aboutsummaryrefslogtreecommitdiffstats
path: root/kernel/slow-work.c
blob: 00889bd3c5903812ed89d722135b15330b513dbb (plain) (blame)
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
/* Worker thread pool for slow items, such as filesystem lookups or mkdirs
 *
 * Copyright (C) 2008 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 *
 * See Documentation/slow-work.txt
 */

#include <linux/module.h>
#include <linux/slow-work.h>
#include <linux/kthread.h>
#include <linux/freezer.h>
#include <linux/wait.h>
#include <linux/debugfs.h>
#include "slow-work.h"

static void slow_work_cull_timeout(unsigned long);
static void slow_work_oom_timeout(unsigned long);

#ifdef CONFIG_SYSCTL
static int slow_work_min_threads_sysctl(struct ctl_table *, int,
					void __user *, size_t *, loff_t *);

static int slow_work_max_threads_sysctl(struct ctl_table *, int ,
					void __user *, size_t *, loff_t *);
#endif

/*
 * The pool of threads has at least min threads in it as long as someone is
 * using the facility, and may have as many as max.
 *
 * A portion of the pool may be processing very slow operations.
 */
static unsigned slow_work_min_threads = 2;
static unsigned slow_work_max_threads = 4;
static unsigned vslow_work_proportion = 50; /* % of threads that may process
					     * very slow work */

#ifdef CONFIG_SYSCTL
static const int slow_work_min_min_threads = 2;
static int slow_work_max_max_threads = SLOW_WORK_THREAD_LIMIT;
static const int slow_work_min_vslow = 1;
static const int slow_work_max_vslow = 99;

ctl_table slow_work_sysctls[] = {
	{
		.ctl_name	= CTL_UNNUMBERED,
		.procname	= "min-threads",
		.data		= &slow_work_min_threads,
		.maxlen		= sizeof(unsigned),
		.mode		= 0644,
		.proc_handler	= slow_work_min_threads_sysctl,
		.extra1		= (void *) &slow_work_min_min_threads,
		.extra2		= &slow_work_max_threads,
	},
	{
		.ctl_name	= CTL_UNNUMBERED,
		.procname	= "max-threads",
		.data		= &slow_work_max_threads,
		.maxlen		= sizeof(unsigned),
		.mode		= 0644,
		.proc_handler	= slow_work_max_threads_sysctl,
		.extra1		= &slow_work_min_threads,
		.extra2		= (void *) &slow_work_max_max_threads,
	},
	{
		.ctl_name	= CTL_UNNUMBERED,
		.procname	= "vslow-percentage",
		.data		= &vslow_work_proportion,
		.maxlen		= sizeof(unsigned),
		.mode		= 0644,
		.proc_handler	= &proc_dointvec_minmax,
		.extra1		= (void *) &slow_work_min_vslow,
		.extra2		= (void *) &slow_work_max_vslow,
	},
	{ .ctl_name = 0 }
};
#endif

/*
 * The active state of the thread pool
 */
static atomic_t slow_work_thread_count;
static atomic_t vslow_work_executing_count;

static bool slow_work_may_not_start_new_thread;
static bool slow_work_cull; /* cull a thread due to lack of activity */
static DEFINE_TIMER(slow_work_cull_timer, slow_work_cull_timeout, 0, 0);
static DEFINE_TIMER(slow_work_oom_timer, slow_work_oom_timeout, 0, 0);
static struct slow_work slow_work_new_thread; /* new thread starter */

/*
 * slow work ID allocation (use slow_work_queue_lock)
 */
static DECLARE_BITMAP(slow_work_ids, SLOW_WORK_THREAD_LIMIT);

/*
 * Unregistration tracking to prevent put_ref() from disappearing during module
 * unload
 */
#ifdef CONFIG_MODULES
static struct module *slow_work_thread_processing[SLOW_WORK_THREAD_LIMIT];
static struct module *slow_work_unreg_module;
static struct slow_work *slow_work_unreg_work_item;
static DECLARE_WAIT_QUEUE_HEAD(slow_work_unreg_wq);
static DEFINE_MUTEX(slow_work_unreg_sync_lock);

static void slow_work_set_thread_processing(int id, struct slow_work *work)
{
	if (work)
		slow_work_thread_processing[id] = work->owner;
}
static void slow_work_done_thread_processing(int id, struct slow_work *work)
{
	struct module *module = slow_work_thread_processing[id];

	slow_work_thread_processing[id] = NULL;
	smp_mb();
	if (slow_work_unreg_work_item == work ||
	    slow_work_unreg_module == module)
		wake_up_all(&slow_work_unreg_wq);
}
static void slow_work_clear_thread_processing(int id)
{
	slow_work_thread_processing[id] = NULL;
}
#else
static void slow_work_set_thread_processing(int id, struct slow_work *work) {}
static void slow_work_done_thread_processing(int id, struct slow_work *work) {}
static void slow_work_clear_thread_processing(int id) {}
#endif

