summaryrefslogtreecommitdiffstats
path: root/fs/bcachefs/six.c
blob: 3a494c5d12478595c76bebc89fd15b517c5ed6d0 (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
// SPDX-License-Identifier: GPL-2.0

#include <linux/export.h>
#include <linux/log2.h>
#include <linux/percpu.h>
#include <linux/preempt.h>
#include <linux/rcupdate.h>
#include <linux/sched.h>
#include <linux/sched/clock.h>
#include <linux/sched/rt.h>
#include <linux/sched/task.h>
#include <linux/slab.h>

#include <trace/events/lock.h>

#include "six.h"

#ifdef DEBUG
#define EBUG_ON(cond)			BUG_ON(cond)
#else
#define EBUG_ON(cond)			do {} while (0)
#endif

#define six_acquire(l, t, r, ip)	lock_acquire(l, 0, t, r, 1, NULL, ip)
#define six_release(l, ip)		lock_release(l, ip)

static void do_six_unlock_type(struct six_lock *lock, enum six_lock_type type);

#define SIX_LOCK_HELD_read_OFFSET	0
#define SIX_LOCK_HELD_read		~(~0U << 26)
#define SIX_LOCK_HELD_intent		(1U << 26)
#define SIX_LOCK_HELD_write		(1U << 27)
#define SIX_LOCK_WAITING_read		(1U << (28 + SIX_LOCK_read))
#define SIX_LOCK_WAITING_write		(1U << (28 + SIX_LOCK_write))
#define SIX_LOCK_NOSPIN			(1U << 31)

struct six_lock_vals {
	/* Value we add to the lock in order to take the lock: */
	u32			lock_val;

	/* If the lock has this value (used as a mask), taking the lock fails: */
	u32			lock_fail;

	/* Mask that indicates lock is held for this type: */
	u32			held_mask;

	/* Waitlist we wakeup when releasing the lock: */
	enum six_lock_type	unlock_wakeup;
};

static const struct six_lock_vals l[] = {
	[SIX_LOCK_read] = {
		.lock_val	= 1U << SIX_LOCK_HELD_read_OFFSET,
		.lock_fail	= SIX_LOCK_HELD_write,
		.held_mask	= SIX_LOCK_HELD_read,
		.unlock_wakeup	= SIX_LOCK_write,
	},
	[SIX_LOCK_intent] = {
		.lock_val	= SIX_LOCK_HELD_intent,
		.lock_fail	= SIX_LOCK_HELD_intent,
		.held_mask	= SIX_LOCK_HELD_intent,
		.unlock_wakeup	= SIX_LOCK_intent,
	},
	[SIX_LOCK_write] = {
		.lock_val	= SIX_LOCK_HELD_write,
		.lock_fail	= SIX_LOCK_HELD_read,
		.held_mask	= SIX_LOCK_HELD_write,
		.unlock_wakeup	= SIX_LOCK_read,
	},
};

static inline void six_set_bitmask(struct six_lock *lock, u32 mask)
{
	if ((atomic_read(&lock->state) & mask) != mask)
		atomic_or(mask, &lock->state);
}

static inline void six_clear_bitmask(struct six_lock *lock, u32 mask)
{
	if (atomic_read(&lock->state) & mask)
		atomic_and(~mask, &lock->state);
}

static inline void six_set_owner(struct six_lock *lock, enum six_lock_type type,
				 u32 old, struct task_struct *owner)
{
	if (type != SIX_LOCK_intent)
		return;

	if (!(old & SIX_LOCK_HELD_intent)) {
		EBUG_ON(lock->owner);
		lock->owner = owner;
	} else {
		EBUG_ON(lock->owner != current);
	}
}

static inline unsigned pcpu_read_count(struct six_lock *lock)
{
	unsigned read_count = 0;
	int cpu;

	for_each_possible_cpu(cpu)
		read_count += *per_cpu_ptr(lock->readers, cpu);
	return read_count;
}

/*
 * __do_six_trylock() - main trylock routine
 *
 * Returns 1 on success, 0 on failure
 *
 * In percpu reader mode, a failed trylock may cause a spurious trylock failure
 * for anoter thread taking the competing lock type, and we may havve to do a
 * wakeup: when a wakeup is required, we return -1 - wakeup_type.
 */
static int __do_six_trylock(struct six_lock *lock, enum six_lock_type type,
			    struct task_struct *task, bool try)
{
	int ret;
	u32 old;

