summaryrefslogtreecommitdiffstats
path: root/kernel/futex/pi.c
blob: 90e5197f4e5696dbd5a79fd0033ed95e4bd32fac (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
// SPDX-License-Identifier: GPL-2.0-or-later

#include <linux/slab.h>
#include <linux/sched/rt.h>
#include <linux/sched/task.h>

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

/*
 * PI code:
 */
int refill_pi_state_cache(void)
{
	struct futex_pi_state *pi_state;

	if (likely(current->pi_state_cache))
		return 0;

	pi_state = kzalloc(sizeof(*pi_state), GFP_KERNEL);

	if (!pi_state)
		return -ENOMEM;

	INIT_LIST_HEAD(&pi_state->list);
	/* pi_mutex gets initialized later */
	pi_state->owner = NULL;
	refcount_set(&pi_state->refcount, 1);
	pi_state->key = FUTEX_KEY_INIT;

	current->pi_state_cache = pi_state;

	return 0;
}

static struct futex_pi_state *alloc_pi_state(void)
{
	struct futex_pi_state *pi_state = current->pi_state_cache;

	WARN_ON(!pi_state);
	current->pi_state_cache = NULL;

	return pi_state;
}

static void pi_state_update_owner(struct futex_pi_state *pi_state,
				  struct task_struct *new_owner)
{
	struct task_struct *old_owner = pi_state->owner;

	lockdep_assert_held(&pi_state->pi_mutex.wait_lock);

	if (old_owner) {
		raw_spin_lock(&old_owner->pi_lock);
		WARN_ON(list_empty(&pi_state->list));
		list_del_init(&pi_state->list);
		raw_spin_unlock(&old_owner->pi_lock);
	}

	if (new_owner) {
		raw_spin_lock(&new_owner->pi_lock);
		WARN_ON(!list_empty(&pi_state->list));
		list_add(&pi_state->list, &new_owner->pi_state_list);
		pi_state->owner = new_owner;
		raw_spin_unlock(&new_owner->pi_lock);
	}
}

void get_pi_state(struct futex_pi_state *pi_state)
{
	WARN_ON_ONCE(!refcount_inc_not_zero(&pi_state->refcount));
}

/*
 * Drops a reference to the pi_state object and frees or caches it
 * when the last reference is gone.
 */
void put_pi_state(struct futex_pi_state *pi_state)
{
	if (!pi_state)
		return;

	if (!refcount_dec_and_test(&pi_state->refcount))
		return;

	/*
	 * If pi_state->owner is NULL, the owner is most probably dying
	 * and has cleaned up the pi_state already
	 */
	if (pi_state->owner) {
		unsigned long flags;

		raw_spin_lock_irqsave(&pi_state->pi_mutex.wait_lock, flags);
		pi_state_update_owner(pi_state, NULL);
		rt_mutex_proxy_unlock(&pi_state->pi_mutex);
		raw_spin_unlock_irqrestore(&pi_state->pi_mutex.wait_lock, flags);
	}

	if (current->pi_state_cache) {
		kfree(pi_state);
	} else {
		/*
		 * pi_state->list is already empty.
		 * clear pi_state->owner.
		 * refcount is at 0 - put it back to 1.
		 */
		pi_state->owner = NULL;
		refcount_set(&pi_state->refcount, 1);
		current->pi_state_cache = pi_state;
	}
}