/*
 * Data for tracking currently executing items for indication through /proc
 */
#ifdef CONFIG_SLOW_WORK_DEBUG
struct slow_work *slow_work_execs[SLOW_WORK_THREAD_LIMIT];
pid_t slow_work_pids[SLOW_WORK_THREAD_LIMIT];
DEFINE_RWLOCK(slow_work_execs_lock);
#endif

/*
 * The queues of work items and the lock governing access to them.  These are
 * shared between all the CPUs.  It doesn't make sense to have per-CPU queues
 * as the number of threads bears no relation to the number of CPUs.
 *
 * There are two queues of work items: one for slow work items, and one for
 * very slow work items.
 */
LIST_HEAD(slow_work_queue);
LIST_HEAD(vslow_work_queue);
DEFINE_SPINLOCK(slow_work_queue_lock);

/*
 * The following are two wait queues that get pinged when a work item is placed
 * on an empty queue.  These allow work items that are hogging a thread by
 * sleeping in a way that could be deferred to yield their thread and enqueue
 * themselves.
 */
static DECLARE_WAIT_QUEUE_HEAD(slow_work_queue_waits_for_occupation);
static DECLARE_WAIT_QUEUE_HEAD(vslow_work_queue_waits_for_occupation);

/*
 * The thread controls.  A variable used to signal to the threads that they
 * should exit when the queue is empty, a waitqueue used by the threads to wait
 * for signals, and a completion set by the last thread to exit.
 */
static bool slow_work_threads_should_exit;
static DECLARE_WAIT_QUEUE_HEAD(slow_work_thread_wq);
static DECLARE_COMPLETION(slow_work_last_thread_exited);

/*
 * The number of users of the thread pool and its lock.  Whilst this is zero we
 * have no threads hanging around, and when this reaches zero, we wait for all
 * active or queued work items to complete and kill all the threads we do have.
 */
static int slow_work_user_count;
static DEFINE_MUTEX(slow_work_user_lock);

static inline int slow_work_get_ref(struct slow_work *work)
{
	if (work->ops->get_ref)
		return work->ops->get_ref(work);

	return 0;
}

static inline void slow_work_put_ref(struct slow_work *work)
{
	if (work->ops->put_ref)
		work->ops->put_ref(work);
}

/*
 * Calculate the maximum number of active threads in the pool that are
 * permitted to process very slow work items.
 *
 * The answer is rounded up to at least 1, but may not equal or exceed the
 * maximum number of the threads in the pool.  This means we always have at
 * least one thread that can process slow work items, and we always have at
 * least one thread that won't get tied up doing so.
 */
static unsigned slow_work_calc_vsmax(void)
{
	unsigned vsmax;

	vsmax = atomic_read(&slow_work_thread_count) * vslow_work_proportion;
	vsmax /= 100;
	vsmax = max(vsmax, 1U);
	return min(vsmax, slow_work_max_threads - 1);
}

/*
 * Attempt to execute stuff queued on a slow thread.  Return true if we managed
 * it, false if there was nothing to do.
 */
static noinline bool slow_work_execute(int id)
{
	struct slow_work *work = NULL;
	unsigned vsmax;
	bool very_slow;

	vsmax = slow_work_calc_vsmax();

	/* see if we can schedule a new thread to be started if we're not
	 * keeping up with the work */
	if (!waitqueue_active(&slow_work_thread_wq) &&
	    (!list_empty(&slow_work_queue) || !list_empty(&vslow_work_queue)) &&
	    atomic_read(&slow_work_thread_count) < slow_work_max_threads &&
	    !slow_work_may_not_start_new_thread)
		slow_work_enqueue(&slow_work_new_thread);

