 */
	t = container_of(tb, struct task_struct, rcu_node_entry);
	rt_mutex_init_proxy_locked(&mtx, t);
	t->rcu_boost_mutex = &mtx;
	raw_local_irq_restore(flags);
	rt_mutex_lock(&mtx);
	rt_mutex_unlock(&mtx);  /* Keep lockdep happy. */

	return ACCESS_ONCE(rcu_preempt_ctrlblk.boost_tasks) != NULL ||
	       ACCESS_ONCE(rcu_preempt_ctrlblk.exp_tasks) != NULL;
}

/*
 * Check to see if it is now time to start boosting RCU readers blocking
 * the current grace period, and, if so, tell the rcu_kthread_task to
 * start boosting them.  If there is an expedited boost in progress,
 * we wait for it to complete.
 *
 * If there are no blocked readers blocking the current grace period,
 * return 0 to let the caller know, otherwise return 1.  Note that this
 * return value is independent of whether or not boosting was done.
 */
static int rcu_initiate_boost(void)
{
	if (!rcu_preempt_blocked_readers_cgp() &&
	    rcu_preempt_ctrlblk.exp_tasks == NULL) {
		RCU_TRACE(rcu_preempt_ctrlblk.n_balk_exp_gp_tasks++);
		return 0;
	}
	if (rcu_preempt_ctrlblk.exp_tasks != NULL ||
	    (rcu_preempt_ctrlblk.gp_tasks != NULL &&
	     rcu_preempt_ctrlblk.boost_tasks == NULL &&
	     ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))) {
		if (rcu_preempt_ctrlblk.exp_tasks == NULL)
			rcu_preempt_ctrlblk.boost_tasks =
				rcu_preempt_ctrlblk.gp_tasks;
		invoke_rcu_callbacks();
	} else
		RCU_TRACE(rcu_initiate_boost_trace());
	return 1;
}

#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(void)
{
	rcu_preempt_ctrlblk.boost_time = jiffies + RCU_BOOST_DELAY_JIFFIES;
}

#else /* #ifdef CONFIG_RCU_BOOST */

/*
 * If there is no RCU priority boosting, we don't initiate boosting,
 * but we do indicate whether there are blocked readers blocking the
 * current grace period.
 */
static int rcu_initiate_boost(void)
{
	return rcu_preempt_blocked_readers_cgp();
}

/*
 * If there is no RCU priority boosting, nothing to do at grace-period start.
 */
static void rcu_preempt_boost_start_gp(void)
{
}

#endif /* else #ifdef CONFIG_RCU_BOOST */

/*
 * Record a preemptible-RCU quiescent state for the specified CPU.  Note
 * that this just means that the task currently running on the CPU is
 * in a quiescent state.  There might be any number of tasks blocked
 * while in an RCU read-side critical section.
 *
 * Unlike the other rcu_*_qs() functions, callers to this function
 * must disable irqs in order to protect the assignment to
 * ->rcu_read_unlock_special.
 *
 * Because this is a single-CPU implementation, the only way a grace
 * period can end is if the CPU is in a quiescent state.  The reason is
 * that a blocked preemptible-RCU reader can exit its critical section
 * only if the CPU is running it at the time.  Therefore, when the
 * last task blocking the current grace period exits its RCU read-side
 * critical section, neither the CPU nor blocked tasks will be stopping
 * the current grace period.  (In contrast, SMP implementations
 * might have CPUs running in RCU read-side critical sections that
 * block later grace periods -- but this is not possible given only
 * one CPU.)
 */
static void rcu_preempt_cpu_qs(void)
{
	/* Record both CPU and task as having responded to current GP. */
	rcu_preempt_ctrlblk.gpcpu = rcu_preempt_ctrlblk.gpnum;
	current->rcu_read_unlock_special &= ~RCU_READ_UNLOCK_NEED_QS;

	/* If there is no GP then there is nothing more to do.  */
	if (!rcu_preempt_gp_in_progress())
		return;
	/*
	 * Check up on boosting.  If there are readers blocking the
	 * current grace period, leave.
	 */
	if (rcu_initiate_boost())
		return;