/*
 * We need to check the following states:
 *
 *      Waiter | pi_state | pi->owner | uTID      | uODIED | ?
 *
 * [1]  NULL   | ---      | ---       | 0         | 0/1    | Valid
 * [2]  NULL   | ---      | ---       | >0        | 0/1    | Valid
 *
 * [3]  Found  | NULL     | --        | Any       | 0/1    | Invalid
 *
 * [4]  Found  | Found    | NULL      | 0         | 1      | Valid
 * [5]  Found  | Found    | NULL      | >0        | 1      | Invalid
 *
 * [6]  Found  | Found    | task      | 0         | 1      | Valid
 *
 * [7]  Found  | Found    | NULL      | Any       | 0      | Invalid
 *
 * [8]  Found  | Found    | task      | ==taskTID | 0/1    | Valid
 * [9]  Found  | Found    | task      | 0         | 0      | Invalid
 * [10] Found  | Found    | task      | !=taskTID | 0/1    | Invalid
 *
 * [1]	Indicates that the kernel can acquire the futex atomically. We
 *	came here due to a stale FUTEX_WAITERS/FUTEX_OWNER_DIED bit.
 *
 * [2]	Valid, if TID does not belong to a kernel thread. If no matching
 *      thread is found then it indicates that the owner TID has died.
 *
 * [3]	Invalid. The waiter is queued on a non PI futex
 *
 * [4]	Valid state after exit_robust_list(), which sets the user space
 *	value to FUTEX_WAITERS | FUTEX_OWNER_DIED.
 *
 * [5]	The user space value got manipulated between exit_robust_list()
 *	and exit_pi_state_list()
 *
 * [6]	Valid state after exit_pi_state_list() which sets the new owner in
 *	the pi_state but cannot access the user space value.
 *
 * [7]	pi_state->owner can only be NULL when the OWNER_DIED bit is set.
 *
 * [8]	Owner and user space value match
 *
 * [9]	There is no transient state which sets the user space TID to 0
 *	except exit_robust_list(), but this is indicated by the
 *	FUTEX_OWNER_DIED bit. See [4]
 *
 * [10] There is no transient state which leaves owner and user space
 *	TID out of sync. Except one error case where the kernel is denied
 *	write access to the user address, see fixup_pi_state_owner().
 *
 *
 * Serialization and lifetime rules:
 *
 * hb->lock:
 *
 *	hb -> futex_q, relation
 *	futex_q -> pi_state, relation
 *
 *	(cannot be raw because hb can contain arbitrary amount
 *	 of futex_q's)
 *
 * pi_mutex->wait_lock:
 *
 *	{uval, pi_state}
 *
 *	(and pi_mutex 'obviously')
 *
 * p->pi_lock:
 *
 *	p->pi_state_list -> pi_state->list, relation
 *	pi_mutex->owner -> pi_state->owner, relation
 *
 * pi_state->refcount:
 *
 *	pi_state lifetime
 *
 *
 * Lock order:
 *
 *   hb->lock
 *     pi_mutex->wait_lock
 *       p->pi_lock
 *
 */

/*
 * Validate that the existing waiter has a pi_state and sanity check
 * the pi_state against the user space value. If correct, attach to
 * it.
 */
static int attach_to_pi_state(u32 __user *uaddr, u32 uval,
			      struct futex_pi_state *pi_state,
			      struct futex_pi_state **ps)
{
	pid_t pid = uval & FUTEX_TID_MASK;
	u32 uval2;
	int ret;

	/*
	 * Userspace might have messed up non-PI and PI futexes [3]
	 */
	if (unlikely(!pi_state))
		return -EINVAL;

	/*
	 * We get here with hb->lock held, and having found a
	 * futex_top_waiter(). This means that futex_lock_pi() of said futex_q
	 * has dropped the hb->lock in between futex_queue() and futex_unqueue_pi(),
	 * which in turn means that futex_lock_pi() still has a reference on
	 * our pi_state.
	 *
	 * The waiter holding a reference on @pi_state also protects against
	 * the unlocked put_pi_state() in futex_unlock_pi(), futex_lock_pi()
	 * and futex_wait_requeue_pi() as it cannot go to 0 and consequently
	 * free pi_state before we can take a reference ourselves.
	 */
	WARN_ON(!refcount_read(&pi_state->refcount));

	/*
	 * Now that we have a pi_state, we can acquire wait_lock
	 * and do the state validation.
	 */
	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