	/* find something to execute */
	spin_lock_irq(&slow_work_queue_lock);
	if (!list_empty(&vslow_work_queue) &&
	    atomic_read(&vslow_work_executing_count) < vsmax) {
		work = list_entry(vslow_work_queue.next,
				  struct slow_work, link);
		if (test_and_set_bit_lock(SLOW_WORK_EXECUTING, &work->flags))
			BUG();
		list_del_init(&work->link);
		atomic_inc(&vslow_work_executing_count);
		very_slow = true;
	} else if (!list_empty(&slow_work_queue)) {
		work = list_entry(slow_work_queue.next,
				  struct slow_work, link);
		if (test_and_set_bit_lock(SLOW_WORK_EXECUTING, &work->flags))
			BUG();
		list_del_init(&work->link);
		very_slow = false;
	} else {
		very_slow = false; /* avoid the compiler warning */
	}

	slow_work_set_thread_processing(id, work);
	if (work) {
		slow_work_mark_time(work);
		slow_work_begin_exec(id, work);
	}

	spin_unlock_irq(&slow_work_queue_lock);

	if (!work)
		return false;

	if (!test_and_clear_bit(SLOW_WORK_PENDING, &work->flags))
		BUG();

	/* don't execute if the work is in the process of being cancelled */
	if (!test_bit(SLOW_WORK_CANCELLING, &work->flags))
		work->ops->execute(work);

	if (very_slow)
		atomic_dec(&vslow_work_executing_count);
	clear_bit_unlock(SLOW_WORK_EXECUTING, &work->flags);

	/* wake up anyone waiting for this work to be complete */
	wake_up_bit(&work->flags, SLOW_WORK_EXECUTING);

	slow_work_end_exec(id, work);

	/* if someone tried to enqueue the item whilst we were executing it,
	 * then it'll be left unenqueued to avoid multiple threads trying to
	 * execute it simultaneously
	 *
	 * there is, however, a race between us testing the pending flag and
	 * getting the spinlock, and between the enqueuer setting the pending
	 * flag and getting the spinlock, so we use a deferral bit to tell us
	 * if the enqueuer got there first
	 */
	if (test_bit(SLOW_WORK_PENDING, &work->flags)) {
		spin_lock_irq(&slow_work_queue_lock);

		if (!test_bit(SLOW_WORK_EXECUTING, &work->flags) &&
		    test_and_clear_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags))
			goto auto_requeue;

		spin_unlock_irq(&slow_work_queue_lock);
	}

	/* sort out the race between module unloading and put_ref() */
	slow_work_put_ref(work);
	slow_work_done_thread_processing(id, work);

	return true;

auto_requeue:
	/* we must complete the enqueue operation
	 * - we transfer our ref on the item back to the appropriate queue
	 * - don't wake another thread up as we're awake already
	 */
	slow_work_mark_time(work);
	if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags))
		list_add_tail(&work->link, &vslow_work_queue);
	else
		list_add_tail(&work->link, &slow_work_queue);
	spin_unlock_irq(&slow_work_queue_lock);
	slow_work_clear_thread_processing(id);
	return true;
}

/**
 * slow_work_sleep_till_thread_needed - Sleep till thread needed by other work
 * work: The work item under execution that wants to sleep
 * _timeout: Scheduler sleep timeout
 *
 * Allow a requeueable work item to sleep on a slow-work processor thread until
 * that thread is needed to do some other work or the sleep is interrupted by
 * some other event.
 *
 * The caller must set up a wake up event before calling this and must have set
 * the appropriate sleep mode (such as TASK_UNINTERRUPTIBLE) and tested its own
 * condition before calling this function as no test is made here.
 *
 * False is returned if there is nothing on the queue; true is returned if the
 * work item should be requeued
 */
bool slow_work_sleep_till_thread_needed(struct slow_work *work,
					signed long *_timeout)
{
	wait_queue_head_t *wfo_wq;
	struct list_head *queue;

	DEFINE_WAIT(wait);

	if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) {
		wfo_wq = &vslow_work_queue_waits_for_occupation;
		queue = &vslow_work_queue;
	} else {
		wfo_wq = &slow_work_queue_waits_for_occupation;
		queue = &slow_work_queue;
	}

	if (!list_empty(queue))
		return true;

	add_wait_queue_exclusive(wfo_wq, &wait);
	if (list_empty(queue))
		*_timeout = schedule_timeout(*_timeout);
	finish_wait(wfo_wq, &wait);

	return !list_empty(queue);
}
EXPORT_SYMBOL(slow_work_sleep_till_thread_needed);