	EBUG_ON(type == SIX_LOCK_write && lock->owner != task);
	EBUG_ON(type == SIX_LOCK_write &&
		(try != !(atomic_read(&lock->state) & SIX_LOCK_HELD_write)));

	/*
	 * Percpu reader mode:
	 *
	 * The basic idea behind this algorithm is that you can implement a lock
	 * between two threads without any atomics, just memory barriers:
	 *
	 * For two threads you'll need two variables, one variable for "thread a
	 * has the lock" and another for "thread b has the lock".
	 *
	 * To take the lock, a thread sets its variable indicating that it holds
	 * the lock, then issues a full memory barrier, then reads from the
	 * other thread's variable to check if the other thread thinks it has
	 * the lock. If we raced, we backoff and retry/sleep.
	 *
	 * Failure to take the lock may cause a spurious trylock failure in
	 * another thread, because we temporarily set the lock to indicate that
	 * we held it. This would be a problem for a thread in six_lock(), when
	 * they are calling trylock after adding themself to the waitlist and
	 * prior to sleeping.
	 *
	 * Therefore, if we fail to get the lock, and there were waiters of the
	 * type we conflict with, we will have to issue a wakeup.
	 *
	 * Since we may be called under wait_lock (and by the wakeup code
	 * itself), we return that the wakeup has to be done instead of doing it
	 * here.
	 */
	if (type == SIX_LOCK_read && lock->readers) {
		preempt_disable();
		this_cpu_inc(*lock->readers); /* signal that we own lock */

		smp_mb();

		old = atomic_read(&lock->state);
		ret = !(old & l[type].lock_fail);

		this_cpu_sub(*lock->readers, !ret);
		preempt_enable();

		if (!ret) {
			smp_mb();
			if (atomic_read(&lock->state) & SIX_LOCK_WAITING_write)
				ret = -1 - SIX_LOCK_write;
		}
	} else if (type == SIX_LOCK_write && lock->readers) {
		if (try) {
			atomic_add(SIX_LOCK_HELD_write, &lock->state);
			smp_mb__after_atomic();
		}

		ret = !pcpu_read_count(lock);

		if (try && !ret) {
			old = atomic_sub_return(SIX_LOCK_HELD_write, &lock->state);
			if (old & SIX_LOCK_WAITING_read)
				ret = -1 - SIX_LOCK_read;
		}
	} else {
		old = atomic_read(&lock->state);
		do {
			ret = !(old & l[type].lock_fail);
			if (!ret || (type == SIX_LOCK_write && !try)) {
				smp_mb();
				break;
			}
		} while (!atomic_try_cmpxchg_acquire(&lock->state, &old, old + l[type].lock_val));

		EBUG_ON(ret && !(atomic_read(&lock->state) & l[type].held_mask));
	}

	if (ret > 0)
		six_set_owner(lock, type, old, task);

	EBUG_ON(type == SIX_LOCK_write && try && ret <= 0 &&
		(atomic_read(&lock->state) & SIX_LOCK_HELD_write));

	return ret;
}

static void __six_lock_wakeup(struct six_lock *lock, enum six_lock_type lock_type)
{
	struct six_lock_waiter *w, *next;
	struct task_struct *task;
	bool saw_one;
	int ret;
again:
	ret = 0;
	saw_one = false;
	raw_spin_lock(&lock->wait_lock);

	list_for_each_entry_safe(w, next, &lock->wait_list, list) {
		if (w->lock_want != lock_type)
			continue;

		if (saw_one && lock_type != SIX_LOCK_read)
			goto unlock;
		saw_one = true;

		ret = __do_six_trylock(lock, lock_type, w->task, false);
		if (ret <= 0)
			goto unlock;