	/* Advance callbacks. */
	rcu_preempt_ctrlblk.completed = rcu_preempt_ctrlblk.gpnum;
	rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.rcb.curtail;
	rcu_preempt_ctrlblk.rcb.curtail = rcu_preempt_ctrlblk.nexttail;

	/* If there are no blocked readers, next GP is done instantly. */
	if (!rcu_preempt_blocked_readers_any())
		rcu_preempt_ctrlblk.rcb.donetail = rcu_preempt_ctrlblk.nexttail;

	/* If there are done callbacks, cause them to be invoked. */
	if (*rcu_preempt_ctrlblk.rcb.donetail != NULL)
		invoke_rcu_callbacks();
}

/*
 * Start a new RCU grace period if warranted.  Hard irqs must be disabled.
 */
static void rcu_preempt_start_gp(void)
{
	if (!rcu_preempt_gp_in_progress() && rcu_preempt_needs_another_gp()) {

		/* Official start of GP. */
		rcu_preempt_ctrlblk.gpnum++;
		RCU_TRACE(rcu_preempt_ctrlblk.n_grace_periods++);

		/* Any blocked RCU readers block new GP. */
		if (rcu_preempt_blocked_readers_any())
			rcu_preempt_ctrlblk.gp_tasks =
				rcu_preempt_ctrlblk.blkd_tasks.next;

		/* Set up for RCU priority boosting. */
		rcu_preempt_boost_start_gp();

		/* If there is no running reader, CPU is done with GP. */
		if (!rcu_preempt_running_reader())
			rcu_preempt_cpu_qs();
	}
}

/*
 * 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.
 * If the task started after the current grace period began, as recorded
 * by ->gpcpu, we enqueue at the beginning of the list.  Otherwise
 * before the element referenced by ->gp_tasks (or at the tail if
 * ->gp_tasks is NULL) and point ->gp_tasks at the newly added element.
 * 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 ->gp_tasks pointer becomes
 * NULL.
 *
 * Caller must disable preemption.
 */
void rcu_preempt_note_context_switch(void)
{
	struct task_struct *t = current;
	unsigned long flags;

	local_irq_save(flags); /* must exclude scheduler_tick(). */
	if (rcu_preempt_running_reader() > 0 &&
	    (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {

		/* Possibly blocking in an RCU read-side critical section. */
		t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;

		/*
		 * If this CPU has already checked in, then this task
		 * will hold up the next grace period rather than the
		 * current grace period.  Queue the task accordingly.
		 * If the task is queued for the current grace period
		 * (i.e., this CPU has not yet passed through a quiescent
		 * state for the current grace period), then as long
		 * as that task remains queued, the current grace period
		 * cannot end.
		 */
		list_add(&t->rcu_node_entry, &rcu_preempt_ctrlblk.blkd_tasks);
		if (rcu_cpu_blocking_cur_gp())
			rcu_preempt_ctrlblk.gp_tasks = &t->rcu_node_entry;
	} else if (rcu_preempt_running_reader() < 0 &&
		   t->rcu_read_unlock_special) {
		/*
		 * Complete exit from RCU read-side critical section on
		 * behalf of preempted instance of __rcu_read_unlock().
		 */
		rcu_read_unlock_special(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 current grace period continues to be blocked.
	 */
	rcu_preempt_cpu_qs();
	local_irq_restore(flags);
}

/*
 * 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.
 */
void rcu_read_unlock_special(struct task_struct *t)
{
	int empty;
	int empty_exp;
	unsigned long flags;
	struct list_head *np;
#ifdef CONFIG_RCU_BOOST
	struct rt_mutex *rbmp = NULL;
#endif /* #ifdef CONFIG_RCU_BOOST */
	int special;

	/*
	 * NMI handlers cannot block and cannot safely manipulate state.
	 * They therefore cannot possibly be special, so just leave.
	 */
	if (in_nmi())
		return;

	local_irq_save(flags);

	/*
	 * If RCU core is waiting for this CPU to exit critical section,
	 * let it know that we have done so.
	 */
	special = t->rcu_read_unlock_special;
	if (special & RCU_READ_UNLOCK_NEED_QS)
		rcu_preempt_cpu_qs();