	/*
	 * Since {uval, pi_state} is serialized by wait_lock, and our current
	 * uval was read without holding it, it can have changed. Verify it
	 * still is what we expect it to be, otherwise retry the entire
	 * operation.
	 */
	if (futex_get_value_locked(&uval2, uaddr))
		goto out_efault;

	if (uval != uval2)
		goto out_eagain;

	/*
	 * Handle the owner died case:
	 */
	if (uval & FUTEX_OWNER_DIED) {
		/*
		 * exit_pi_state_list sets owner to NULL and wakes the
		 * topmost waiter. The task which acquires the
		 * pi_state->rt_mutex will fixup owner.
		 */
		if (!pi_state->owner) {
			/*
			 * No pi state owner, but the user space TID
			 * is not 0. Inconsistent state. [5]
			 */
			if (pid)
				goto out_einval;
			/*
			 * Take a ref on the state and return success. [4]
			 */
			goto out_attach;
		}

		/*
		 * If TID is 0, then either the dying owner has not
		 * yet executed exit_pi_state_list() or some waiter
		 * acquired the rtmutex in the pi state, but did not
		 * yet fixup the TID in user space.
		 *
		 * Take a ref on the state and return success. [6]
		 */
		if (!pid)
			goto out_attach;
	} else {
		/*
		 * If the owner died bit is not set, then the pi_state
		 * must have an owner. [7]
		 */
		if (!pi_state->owner)
			goto out_einval;
	}

	/*
	 * Bail out if user space manipulated the futex value. If pi
	 * state exists then the owner TID must be the same as the
	 * user space TID. [9/10]
	 */
	if (pid != task_pid_vnr(pi_state->owner))
		goto out_einval;

out_attach:
	get_pi_state(pi_state);
	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
	*ps = pi_state;
	return 0;

out_einval:
	ret = -EINVAL;
	goto out_error;

out_eagain:
	ret = -EAGAIN;
	goto out_error;

out_efault:
	ret = -EFAULT;
	goto out_error;

out_error:
	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
	return ret;
}

static int handle_exit_race(u32 __user *uaddr, u32 uval,
			    struct task_struct *tsk)
{
	u32 uval2;

	/*
	 * If the futex exit state is not yet FUTEX_STATE_DEAD, tell the
	 * caller that the alleged owner is busy.
	 */
	if (tsk && tsk->futex_state != FUTEX_STATE_DEAD)
		return -EBUSY;

	/*
	 * Reread the user space value to handle the following situation:
	 *
	 * CPU0				CPU1
	 *
	 * sys_exit()			sys_futex()
	 *  do_exit()			 futex_lock_pi()
	 *                                futex_lock_pi_atomic()
	 *   exit_signals(tsk)		    No waiters:
	 *    tsk->flags |= PF_EXITING;	    *uaddr == 0x00000PID
	 *  mm_release(tsk)		    Set waiter bit
	 *   exit_robust_list(tsk) {	    *uaddr = 0x80000PID;
	 *      Set owner died		    attach_to_pi_owner() {
	 *    *uaddr = 0xC0000000;	     tsk = get_task(PID);
	 *   }				     if (!tsk->flags & PF_EXITING) {
	 *  ...				       attach();
	 *  tsk->futex_state =               } else {
	 *	FUTEX_STATE_DEAD;              if (tsk->futex_state !=
	 *					  FUTEX_STATE_DEAD)
	 *				         return -EAGAIN;
	 *				       return -ESRCH; <--- FAIL
	 *				     }
	 *
	 * Returning ESRCH unconditionally is wrong here because the
	 * user space value has been changed by the exiting task.
	 *
	 * The same logic applies to the case where the exiting task is
	 * already gone.
	 */
	if (futex_get_value_locked(&uval2, uaddr))
		return -EFAULT;

	/* If the user space value has changed, try again. */
	if (uval2 != uval)
		return -EAGAIN;