		/*
		 * Similar to percpu_rwsem_wake_function(), we need to guard
		 * against the wakee noticing w->lock_acquired, returning, and
		 * then exiting before we do the wakeup:
		 */
		task = get_task_struct(w->task);
		__list_del(w->list.prev, w->list.next);
		/*
		 * The release barrier here ensures the ordering of the
		 * __list_del before setting w->lock_acquired; @w is on the
		 * stack of the thread doing the waiting and will be reused
		 * after it sees w->lock_acquired with no other locking:
		 * pairs with smp_load_acquire() in six_lock_slowpath()
		 */
		smp_store_release(&w->lock_acquired, true);
		wake_up_process(task);
		put_task_struct(task);
	}

	six_clear_bitmask(lock, SIX_LOCK_WAITING_read << lock_type);
unlock:
	raw_spin_unlock(&lock->wait_lock);

	if (ret < 0) {
		lock_type = -ret - 1;
		goto again;
	}
}

__always_inline
static void six_lock_wakeup(struct six_lock *lock, u32 state,
			    enum six_lock_type lock_type)
{
	if (lock_type == SIX_LOCK_write && (state & SIX_LOCK_HELD_read))
		return;

	if (!(state & (SIX_LOCK_WAITING_read << lock_type)))
		return;

	__six_lock_wakeup(lock, lock_type);
}

__always_inline
static bool do_six_trylock(struct six_lock *lock, enum six_lock_type type, bool try)
{
	int ret;

	ret = __do_six_trylock(lock, type, current, try);
	if (ret < 0)
		__six_lock_wakeup(lock, -ret - 1);

	return ret > 0;
}

/**
 * six_trylock_ip - attempt to take a six lock without blocking
 * @lock:	lock to take
 * @type:	SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
 * @ip:		ip parameter for lockdep/lockstat, i.e. _THIS_IP_
 *
 * Return: true on success, false on failure.
 */
bool six_trylock_ip(struct six_lock *lock, enum six_lock_type type, unsigned long ip)
{
	if (!do_six_trylock(lock, type, true))
		return false;

	if (type != SIX_LOCK_write)
		six_acquire(&lock->dep_map, 1, type == SIX_LOCK_read, ip);
	return true;
}
EXPORT_SYMBOL_GPL(six_trylock_ip);

/**
 * six_relock_ip - attempt to re-take a lock that was held previously
 * @lock:	lock to take
 * @type:	SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
 * @seq:	lock sequence number obtained from six_lock_seq() while lock was
 *		held previously
 * @ip:		ip parameter for lockdep/lockstat, i.e. _THIS_IP_
 *
 * Return: true on success, false on failure.
 */
bool six_relock_ip(struct six_lock *lock, enum six_lock_type type,
		   unsigned seq, unsigned long ip)
{
	if (six_lock_seq(lock) != seq || !six_trylock_ip(lock, type, ip))
		return false;

	if (six_lock_seq(lock) != seq) {
		six_unlock_ip(lock, type, ip);
		return false;
	}

	return true;
}
EXPORT_SYMBOL_GPL(six_relock_ip);

#ifdef CONFIG_BCACHEFS_SIX_OPTIMISTIC_SPIN

static inline bool six_owner_running(struct six_lock *lock)
{
	/*
	 * When there's no owner, we might have preempted between the owner
	 * acquiring the lock and setting the owner field. If we're an RT task
	 * that will live-lock because we won't let the owner complete.
	 */
	rcu_read_lock();
	struct task_struct *owner = READ_ONCE(lock->owner);
	bool ret = owner ? owner_on_cpu(owner) : !rt_task(current);
	rcu_read_unlock();

	return ret;
}

static inline bool six_optimistic_spin(struct six_lock *lock,
				       struct six_lock_waiter *wait,
				       enum six_lock_type type)
{
	unsigned loop = 0;
	u64 end_time;

	if (type == SIX_LOCK_write)
		return false;

	if (lock->wait_list.next != &wait->list)
		return false;

	if (atomic_read(&lock->state) & SIX_LOCK_NOSPIN)
		return false;

	preempt_disable();
	end_time = sched_clock() + 10 * NSEC_PER_USEC;

	while (!need_resched() && six_owner_running(lock)) {
		/*
		 * Ensures that writes to the waitlist entry happen after we see
		 * wait->lock_acquired: pairs with the smp_store_release in
		 * __six_lock_wakeup
		 */
		if (smp_load_acquire(&wait->lock_acquired)) {
			preempt_enable();
			return true;
		}

		if (!(++loop & 0xf) && (time_after64(sched_clock(), end_time))) {
			six_set_bitmask(lock, SIX_LOCK_NOSPIN);
			break;
		}