/**
 * slow_work_enqueue - Schedule a slow work item for processing
 * @work: The work item to queue
 *
 * Schedule a slow work item for processing.  If the item is already undergoing
 * execution, this guarantees not to re-enter the execution routine until the
 * first execution finishes.
 *
 * The item is pinned by this function as it retains a reference to it, managed
 * through the item operations.  The item is unpinned once it has been
 * executed.
 *
 * An item may hog the thread that is running it for a relatively large amount
 * of time, sufficient, for example, to perform several lookup, mkdir, create
 * and setxattr operations.  It may sleep on I/O and may sleep to obtain locks.
 *
 * Conversely, if a number of items are awaiting processing, it may take some
 * time before any given item is given attention.  The number of threads in the
 * pool may be increased to deal with demand, but only up to a limit.
 *
 * If SLOW_WORK_VERY_SLOW is set on the work item, then it will be placed in
 * the very slow queue, from which only a portion of the threads will be
 * allowed to pick items to execute.  This ensures that very slow items won't
 * overly block ones that are just ordinarily slow.
 *
 * Returns 0 if successful, -EAGAIN if not (or -ECANCELED if cancelled work is
 * attempted queued)
 */
int slow_work_enqueue(struct slow_work *work)
{
	wait_queue_head_t *wfo_wq;
	struct list_head *queue;
	unsigned long flags;
	int ret;

	if (test_bit(SLOW_WORK_CANCELLING, &work->flags))
		return -ECANCELED;

	BUG_ON(slow_work_user_count <= 0);
	BUG_ON(!work);
	BUG_ON(!work->ops);

	/* when honouring an enqueue request, we only promise that we will run
	 * the work function in the future; we do not promise to run it once
	 * per enqueue request
	 *
	 * we use the PENDING bit to merge together repeat requests without
	 * having to disable IRQs and take the spinlock, whilst still
	 * maintaining our promise
	 */
	if (!test_and_set_bit_lock(SLOW_WORK_PENDING, &work->flags)) {
		if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) {
			wfo_wq = &vslow_work_queue_waits_for_occupation;
			queue = &vslow_work_queue;
		} else {
			wfo_wq = &slow_work_queue_waits_for_occupation;
			queue = &slow_work_queue;
		}

		spin_lock_irqsave(&slow_work_queue_lock, flags);

		if (unlikely(test_bit(SLOW_WORK_CANCELLING, &work->flags)))
			goto cancelled;

		/* we promise that we will not attempt to execute the work
		 * function in more than one thread simultaneously
		 *
		 * this, however, leaves us with a problem if we're asked to
		 * enqueue the work whilst someone is executing the work
		 * function as simply queueing the work immediately means that
		 * another thread may try executing it whilst it is already
		 * under execution
		 *
		 * to deal with this, we set the ENQ_DEFERRED bit instead of
		 * enqueueing, and the thread currently executing the work
		 * function will enqueue the work item when the work function
		 * returns and it has cleared the EXECUTING bit
		 */
		if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) {
			set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags);
		} else {
			ret = slow_work_get_ref(work);
			if (ret < 0)
				goto failed;
			slow_work_mark_time(work);
			list_add_tail(&work->link, queue);
			wake_up(&slow_work_thread_wq);

			/* if someone who could be requeued is sleeping on a
			 * thread, then ask them to yield their thread */
			if (work->link.prev == queue)
				wake_up(wfo_wq);
		}

		spin_unlock_irqrestore(&slow_work_queue_lock, flags);
	}
	return 0;

cancelled:
	ret = -ECANCELED;
failed:
	spin_unlock_irqrestore(&slow_work_queue_lock, flags);
	return ret;
}
EXPORT_SYMBOL(slow_work_enqueue);

static int slow_work_wait(void *word)
{
	schedule();
	return 0;
}

/**
 * slow_work_cancel - Cancel a slow work item
 * @work: The work item to cancel
 *
 * This function will cancel a previously enqueued work item. If we cannot
 * cancel the work item, it is guarenteed to have run when this function
 * returns.
 */
void slow_work_cancel(struct slow_work *work)
{
	bool wait = true, put = false;

	set_bit(SLOW_WORK_CANCELLING, &work->flags);
	smp_mb();

	/* if the work item is a delayed work item with an active timer, we
	 * need to wait for the timer to finish _before_ getting the spinlock,
	 * lest we deadlock against the timer routine
	 *
	 * the timer routine will leave DELAYED set if it notices the
	 * CANCELLING flag in time
	 */
	if (test_bit(SLOW_WORK_DELAYED, &work->flags)) {
		struct delayed_slow_work *dwork =
			container_of(work, struct delayed_slow_work, work);
		del_timer_sync(&dwork->timer);
	}

	spin_lock_irq(&slow_work_queue_lock);

	if (test_bit(SLOW_WORK_DELAYED, &work->flags)) {
		/* the timer routine aborted or never happened, so we are left
		 * holding the timer's reference on the item and should just
		 * drop the pending flag and wait for any ongoing execution to
		 * finish */
		struct delayed_slow_work *dwork =
			container_of(work, struct delayed_slow_work, work);