	/* Hardware IRQ handlers cannot block. */
	if (in_irq() || in_serving_softirq()) {
		local_irq_restore(flags);
		return;
	}

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

		/*
		 * Remove this task from the ->blkd_tasks list and adjust
		 * any pointers that might have been referencing it.
		 */
		empty = !rcu_preempt_blocked_readers_cgp();
		empty_exp = rcu_preempt_ctrlblk.exp_tasks == NULL;
		np = rcu_next_node_entry(t);
		list_del_init(&t->rcu_node_entry);
		if (&t->rcu_node_entry == rcu_preempt_ctrlblk.gp_tasks)
			rcu_preempt_ctrlblk.gp_tasks = np;
		if (&t->rcu_node_entry == rcu_preempt_ctrlblk.exp_tasks)
			rcu_preempt_ctrlblk.exp_tasks = np;
#ifdef CONFIG_RCU_BOOST
		if (&t->rcu_node_entry == rcu_preempt_ctrlblk.boost_tasks)
			rcu_preempt_ctrlblk.boost_tasks = np;
#endif /* #ifdef CONFIG_RCU_BOOST */

		/*
		 * If this was the last task on the current list, and if
		 * we aren't waiting on the CPU, report the quiescent state
		 * and start a new grace period if needed.
		 */
		if (!empty && !rcu_preempt_blocked_readers_cgp()) {
			rcu_preempt_cpu_qs();
			rcu_preempt_start_gp();
		}

		/*
		 * If this was the last task on the expedited lists,
		 * then we need wake up the waiting task.
		 */
		if (!empty_exp && rcu_preempt_ctrlblk.exp_tasks == NULL)
			rcu_report_exp_done();
	}
#ifdef CONFIG_RCU_BOOST
	/* Unboost self if was boosted. */
	if (t->rcu_boost_mutex != NULL) {
		rbmp = t->rcu_boost_mutex;
		t->rcu_boost_mutex = NULL;
		rt_mutex_unlock(rbmp);
	}
#endif /* #ifdef CONFIG_RCU_BOOST */
	local_irq_restore(flags);
}

/*
 * Check for a quiescent state from the current CPU.  When a task blocks,
 * the task is recorded in the rcu_preempt_ctrlblk structure, which is
 * checked elsewhere.  This is called from the scheduling-clock interrupt.
 *
 * Caller must disable hard irqs.
 */
static void rcu_preempt_check_callbacks(void)
{
	struct task_struct *t = current;

	if (rcu_preempt_gp_in_progress() &&
	    (!rcu_preempt_running_reader() ||
	     !rcu_cpu_blocking_cur_gp()))
		rcu_preempt_cpu_qs();
	if (&rcu_preempt_ctrlblk.rcb.rcucblist !=
	    rcu_preempt_ctrlblk.rcb.donetail)
		invoke_rcu_callbacks();
	if (rcu_preempt_gp_in_progress() &&
	    rcu_cpu_blocking_cur_gp() &&
	    rcu_preempt_running_reader() > 0)
		t->rcu_read_unlock_special |= RCU_READ_UNLOCK_NEED_QS;
}

/*
 * TINY_PREEMPT_RCU has an extra callback-list tail pointer to
 * update, so this is invoked from rcu_process_callbacks() to
 * handle that case.  Of course, it is invoked for all flavors of
 * RCU, but RCU callbacks can appear only on one of the lists, and
 * neither ->nexttail nor ->donetail can possibly be NULL, so there
 * is no need for an explicit check.
 */
static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
{
	if (rcu_preempt_ctrlblk.nexttail == rcp->donetail)
		rcu_preempt_ctrlblk.nexttail = &rcp->rcucblist;
}

/*
 * Process callbacks for preemptible RCU.
 */
static void rcu_preempt_process_callbacks(void)
{
	__rcu_process_callbacks(&rcu_preempt_ctrlblk.rcb);
}