		/*
		 * The cpu_relax() call is a compiler barrier which forces
		 * everything in this loop to be re-loaded. We don't need
		 * memory barriers as we'll eventually observe the right
		 * values at the cost of a few extra spins.
		 */
		cpu_relax();
	}

	preempt_enable();
	return false;
}

#else /* CONFIG_LOCK_SPIN_ON_OWNER */

static inline bool six_optimistic_spin(struct six_lock *lock,
				       struct six_lock_waiter *wait,
				       enum six_lock_type type)
{
	return false;
}

#endif

noinline
static int six_lock_slowpath(struct six_lock *lock, enum six_lock_type type,
			     struct six_lock_waiter *wait,
			     six_lock_should_sleep_fn should_sleep_fn, void *p,
			     unsigned long ip)
{
	int ret = 0;

	if (type == SIX_LOCK_write) {
		EBUG_ON(atomic_read(&lock->state) & SIX_LOCK_HELD_write);
		atomic_add(SIX_LOCK_HELD_write, &lock->state);
		smp_mb__after_atomic();
	}

	trace_contention_begin(lock, 0);
	lock_contended(&lock->dep_map, ip);

	wait->task		= current;
	wait->lock_want		= type;
	wait->lock_acquired	= false;

	raw_spin_lock(&lock->wait_lock);
	six_set_bitmask(lock, SIX_LOCK_WAITING_read << type);
	/*
	 * Retry taking the lock after taking waitlist lock, in case we raced
	 * with an unlock:
	 */
	ret = __do_six_trylock(lock, type, current, false);
	if (ret <= 0) {
		wait->start_time = local_clock();

		if (!list_empty(&lock->wait_list)) {
			struct six_lock_waiter *last =
				list_last_entry(&lock->wait_list,
					struct six_lock_waiter, list);

			if (time_before_eq64(wait->start_time, last->start_time))
				wait->start_time = last->start_time + 1;
		}

		list_add_tail(&wait->list, &lock->wait_list);
	}
	raw_spin_unlock(&lock->wait_lock);

	if (unlikely(ret > 0)) {
		ret = 0;
		goto out;
	}

	if (unlikely(ret < 0)) {
		__six_lock_wakeup(lock, -ret - 1);
		ret = 0;
	}

	if (six_optimistic_spin(lock, wait, type))
		goto out;

	while (1) {
		set_current_state(TASK_UNINTERRUPTIBLE);

		/*
		 * Ensures that writes to the waitlist entry happen after we see
		 * wait->lock_acquired: pairs with the smp_store_release in
		 * __six_lock_wakeup
		 */
		if (smp_load_acquire(&wait->lock_acquired))
			break;

		ret = should_sleep_fn ? should_sleep_fn(lock, p) : 0;
		if (unlikely(ret)) {
			bool acquired;

			/*
			 * If should_sleep_fn() returns an error, we are
			 * required to return that error even if we already
			 * acquired the lock - should_sleep_fn() might have
			 * modified external state (e.g. when the deadlock cycle
			 * detector in bcachefs issued a transaction restart)
			 */
			raw_spin_lock(&lock->wait_lock);
			acquired = wait->lock_acquired;
			if (!acquired)
				list_del(&wait->list);
			raw_spin_unlock(&lock->wait_lock);

			if (unlikely(acquired))
				do_six_unlock_type(lock, type);
			break;
		}

		schedule();
	}

	__set_current_state(TASK_RUNNING);
out:
	if (ret && type == SIX_LOCK_write) {
		six_clear_bitmask(lock, SIX_LOCK_HELD_write);
		six_lock_wakeup(lock, atomic_read(&lock->state), SIX_LOCK_read);
	}
	trace_contention_end(lock, 0);