		BUG_ON(timer_pending(&dwork->timer));
		BUG_ON(!list_empty(&work->link));

		clear_bit(SLOW_WORK_DELAYED, &work->flags);
		put = true;
		clear_bit(SLOW_WORK_PENDING, &work->flags);

	} else if (test_bit(SLOW_WORK_PENDING, &work->flags) &&
		   !list_empty(&work->link)) {
		/* the link in the pending queue holds a reference on the item
		 * that we will need to release */
		list_del_init(&work->link);
		wait = false;
		put = true;
		clear_bit(SLOW_WORK_PENDING, &work->flags);

	} else if (test_and_clear_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags)) {
		/* the executor is holding our only reference on the item, so
		 * we merely need to wait for it to finish executing */
		clear_bit(SLOW_WORK_PENDING, &work->flags);
	}

	spin_unlock_irq(&slow_work_queue_lock);

	/* the EXECUTING flag is set by the executor whilst the spinlock is set
	 * and before the item is dequeued - so assuming the above doesn't
	 * actually dequeue it, simply waiting for the EXECUTING flag to be
	 * released here should be sufficient */
	if (wait)
		wait_on_bit(&work->flags, SLOW_WORK_EXECUTING, slow_work_wait,
			    TASK_UNINTERRUPTIBLE);

	clear_bit(SLOW_WORK_CANCELLING, &work->flags);
	if (put)
		slow_work_put_ref(work);
}
EXPORT_SYMBOL(slow_work_cancel);

/*
 * Handle expiry of the delay timer, indicating that a delayed slow work item
 * should now be queued if not cancelled
 */
static void delayed_slow_work_timer(unsigned long data)
{
	wait_queue_head_t *wfo_wq;
	struct list_head *queue;
	struct slow_work *work = (struct slow_work *) data;
	unsigned long flags;
	bool queued = false, put = false, first = false;

	if (test_bit(SLOW_WORK_VERY_SLOW, &work->flags)) {
		wfo_wq = &vslow_work_queue_waits_for_occupation;
		queue = &vslow_work_queue;
	} else {
		wfo_wq = &slow_work_queue_waits_for_occupation;
		queue = &slow_work_queue;
	}

	spin_lock_irqsave(&slow_work_queue_lock, flags);
	if (likely(!test_bit(SLOW_WORK_CANCELLING, &work->flags))) {
		clear_bit(SLOW_WORK_DELAYED, &work->flags);

		if (test_bit(SLOW_WORK_EXECUTING, &work->flags)) {
			/* we discard the reference the timer was holding in
			 * favour of the one the executor holds */
			set_bit(SLOW_WORK_ENQ_DEFERRED, &work->flags);
			put = true;
		} else {
			slow_work_mark_time(work);
			list_add_tail(&work->link, queue);
			queued = true;
			if (work->link.prev == queue)
				first = true;
		}
	}

	spin_unlock_irqrestore(&slow_work_queue_lock, flags);
	if (put)
		slow_work_put_ref(work);
	if (first)
		wake_up(wfo_wq);
	if (queued)
		wake_up(&slow_work_thread_wq);
}

/**
 * delayed_slow_work_enqueue - Schedule a delayed slow work item for processing
 * @dwork: The delayed work item to queue
 * @delay: When to start executing the work, in jiffies from now
 *
 * This is similar to slow_work_enqueue(), but it adds a delay before the work
 * is actually queued for processing.
 *
 * The item can have delayed processing requested on it whilst it is being
 * executed.  The delay will begin immediately, and if it expires before the
 * item finishes executing, the item will be placed back on the queue when it
 * has done executing.
 */
int delayed_slow_work_enqueue(struct delayed_slow_work *dwork,
			      unsigned long delay)
{
	struct slow_work *work = &dwork->work;
	unsigned long flags;
	int ret;

	if (delay == 0)
		return slow_work_enqueue(&dwork->work);

	BUG_ON(slow_work_user_count <= 0);
	BUG_ON(!work);
	BUG_ON(!work->ops);

	if (test_bit(SLOW_WORK_CANCELLING, &work->flags))
		return -ECANCELED;

	if (!test_and_set_bit_lock(SLOW_WORK_PENDING, &work->flags)) {
		spin_lock_irqsave(&slow_work_queue_lock, flags);

		if (test_bit(SLOW_WORK_CANCELLING, &work->flags))
			goto cancelled;