/*
 * Queue a preemptible -RCU callback for invocation after a grace period.
 */
void call_rcu(struct rcu_head *head, void (*func)(struct rcu_head *rcu))
{
	unsigned long flags;

	debug_rcu_head_queue(head);
	head->func = func;
	head->next = NULL;

	local_irq_save(flags);
	*rcu_preempt_ctrlblk.nexttail = head;
	rcu_preempt_ctrlblk.nexttail = &head->next;
	RCU_TRACE(rcu_preempt_ctrlblk.rcb.qlen++);
	rcu_preempt_start_gp();  /* checks to see if GP needed. */
	local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(call_rcu);

/*
 * synchronize_rcu - wait until a grace period has elapsed.
 *
 * Control will return to the caller some time after a full grace
 * period has elapsed, in other words after all currently executing RCU
 * read-side critical sections have completed.  RCU read-side critical
 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 * and may be nested.
 */
void synchronize_rcu(void)
{
	rcu_lockdep_assert(!lock_is_held(&rcu_bh_lock_map) &&
			   !lock_is_held(&rcu_lock_map) &&
			   !lock_is_held(&rcu_sched_lock_map),
			   "Illegal synchronize_rcu() in RCU read-side critical section");

#ifdef CONFIG_DEBUG_LOCK_ALLOC
	if (!rcu_scheduler_active)
		return;
#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

	WARN_ON_ONCE(rcu_preempt_running_reader());
	if (!rcu_preempt_blocked_readers_any())
		return;

	/* Once we get past the fastpath checks, same code as rcu_barrier(). */
	rcu_barrier();
}
EXPORT_SYMBOL_GPL(synchronize_rcu);

static DECLARE_WAIT_QUEUE_HEAD(sync_rcu_preempt_exp_wq);
static unsigned long sync_rcu_preempt_exp_count;
static DEFINE_MUTEX(sync_rcu_preempt_exp_mutex);

/*
 * Return non-zero if there are any tasks in RCU read-side critical
 * sections blocking the current preemptible-RCU expedited grace period.
 * If there is no preemptible-RCU expedited grace period currently in
 * progress, returns zero unconditionally.
 */
static int rcu_preempted_readers_exp(void)
{
	return rcu_preempt_ctrlblk.exp_tasks != NULL;
}

/*
 * Report the exit from RCU read-side critical section for the last task
 * that queued itself during or before the current expedited preemptible-RCU
 * grace period.
 */
static void rcu_report_exp_done(void)
{
	wake_up(&sync_rcu_preempt_exp_wq);
}

/*
 * Wait for an rcu-preempt grace period, but expedite it.  The basic idea
 * is to rely in the fact that there is but one CPU, and that it is
 * illegal for a task to invoke synchronize_rcu_expedited() while in a
 * preemptible-RCU read-side critical section.  Therefore, any such
 * critical sections must correspond to blocked tasks, which must therefore
 * be on the ->blkd_tasks list.  So just record the current head of the
 * list in the ->exp_tasks pointer, and wait for all tasks including and
 * after the task pointed to by ->exp_tasks to drain.
 */
void synchronize_rcu_expedited(void)
{
	unsigned long flags;
	struct rcu_preempt_ctrlblk *rpcp = &rcu_preempt_ctrlblk;
	unsigned long snap;

	barrier(); /* ensure prior action seen before grace period. */

	WARN_ON_ONCE(rcu_preempt_running_reader());

	/*
	 * Acquire lock so that there is only one preemptible RCU grace
	 * period in flight.  Of course, if someone does the expedited
	 * grace period for us while we are acquiring the lock, just leave.
	 */
	snap = sync_rcu_preempt_exp_count + 1;
	mutex_lock(&sync_rcu_preempt_exp_mutex);
	if (ULONG_CMP_LT(snap, sync_rcu_preempt_exp_count))
		goto unlock_mb_ret; /* Others did our work for us. */

	local_irq_save(flags);

	/*
	 * All RCU readers have to already be on blkd_tasks because
	 * we cannot legally be executing in an RCU read-side critical
	 * section.
	 */

	/* Snapshot current head of ->blkd_tasks list. */
	rpcp->exp_tasks = rpcp->blkd_tasks.next;
	if (rpcp->exp_tasks == &rpcp->blkd_tasks)
		rpcp->exp_tasks = NULL;