	return ret;
}

/**
 * six_lock_ip_waiter - take a lock, with full waitlist interface
 * @lock:	lock to take
 * @type:	SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
 * @wait:	pointer to wait object, which will be added to lock's waitlist
 * @should_sleep_fn: callback run after adding to waitlist, immediately prior
 *		to scheduling
 * @p:		passed through to @should_sleep_fn
 * @ip:		ip parameter for lockdep/lockstat, i.e. _THIS_IP_
 *
 * This is the most general six_lock() variant, with parameters to support full
 * cycle detection for deadlock avoidance.
 *
 * The code calling this function must implement tracking of held locks, and the
 * @wait object should be embedded into the struct that tracks held locks -
 * which must also be accessible in a thread-safe way.
 *
 * @should_sleep_fn should invoke the cycle detector; it should walk each
 * lock's waiters, and for each waiter recursively walk their held locks.
 *
 * When this function must block, @wait will be added to @lock's waitlist before
 * calling trylock, and before calling @should_sleep_fn, and @wait will not be
 * removed from the lock waitlist until the lock has been successfully acquired,
 * or we abort.
 *
 * @wait.start_time will be monotonically increasing for any given waitlist, and
 * thus may be used as a loop cursor.
 *
 * Return: 0 on success, or the return code from @should_sleep_fn on failure.
 */
int six_lock_ip_waiter(struct six_lock *lock, enum six_lock_type type,
		       struct six_lock_waiter *wait,
		       six_lock_should_sleep_fn should_sleep_fn, void *p,
		       unsigned long ip)
{
	int ret;

	wait->start_time = 0;

	if (type != SIX_LOCK_write)
		six_acquire(&lock->dep_map, 0, type == SIX_LOCK_read, ip);

	ret = do_six_trylock(lock, type, true) ? 0
		: six_lock_slowpath(lock, type, wait, should_sleep_fn, p, ip);

	if (ret && type != SIX_LOCK_write)
		six_release(&lock->dep_map, ip);
	if (!ret)
		lock_acquired(&lock->dep_map, ip);

	return ret;
}
EXPORT_SYMBOL_GPL(six_lock_ip_waiter);

__always_inline
static void do_six_unlock_type(struct six_lock *lock, enum six_lock_type type)
{
	u32 state;

	if (type == SIX_LOCK_intent)
		lock->owner = NULL;

	if (type == SIX_LOCK_read &&
	    lock->readers) {
		smp_mb(); /* unlock barrier */
		this_cpu_dec(*lock->readers);
		smp_mb(); /* between unlocking and checking for waiters */
		state = atomic_read(&lock->state);
	} else {
		u32 v = l[type].lock_val;

		if (type != SIX_LOCK_read)
			v += atomic_read(&lock->state) & SIX_LOCK_NOSPIN;

		EBUG_ON(!(atomic_read(&lock->state) & l[type].held_mask));
		state = atomic_sub_return_release(v, &lock->state);
	}

	six_lock_wakeup(lock, state, l[type].unlock_wakeup);
}

/**
 * six_unlock_ip - drop a six lock
 * @lock:	lock to unlock
 * @type:	SIX_LOCK_read, SIX_LOCK_intent, or SIX_LOCK_write
 * @ip:		ip parameter for lockdep/lockstat, i.e. _THIS_IP_
 *
 * When a lock is held multiple times (because six_lock_incement()) was used),
 * this decrements the 'lock held' counter by one.
 *
 * For example:
 * six_lock_read(&foo->lock);				read count 1
 * six_lock_increment(&foo->lock, SIX_LOCK_read);	read count 2
 * six_lock_unlock(&foo->lock, SIX_LOCK_read);		read count 1
 * six_lock_unlock(&foo->lock, SIX_LOCK_read);		read count 0
 */
void six_unlock_ip(struct six_lock *lock, enum six_lock_type type, unsigned long ip)
{
	EBUG_ON(type == SIX_LOCK_write &&
		!(atomic_read(&lock->state) & SIX_LOCK_HELD_intent));
	EBUG_ON((type == SIX_LOCK_write ||
		 type == SIX_LOCK_intent) &&
		lock->owner != current);

	if (type != SIX_LOCK_write)
		six_release(&lock->dep_map, ip);
	else
		lock->seq++;

	if (type == SIX_LOCK_intent &&
	    lock->intent_lock_recurse) {
		--lock->intent_lock_recurse;
		return;
	}

	do_six_unlock_type(lock, type);
}
EXPORT_SYMBOL_GPL(six_unlock_ip);