	/*
	 * The exiting task did not have a robust list, the robust list was
	 * corrupted or the user space value in *uaddr is simply bogus.
	 * Give up and tell user space.
	 */
	return -ESRCH;
}

static void __attach_to_pi_owner(struct task_struct *p, union futex_key *key,
				 struct futex_pi_state **ps)
{
	/*
	 * No existing pi state. First waiter. [2]
	 *
	 * This creates pi_state, we have hb->lock held, this means nothing can
	 * observe this state, wait_lock is irrelevant.
	 */
	struct futex_pi_state *pi_state = alloc_pi_state();

	/*
	 * Initialize the pi_mutex in locked state and make @p
	 * the owner of it:
	 */
	rt_mutex_init_proxy_locked(&pi_state->pi_mutex, p);

	/* Store the key for possible exit cleanups: */
	pi_state->key = *key;

	WARN_ON(!list_empty(&pi_state->list));
	list_add(&pi_state->list, &p->pi_state_list);
	/*
	 * Assignment without holding pi_state->pi_mutex.wait_lock is safe
	 * because there is no concurrency as the object is not published yet.
	 */
	pi_state->owner = p;

	*ps = pi_state;
}
/*
 * Lookup the task for the TID provided from user space and attach to
 * it after doing proper sanity checks.
 */
static int attach_to_pi_owner(u32 __user *uaddr, u32 uval, union futex_key *key,
			      struct futex_pi_state **ps,
			      struct task_struct **exiting)
{
	pid_t pid = uval & FUTEX_TID_MASK;
	struct task_struct *p;

	/*
	 * We are the first waiter - try to look up the real owner and attach
	 * the new pi_state to it, but bail out when TID = 0 [1]
	 *
	 * The !pid check is paranoid. None of the call sites should end up
	 * with pid == 0, but better safe than sorry. Let the caller retry
	 */
	if (!pid)
		return -EAGAIN;
	p = find_get_task_by_vpid(pid);
	if (!p)
		return handle_exit_race(uaddr, uval, NULL);

	if (unlikely(p->flags & PF_KTHREAD)) {
		put_task_struct(p);
		return -EPERM;
	}

	/*
	 * We need to look at the task state to figure out, whether the
	 * task is exiting. To protect against the change of the task state
	 * in futex_exit_release(), we do this protected by p->pi_lock:
	 */
	raw_spin_lock_irq(&p->pi_lock);
	if (unlikely(p->futex_state != FUTEX_STATE_OK)) {
		/*
		 * The task is on the way out. When the futex state is
		 * FUTEX_STATE_DEAD, we know that the task has finished
		 * the cleanup:
		 */
		int ret = handle_exit_race(uaddr, uval, p);

		raw_spin_unlock_irq(&p->pi_lock);
		/*
		 * If the owner task is between FUTEX_STATE_EXITING and
		 * FUTEX_STATE_DEAD then store the task pointer and keep
		 * the reference on the task struct. The calling code will
		 * drop all locks, wait for the task to reach
		 * FUTEX_STATE_DEAD and then drop the refcount. This is
		 * required to prevent a live lock when the current task
		 * preempted the exiting task between the two states.
		 */
		if (ret == -EBUSY)
			*exiting = p;
		else
			put_task_struct(p);
		return ret;
	}

	__attach_to_pi_owner(p, key, ps);
	raw_spin_unlock_irq(&p->pi_lock);

	put_task_struct(p);

	return 0;
}

static int lock_pi_update_atomic(u32 __user *uaddr, u32 uval, u32 newval)
{
	int err;
	u32 curval;

	if (unlikely(should_fail_futex(true)))
		return -EFAULT;

	err = futex_cmpxchg_value_locked(&curval, uaddr, uval, newval);
	if (unlikely(err))
		return err;

	/* If user space value changed, let the caller retry */
	return curval != uval ? -EAGAIN : 0;
}