	/* Wait for tail of ->blkd_tasks list to drain. */
	if (!rcu_preempted_readers_exp())
		local_irq_restore(flags);
	else {
		rcu_initiate_boost();
		local_irq_restore(flags);
		wait_event(sync_rcu_preempt_exp_wq,
			   !rcu_preempted_readers_exp());
	}

	/* Clean up and exit. */
	barrier(); /* ensure expedited GP seen before counter increment. */
	sync_rcu_preempt_exp_count++;
unlock_mb_ret:
	mutex_unlock(&sync_rcu_preempt_exp_mutex);
	barrier(); /* ensure subsequent action seen after grace period. */
}
EXPORT_SYMBOL_GPL(synchronize_rcu_expedited);

/*
 * Does preemptible RCU need the CPU to stay out of dynticks mode?
 */
int rcu_preempt_needs_cpu(void)
{
	return rcu_preempt_ctrlblk.rcb.rcucblist != NULL;
}

#else /* #ifdef CONFIG_TINY_PREEMPT_RCU */

#ifdef CONFIG_RCU_TRACE

/*
 * Because preemptible RCU does not exist, it is not necessary to
 * dump out its statistics.
 */
static void show_tiny_preempt_stats(struct seq_file *m)
{
}

#endif /* #ifdef CONFIG_RCU_TRACE */

/*
 * Because preemptible RCU does not exist, it never has any callbacks
 * to check.
 */
static void rcu_preempt_check_callbacks(void)
{
}

/*
 * Because preemptible RCU does not exist, it never has any callbacks
 * to remove.
 */
static void rcu_preempt_remove_callbacks(struct rcu_ctrlblk *rcp)
{
}

/*
 * Because preemptible RCU does not exist, it never has any callbacks
 * to process.
 */
static void rcu_preempt_process_callbacks(void)
{
}

#endif /* #else #ifdef CONFIG_TINY_PREEMPT_RCU */

#ifdef CONFIG_RCU_BOOST

/*
 * Wake up rcu_kthread() to process callbacks now eligible for invocation
 * or to boost readers.
 */
static void invoke_rcu_callbacks(void)
{
	have_rcu_kthread_work = 1;
	if (rcu_kthread_task != NULL)
		wake_up(&rcu_kthread_wq);
}

#ifdef CONFIG_RCU_TRACE

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

#endif /* #ifdef CONFIG_RCU_TRACE */

/*
 * This kthread invokes RCU callbacks whose grace periods have
 * elapsed.  It is awakened as needed, and takes the place of the
 * RCU_SOFTIRQ that is used for this purpose when boosting is disabled.
 * This is a kthread, but it is never stopped, at least not until
 * the system goes down.
 */
static int rcu_kthread(void *arg)
{
	unsigned long work;
	unsigned long morework;
	unsigned long flags;

	for (;;) {
		wait_event_interruptible(rcu_kthread_wq,
					 have_rcu_kthread_work != 0);
		morework = rcu_boost();
		local_irq_save(flags);
		work = have_rcu_kthread_work;
		have_rcu_kthread_work = morework;
		local_irq_restore(flags);
		if (work)
			rcu_process_callbacks(NULL);
		schedule_timeout_interruptible(1); /* Leave CPU for others. */
	}

	return 0;  /* Not reached, but needed to shut gcc up. */
}

/*
 * Spawn the kthread that invokes RCU callbacks.
 */
static int __init rcu_spawn_kthreads(void)
{
	struct sched_param sp;

	rcu_kthread_task = kthread_run(rcu_kthread, NULL, "rcu_kthread");
	sp.sched_priority = RCU_BOOST_PRIO;
	sched_setscheduler_nocheck(rcu_kthread_task, SCHED_FIFO, &sp);
	return 0;
}
early_initcall(rcu_spawn_kthreads);

#else /* #ifdef CONFIG_RCU_BOOST */

/* Hold off callback invocation until early_initcall() time. */
static int rcu_scheduler_fully_active __read_mostly;

/*
 * Start up softirq processing of callbacks.
 */
void invoke_rcu_callbacks(void)
{
	if (rcu_scheduler_fully_active)
		raise_softirq(RCU_SOFTIRQ);
}