/**
 * six_lock_downgrade - convert an intent lock to a read lock
 * @lock:	lock to dowgrade
 *
 * @lock will have read count incremented and intent count decremented
 */
void six_lock_downgrade(struct six_lock *lock)
{
	six_lock_increment(lock, SIX_LOCK_read);
	six_unlock_intent(lock);
}
EXPORT_SYMBOL_GPL(six_lock_downgrade);

/**
 * six_lock_tryupgrade - attempt to convert read lock to an intent lock
 * @lock:	lock to upgrade
 *
 * On success, @lock will have intent count incremented and read count
 * decremented
 *
 * Return: true on success, false on failure
 */
bool six_lock_tryupgrade(struct six_lock *lock)
{
	u32 old = atomic_read(&lock->state), new;

	do {
		new = old;

		if (new & SIX_LOCK_HELD_intent)
			return false;

		if (!lock->readers) {
			EBUG_ON(!(new & SIX_LOCK_HELD_read));
			new -= l[SIX_LOCK_read].lock_val;
		}

		new |= SIX_LOCK_HELD_intent;
	} while (!atomic_try_cmpxchg_acquire(&lock->state, &old, new));

	if (lock->readers)
		this_cpu_dec(*lock->readers);

	six_set_owner(lock, SIX_LOCK_intent, old, current);

	return true;
}
EXPORT_SYMBOL_GPL(six_lock_tryupgrade);

/**
 * six_trylock_convert - attempt to convert a held lock from one type to another
 * @lock:	lock to upgrade
 * @from:	SIX_LOCK_read or SIX_LOCK_intent
 * @to:		SIX_LOCK_read or SIX_LOCK_intent
 *
 * On success, @lock will have intent count incremented and read count
 * decremented
 *
 * Return: true on success, false on failure
 */
bool six_trylock_convert(struct six_lock *lock,
			 enum six_lock_type from,
			 enum six_lock_type to)
{
	EBUG_ON(to == SIX_LOCK_write || from == SIX_LOCK_write);

	if (to == from)
		return true;

	if (to == SIX_LOCK_read) {
		six_lock_downgrade(lock);
		return true;
	} else {
		return six_lock_tryupgrade(lock);
	}
}
EXPORT_SYMBOL_GPL(six_trylock_convert);

/**
 * six_lock_increment - increase held lock count on a lock that is already held
 * @lock:	lock to increment
 * @type:	SIX_LOCK_read or SIX_LOCK_intent
 *
 * @lock must already be held, with a lock type that is greater than or equal to
 * @type
 *
 * A corresponding six_unlock_type() call will be required for @lock to be fully
 * unlocked.
 */
void six_lock_increment(struct six_lock *lock, enum six_lock_type type)
{
	six_acquire(&lock->dep_map, 0, type == SIX_LOCK_read, _RET_IP_);

	/* XXX: assert already locked, and that we don't overflow: */

	switch (type) {
	case SIX_LOCK_read:
		if (lock->readers) {
			this_cpu_inc(*lock->readers);
		} else {
			EBUG_ON(!(atomic_read(&lock->state) &
				  (SIX_LOCK_HELD_read|
				   SIX_LOCK_HELD_intent)));
			atomic_add(l[type].lock_val, &lock->state);
		}
		break;
	case SIX_LOCK_intent:
		EBUG_ON(!(atomic_read(&lock->state) & SIX_LOCK_HELD_intent));
		lock->intent_lock_recurse++;
		break;
	case SIX_LOCK_write:
		BUG();
		break;
	}
}
EXPORT_SYMBOL_GPL(six_lock_increment);

/**
 * six_lock_wakeup_all - wake up all waiters on @lock
 * @lock:	lock to wake up waiters for
 *
 * Wakeing up waiters will cause them to re-run should_sleep_fn, which may then
 * abort the lock operation.
 *
 * This function is never needed in a bug-free program; it's only useful in
 * debug code, e.g. to determine if a cycle detector is at fault.
 */
void six_lock_wakeup_all(struct six_lock *lock)
{
	u32 state = atomic_read(&lock->state);
	struct six_lock_waiter *w;

	six_lock_wakeup(lock, state, SIX_LOCK_read);
	six_lock_wakeup(lock, state, SIX_LOCK_intent);
	six_lock_wakeup(lock, state, SIX_LOCK_write);

	raw_spin_lock(&lock->wait_lock);
	list_for_each_entry(w, &lock->wait_list, list)
		wake_up_process(w->task);
	raw_spin_unlock(&lock->wait_lock);
}
EXPORT_SYMBOL_GPL(six_lock_wakeup_all);