		/* the timer holds a reference whilst it is pending */
		ret = work->ops->get_ref(work);
		if (ret < 0)
			goto cant_get_ref;

		if (test_and_set_bit(SLOW_WORK_DELAYED, &work->flags))
			BUG();
		dwork->timer.expires = jiffies + delay;
		dwork->timer.data = (unsigned long) work;
		dwork->timer.function = delayed_slow_work_timer;
		add_timer(&dwork->timer);

		spin_unlock_irqrestore(&slow_work_queue_lock, flags);
	}

	return 0;

cancelled:
	ret = -ECANCELED;
cant_get_ref:
	spin_unlock_irqrestore(&slow_work_queue_lock, flags);
	return ret;
}
EXPORT_SYMBOL(delayed_slow_work_enqueue);

/*
 * Schedule a cull of the thread pool at some time in the near future
 */
static void slow_work_schedule_cull(void)
{
	mod_timer(&slow_work_cull_timer,
		  round_jiffies(jiffies + SLOW_WORK_CULL_TIMEOUT));
}

/*
 * Worker thread culling algorithm
 */
static bool slow_work_cull_thread(void)
{
	unsigned long flags;
	bool do_cull = false;

	spin_lock_irqsave(&slow_work_queue_lock, flags);

	if (slow_work_cull) {
		slow_work_cull = false;

		if (list_empty(&slow_work_queue) &&
		    list_empty(&vslow_work_queue) &&
		    atomic_read(&slow_work_thread_count) >
		    slow_work_min_threads) {
			slow_work_schedule_cull();
			do_cull = true;
		}
	}

	spin_unlock_irqrestore(&slow_work_queue_lock, flags);
	return do_cull;
}

/*
 * Determine if there is slow work available for dispatch
 */
static inline bool slow_work_available(int vsmax)
{
	return !list_empty(&slow_work_queue) ||
		(!list_empty(&vslow_work_queue) &&
		 atomic_read(&vslow_work_executing_count) < vsmax);
}

/*
 * Worker thread dispatcher
 */
static int slow_work_thread(void *_data)
{
	int vsmax, id;

	DEFINE_WAIT(wait);

	set_freezable();
	set_user_nice(current, -5);

	/* allocate ourselves an ID */
	spin_lock_irq(&slow_work_queue_lock);
	id = find_first_zero_bit(slow_work_ids, SLOW_WORK_THREAD_LIMIT);
	BUG_ON(id < 0 || id >= SLOW_WORK_THREAD_LIMIT);
	__set_bit(id, slow_work_ids);
	slow_work_set_thread_pid(id, current->pid);
	spin_unlock_irq(&slow_work_queue_lock);

	sprintf(current->comm, "kslowd%03u", id);

	for (;;) {
		vsmax = vslow_work_proportion;
		vsmax *= atomic_read(&slow_work_thread_count);
		vsmax /= 100;

		prepare_to_wait_exclusive(&slow_work_thread_wq, &wait,
					  TASK_INTERRUPTIBLE);
		if (!freezing(current) &&
		    !slow_work_threads_should_exit &&
		    !slow_work_available(vsmax) &&
		    !slow_work_cull)
			schedule();
		finish_wait(&slow_work_thread_wq, &wait);

		try_to_freeze();

		vsmax = vslow_work_proportion;
		vsmax *= atomic_read(&slow_work_thread_count);
		vsmax /= 100;

		if (slow_work_available(vsmax) && slow_work_execute(id)) {
			cond_resched();
			if (list_empty(&slow_work_queue) &&
			    list_empty(&vslow_work_queue) &&
			    atomic_read(&slow_work_thread_count) >
			    slow_work_min_threads)
				slow_work_schedule_cull();
			continue;
		}

		if (slow_work_threads_should_exit)
			break;

		if (slow_work_cull && slow_work_cull_thread())
			break;
	}

	spin_lock_irq(&slow_work_queue_lock);
	slow_work_set_thread_pid(id, 0);
	__clear_bit(id, slow_work_ids);
	spin_unlock_irq(&slow_work_queue_lock);

	if (atomic_dec_and_test(&slow_work_thread_count))
		complete_and_exit(&slow_work_last_thread_exited, 0);
	return 0;
}

/*
 * Handle thread cull timer expiration
 */
static void slow_work_cull_timeout(unsigned long data)
{
	slow_work_cull = true;
	wake_up(&slow_work_thread_wq);
}