#ifdef CONFIG_RCU_TRACE

/*
 * There is no callback kthread, so this thread is never it.
 */
static bool rcu_is_callbacks_kthread(void)
{
	return false;
}

#endif /* #ifdef CONFIG_RCU_TRACE */

static int __init rcu_scheduler_really_started(void)
{
	rcu_scheduler_fully_active = 1;
	open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
	raise_softirq(RCU_SOFTIRQ);  /* Invoke any callbacks from early boot. */
	return 0;
}
early_initcall(rcu_scheduler_really_started);

#endif /* #else #ifdef CONFIG_RCU_BOOST */

#ifdef CONFIG_DEBUG_LOCK_ALLOC
#include <linux/kernel_stat.h>

/*
 * During boot, we forgive RCU lockdep issues.  After this function is
 * invoked, we start taking RCU lockdep issues seriously.
 */
void __init rcu_scheduler_starting(void)
{
	WARN_ON(nr_context_switches() > 0);
	rcu_scheduler_active = 1;
}

#endif /* #ifdef CONFIG_DEBUG_LOCK_ALLOC */

#ifdef CONFIG_RCU_TRACE

#ifdef CONFIG_RCU_BOOST

static void rcu_initiate_boost_trace(void)
{
	if (list_empty(&rcu_preempt_ctrlblk.blkd_tasks))
		rcu_preempt_ctrlblk.n_balk_blkd_tasks++;
	else if (rcu_preempt_ctrlblk.gp_tasks == NULL &&
		 rcu_preempt_ctrlblk.exp_tasks == NULL)
		rcu_preempt_ctrlblk.n_balk_exp_gp_tasks++;
	else if (rcu_preempt_ctrlblk.boost_tasks != NULL)
		rcu_preempt_ctrlblk.n_balk_boost_tasks++;
	else if (!ULONG_CMP_GE(jiffies, rcu_preempt_ctrlblk.boost_time))
		rcu_preempt_ctrlblk.n_balk_notyet++;
	else
		rcu_preempt_ctrlblk.n_balk_nos++;
}

#endif /* #ifdef CONFIG_RCU_BOOST */

static void rcu_trace_sub_qlen(struct rcu_ctrlblk *rcp, int n)
{
	unsigned long flags;

	raw_local_irq_save(flags);
	rcp->qlen -= n;
	raw_local_irq_restore(flags);
}

/*
 * Dump statistics for TINY_RCU, such as they are.
 */
static int show_tiny_stats(struct seq_file *m, void *unused)
{
	show_tiny_preempt_stats(m);
	seq_printf(m, "rcu_sched: qlen: %ld\n", rcu_sched_ctrlblk.qlen);
	seq_printf(m, "rcu_bh: qlen: %ld\n", rcu_bh_ctrlblk.qlen);
	return 0;
}

static int show_tiny_stats_open(struct inode *inode, struct file *file)
{
	return single_open(file, show_tiny_stats, NULL);
}

static const struct file_operations show_tiny_stats_fops = {
	.owner = THIS_MODULE,
	.open = show_tiny_stats_open,
	.read = seq_read,
	.llseek = seq_lseek,
	.release = single_release,
};

static struct dentry *rcudir;

static int __init rcutiny_trace_init(void)
{
	struct dentry *retval;

	rcudir = debugfs_create_dir("rcu", NULL);
	if (!rcudir)
		goto free_out;
	retval = debugfs_create_file("rcudata", 0444, rcudir,
				     NULL, &show_tiny_stats_fops);
	if (!retval)
		goto free_out;
	return 0;
free_out:
	debugfs_remove_recursive(rcudir);
	return 1;
}

static void __exit rcutiny_trace_cleanup(void)
{
	debugfs_remove_recursive(rcudir);
}

module_init(rcutiny_trace_init);
module_exit(rcutiny_trace_cleanup);

MODULE_AUTHOR("Paul E. McKenney");
MODULE_DESCRIPTION("Read-Copy Update tracing for tiny implementation");
MODULE_LICENSE("GPL");

#endif /* #ifdef CONFIG_RCU_TRACE */