/**
 * six_lock_counts - return held lock counts, for each lock type
 * @lock:	lock to return counters for
 *
 * Return: the number of times a lock is held for read, intent and write.
 */
struct six_lock_count six_lock_counts(struct six_lock *lock)
{
	struct six_lock_count ret;

	ret.n[SIX_LOCK_read]	= !lock->readers
		? atomic_read(&lock->state) & SIX_LOCK_HELD_read
		: pcpu_read_count(lock);
	ret.n[SIX_LOCK_intent]	= !!(atomic_read(&lock->state) & SIX_LOCK_HELD_intent) +
		lock->intent_lock_recurse;
	ret.n[SIX_LOCK_write]	= !!(atomic_read(&lock->state) & SIX_LOCK_HELD_write);

	return ret;
}
EXPORT_SYMBOL_GPL(six_lock_counts);

/**
 * six_lock_readers_add - directly manipulate reader count of a lock
 * @lock:	lock to add/subtract readers for
 * @nr:		reader count to add/subtract
 *
 * When an upper layer is implementing lock reentrency, we may have both read
 * and intent locks on the same lock.
 *
 * When we need to take a write lock, the read locks will cause self-deadlock,
 * because six locks themselves do not track which read locks are held by the
 * current thread and which are held by a different thread - it does no
 * per-thread tracking of held locks.
 *
 * The upper layer that is tracking held locks may however, if trylock() has
 * failed, count up its own read locks, subtract them, take the write lock, and
 * then re-add them.
 *
 * As in any other situation when taking a write lock, @lock must be held for
 * intent one (or more) times, so @lock will never be left unlocked.
 */
void six_lock_readers_add(struct six_lock *lock, int nr)
{
	if (lock->readers) {
		this_cpu_add(*lock->readers, nr);
	} else {
		EBUG_ON((int) (atomic_read(&lock->state) & SIX_LOCK_HELD_read) + nr < 0);
		/* reader count starts at bit 0 */
		atomic_add(nr, &lock->state);
	}
}
EXPORT_SYMBOL_GPL(six_lock_readers_add);

/**
 * six_lock_exit - release resources held by a lock prior to freeing
 * @lock:	lock to exit
 *
 * When a lock was initialized in percpu mode (SIX_OLCK_INIT_PCPU), this is
 * required to free the percpu read counts.
 */
void six_lock_exit(struct six_lock *lock)
{
	WARN_ON(lock->readers && pcpu_read_count(lock));
	WARN_ON(atomic_read(&lock->state) & SIX_LOCK_HELD_read);

	free_percpu(lock->readers);
	lock->readers = NULL;
}
EXPORT_SYMBOL_GPL(six_lock_exit);

void __six_lock_init(struct six_lock *lock, const char *name,
		     struct lock_class_key *key, enum six_lock_init_flags flags)
{
	atomic_set(&lock->state, 0);
	raw_spin_lock_init(&lock->wait_lock);
	INIT_LIST_HEAD(&lock->wait_list);
#ifdef CONFIG_DEBUG_LOCK_ALLOC
	debug_check_no_locks_freed((void *) lock, sizeof(*lock));
	lockdep_init_map(&lock->dep_map, name, key, 0);
#endif

	/*
	 * Don't assume that we have real percpu variables available in
	 * userspace:
	 */
#ifdef __KERNEL__
	if (flags & SIX_LOCK_INIT_PCPU) {
		/*
		 * We don't return an error here on memory allocation failure
		 * since percpu is an optimization, and locks will work with the
		 * same semantics in non-percpu mode: callers can check for
		 * failure if they wish by checking lock->readers, but generally
		 * will not want to treat it as an error.
		 */
		lock->readers = alloc_percpu(unsigned);
	}
#endif
}
EXPORT_SYMBOL_GPL(__six_lock_init);