/**
 * futex_lock_pi_atomic() - Atomic work required to acquire a pi aware futex
 * @uaddr:		the pi futex user address
 * @hb:			the pi futex hash bucket
 * @key:		the futex key associated with uaddr and hb
 * @ps:			the pi_state pointer where we store the result of the
 *			lookup
 * @task:		the task to perform the atomic lock work for.  This will
 *			be "current" except in the case of requeue pi.
 * @exiting:		Pointer to store the task pointer of the owner task
 *			which is in the middle of exiting
 * @set_waiters:	force setting the FUTEX_WAITERS bit (1) or not (0)
 *
 * Return:
 *  -  0 - ready to wait;
 *  -  1 - acquired the lock;
 *  - <0 - error
 *
 * The hb->lock must be held by the caller.
 *
 * @exiting is only set when the return value is -EBUSY. If so, this holds
 * a refcount on the exiting task on return and the caller needs to drop it
 * after waiting for the exit to complete.
 */
int futex_lock_pi_atomic(u32 __user *uaddr, struct futex_hash_bucket *hb,
			 union futex_key *key,
			 struct futex_pi_state **ps,
			 struct task_struct *task,
			 struct task_struct **exiting,
			 int set_waiters)
{
	u32 uval, newval, vpid = task_pid_vnr(task);
	struct futex_q *top_waiter;
	int ret;

	/*
	 * Read the user space value first so we can validate a few
	 * things before proceeding further.
	 */
	if (futex_get_value_locked(&uval, uaddr))
		return -EFAULT;

	if (unlikely(should_fail_futex(true)))
		return -EFAULT;

	/*
	 * Detect deadlocks.
	 */
	if ((unlikely((uval & FUTEX_TID_MASK) == vpid)))
		return -EDEADLK;

	if ((unlikely(should_fail_futex(true))))
		return -EDEADLK;

	/*
	 * Lookup existing state first. If it exists, try to attach to
	 * its pi_state.
	 */
	top_waiter = futex_top_waiter(hb, key);
	if (top_waiter)
		return attach_to_pi_state(uaddr, uval, top_waiter->pi_state, ps);

	/*
	 * No waiter and user TID is 0. We are here because the
	 * waiters or the owner died bit is set or called from
	 * requeue_cmp_pi or for whatever reason something took the
	 * syscall.
	 */
	if (!(uval & FUTEX_TID_MASK)) {
		/*
		 * We take over the futex. No other waiters and the user space
		 * TID is 0. We preserve the owner died bit.
		 */
		newval = uval & FUTEX_OWNER_DIED;
		newval |= vpid;

		/* The futex requeue_pi code can enforce the waiters bit */
		if (set_waiters)
			newval |= FUTEX_WAITERS;

		ret = lock_pi_update_atomic(uaddr, uval, newval);
		if (ret)
			return ret;

		/*
		 * If the waiter bit was requested the caller also needs PI
		 * state attached to the new owner of the user space futex.
		 *
		 * @task is guaranteed to be alive and it cannot be exiting
		 * because it is either sleeping or waiting in
		 * futex_requeue_pi_wakeup_sync().
		 *
		 * No need to do the full attach_to_pi_owner() exercise
		 * because @task is known and valid.
		 */
		if (set_waiters) {
			raw_spin_lock_irq(&task->pi_lock);
			__attach_to_pi_owner(task, key, ps);
			raw_spin_unlock_irq(&task->pi_lock);
		}
		return 1;
	}

	/*
	 * First waiter. Set the waiters bit before attaching ourself to
	 * the owner. If owner tries to unlock, it will be forced into
	 * the kernel and blocked on hb->lock.
	 */
	newval = uval | FUTEX_WAITERS;
	ret = lock_pi_update_atomic(uaddr, uval, newval);
	if (ret)
		return ret;
	/*
	 * If the update of the user space value succeeded, we try to
	 * attach to the owner. If that fails, no harm done, we only
	 * set the FUTEX_WAITERS bit in the user space variable.
	 */
	return attach_to_pi_owner(uaddr, newval, key, ps, exiting);
}