/*
 * Start a new slow work thread
 */
static void slow_work_new_thread_execute(struct slow_work *work)
{
	struct task_struct *p;

	if (slow_work_threads_should_exit)
		return;

	if (atomic_read(&slow_work_thread_count) >= slow_work_max_threads)
		return;

	if (!mutex_trylock(&slow_work_user_lock))
		return;

	slow_work_may_not_start_new_thread = true;
	atomic_inc(&slow_work_thread_count);
	p = kthread_run(slow_work_thread, NULL, "kslowd");
	if (IS_ERR(p)) {
		printk(KERN_DEBUG "Slow work thread pool: OOM\n");
		if (atomic_dec_and_test(&slow_work_thread_count))
			BUG(); /* we're running on a slow work thread... */
		mod_timer(&slow_work_oom_timer,
			  round_jiffies(jiffies + SLOW_WORK_OOM_TIMEOUT));
	} else {
		/* ratelimit the starting of new threads */
		mod_timer(&slow_work_oom_timer, jiffies + 1);
	}

	mutex_unlock(&slow_work_user_lock);
}

static const struct slow_work_ops slow_work_new_thread_ops = {
	.owner		= THIS_MODULE,
	.execute	= slow_work_new_thread_execute,
#ifdef CONFIG_SLOW_WORK_DEBUG
	.desc		= slow_work_new_thread_desc,
#endif
};

/*
 * post-OOM new thread start suppression expiration
 */
static void slow_work_oom_timeout(unsigned long data)
{
	slow_work_may_not_start_new_thread = false;
}

#ifdef CONFIG_SYSCTL
/*
 * Handle adjustment of the minimum number of threads
 */
static int slow_work_min_threads_sysctl(struct ctl_table *table, int write,
					void __user *buffer,
					size_t *lenp, loff_t *ppos)
{
	int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
	int n;

	if (ret == 0) {
		mutex_lock(&slow_work_user_lock);
		if (slow_work_user_count > 0) {
			/* see if we need to start or stop threads */
			n = atomic_read(&slow_work_thread_count) -
				slow_work_min_threads;

			if (n < 0 && !slow_work_may_not_start_new_thread)
				slow_work_enqueue(&slow_work_new_thread);
			else if (n > 0)
				slow_work_schedule_cull();
		}
		mutex_unlock(&slow_work_user_lock);
	}

	return ret;
}

/*
 * Handle adjustment of the maximum number of threads
 */
static int slow_work_max_threads_sysctl(struct ctl_table *table, int write,
					void __user *buffer,
					size_t *lenp, loff_t *ppos)
{
	int ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
	int n;

	if (ret == 0) {
		mutex_lock(&slow_work_user_lock);
		if (slow_work_user_count > 0) {
			/* see if we need to stop threads */
			n = slow_work_max_threads -
				atomic_read(&slow_work_thread_count);

			if (n < 0)
				slow_work_schedule_cull();
		}
		mutex_unlock(&slow_work_user_lock);
	}

	return ret;
}
#endif /* CONFIG_SYSCTL */

/**
 * slow_work_register_user - Register a user of the facility
 * @module: The module about to make use of the facility
 *
 * Register a user of the facility, starting up the initial threads if there
 * aren't any other users at this point.  This will return 0 if successful, or
 * an error if not.
 */
int slow_work_register_user(struct module *module)
{
	struct task_struct *p;
	int loop;

	mutex_lock(&slow_work_user_lock);

	if (slow_work_user_count == 0) {
		printk(KERN_NOTICE "Slow work thread pool: Starting up\n");
		init_completion(&slow_work_last_thread_exited);

		slow_work_threads_should_exit = false;
		slow_work_init(&slow_work_new_thread,
			       &slow_work_new_thread_ops);
		slow_work_may_not_start_new_thread = false;
		slow_work_cull = false;

		/* start the minimum number of threads */
		for (loop = 0; loop < slow_work_min_threads; loop++) {
			atomic_inc(&slow_work_thread_count);
			p = kthread_run(slow_work_thread, NULL, "kslowd");
			if (IS_ERR(p))
				goto error;
		}
		printk(KERN_NOTICE "Slow work thread pool: Ready\n");
	}

	slow_work_user_count++;
	mutex_unlock(&slow_work_user_lock);
	return 0;

error:
	if (atomic_dec_and_test(&slow_work_thread_count))
		complete(&slow_work_last_thread_exited);
	if (loop > 0) {
		printk(KERN_ERR "Slow work thread pool:"
		       " Aborting startup on ENOMEM\n");
		slow_work_threads_should_exit = true;
		wake_up_all(&slow_work_thread_wq);
		wait_for_completion(&slow_work_last_thread_exited);
		printk(KERN_ERR "Slow work thread pool: Aborted\n");
	}
	mutex_unlock(&slow_work_user_lock);
	return PTR_ERR(p);
}
EXPORT_SYMBOL(slow_work_register_user);

/*
 * wait for all outstanding items from the calling module to complete
 * - note that more items may be queued whilst we're waiting
 */
static void slow_work_wait_for_items(struct module *module)
{
#ifdef CONFIG_MODULES
	DECLARE_WAITQUEUE(myself, current);
	struct slow_work *work;
	int loop;

	mutex_lock(&slow_work_unreg_sync_lock);
	add_wait_queue(&slow_work_unreg_wq, &myself);

	for (;;) {
		spin_lock_irq(&slow_work_queue_lock);

		/* first of all, we wait for the last queued item in each list
		 * to be processed */
		list_for_each_entry_reverse(work, &vslow_work_queue, link) {
			if (work->owner == module) {
				set_current_state(TASK_UNINTERRUPTIBLE);
				slow_work_unreg_work_item = work;
				goto do_wait;
			}
		}
		list_for_each_entry_reverse(work, &slow_work_queue, link) {
			if (work->owner == module) {
				set_current_state(TASK_UNINTERRUPTIBLE);
				slow_work_unreg_work_item = work;
				goto do_wait;
			}
		}

		/* then we wait for the items being processed to finish */
		slow_work_unreg_module = module;
		smp_mb();
		for (loop = 0; loop < SLOW_WORK_THREAD_LIMIT; loop++) {
			if (slow_work_thread_processing[loop] == module)
				goto do_wait;
		}
		spin_unlock_irq(&slow_work_queue_lock);
		break; /* okay, we're done */

	do_wait:
		spin_unlock_irq(&slow_work_queue_lock);
		schedule();
		slow_work_unreg_work_item = NULL;
		slow_work_unreg_module = NULL;
	}

	remove_wait_queue(&slow_work_unreg_wq, &myself);
	mutex_unlock(&slow_work_unreg_sync_lock);
#endif /* CONFIG_MODULES */
}

/**
 * slow_work_unregister_user - Unregister a user of the facility
 * @module: The module whose items should be cleared
 *
 * Unregister a user of the facility, killing all the threads if this was the
 * last one.
 *
 * This waits for all the work items belonging to the nominated module to go
 * away before proceeding.
 */
void slow_work_unregister_user(struct module *module)
{
	/* first of all, wait for all outstanding items from the calling module
	 * to complete */
	if (module)
		slow_work_wait_for_items(module);

	/* then we can actually go about shutting down the facility if need
	 * be */
	mutex_lock(&slow_work_user_lock);

	BUG_ON(slow_work_user_count <= 0);

	slow_work_user_count--;
	if (slow_work_user_count == 0) {
		printk(KERN_NOTICE "Slow work thread pool: Shutting down\n");
		slow_work_threads_should_exit = true;
		del_timer_sync(&slow_work_cull_timer);
		del_timer_sync(&slow_work_oom_timer);
		wake_up_all(&slow_work_thread_wq);
		wait_for_completion(&slow_work_last_thread_exited);
		printk(KERN_NOTICE "Slow work thread pool:"
		       " Shut down complete\n");
	}

	mutex_unlock(&slow_work_user_lock);
}
EXPORT_SYMBOL(slow_work_unregister_user);

/*
 * Initialise the slow work facility
 */
static int __init init_slow_work(void)
{
	unsigned nr_cpus = num_possible_cpus();

	if (slow_work_max_threads < nr_cpus)
		slow_work_max_threads = nr_cpus;
#ifdef CONFIG_SYSCTL
	if (slow_work_max_max_threads < nr_cpus * 2)
		slow_work_max_max_threads = nr_cpus * 2;
#endif
#ifdef CONFIG_SLOW_WORK_DEBUG
	{
		struct dentry *dbdir;

		dbdir = debugfs_create_dir("slow_work", NULL);
		if (dbdir && !IS_ERR(dbdir))
			debugfs_create_file("runqueue", S_IFREG | 0400, dbdir,
					    NULL, &slow_work_runqueue_fops);
	}
#endif
	return 0;
}

subsys_initcall(init_slow_work);