/*
 * Caller must hold a reference on @pi_state.
 */
static int wake_futex_pi(u32 __user *uaddr, u32 uval,
			 struct futex_pi_state *pi_state,
			 struct rt_mutex_waiter *top_waiter)
{
	struct task_struct *new_owner;
	bool postunlock = false;
	DEFINE_RT_WAKE_Q(wqh);
	u32 curval, newval;
	int ret = 0;

	new_owner = top_waiter->task;

	/*
	 * We pass it to the next owner. The WAITERS bit is always kept
	 * enabled while there is PI state around. We cleanup the owner
	 * died bit, because we are the owner.
	 */
	newval = FUTEX_WAITERS | task_pid_vnr(new_owner);

	if (unlikely(should_fail_futex(true))) {
		ret = -EFAULT;
		goto out_unlock;
	}

	ret = futex_cmpxchg_value_locked(&curval, uaddr, uval, newval);
	if (!ret && (curval != uval)) {
		/*
		 * If a unconditional UNLOCK_PI operation (user space did not
		 * try the TID->0 transition) raced with a waiter setting the
		 * FUTEX_WAITERS flag between get_user() and locking the hash
		 * bucket lock, retry the operation.
		 */
		if ((FUTEX_TID_MASK & curval) == uval)
			ret = -EAGAIN;
		else
			ret = -EINVAL;
	}

	if (!ret) {
		/*
		 * This is a point of no return; once we modified the uval
		 * there is no going back and subsequent operations must
		 * not fail.
		 */
		pi_state_update_owner(pi_state, new_owner);
		postunlock = __rt_mutex_futex_unlock(&pi_state->pi_mutex, &wqh);
	}

out_unlock:
	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);

	if (postunlock)
		rt_mutex_postunlock(&wqh);

	return ret;
}

static int __fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
				  struct task_struct *argowner)
{
	struct futex_pi_state *pi_state = q->pi_state;
	struct task_struct *oldowner, *newowner;
	u32 uval, curval, newval, newtid;
	int err = 0;

	oldowner = pi_state->owner;

	/*
	 * We are here because either:
	 *
	 *  - we stole the lock and pi_state->owner needs updating to reflect
	 *    that (@argowner == current),
	 *
	 * or:
	 *
	 *  - someone stole our lock and we need to fix things to point to the
	 *    new owner (@argowner == NULL).
	 *
	 * Either way, we have to replace the TID in the user space variable.
	 * This must be atomic as we have to preserve the owner died bit here.
	 *
	 * Note: We write the user space value _before_ changing the pi_state
	 * because we can fault here. Imagine swapped out pages or a fork
	 * that marked all the anonymous memory readonly for cow.
	 *
	 * Modifying pi_state _before_ the user space value would leave the
	 * pi_state in an inconsistent state when we fault here, because we
	 * need to drop the locks to handle the fault. This might be observed
	 * in the PID checks when attaching to PI state .
	 */
retry:
	if (!argowner) {
		if (oldowner != current) {
			/*
			 * We raced against a concurrent self; things are
			 * already fixed up. Nothing to do.
			 */
			return 0;
		}

		if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
			/* We got the lock. pi_state is correct. Tell caller. */
			return 1;
		}

		/*
		 * The trylock just failed, so either there is an owner or
		 * there is a higher priority waiter than this one.
		 */
		newowner = rt_mutex_owner(&pi_state->pi_mutex);
		/*
		 * If the higher priority waiter has not yet taken over the
		 * rtmutex then newowner is NULL. We can't return here with
		 * that state because it's inconsistent vs. the user space
		 * state. So drop the locks and try again. It's a valid
		 * situation and not any different from the other retry
		 * conditions.
		 */
		if (unlikely(!newowner)) {
			err = -EAGAIN;
			goto handle_err;
		}
	} else {
		WARN_ON_ONCE(argowner != current);
		if (oldowner == current) {
			/*
			 * We raced against a concurrent self; things are
			 * already fixed up. Nothing to do.
			 */
			return 1;
		}
		newowner = argowner;
	}

	newtid = task_pid_vnr(newowner) | FUTEX_WAITERS;
	/* Owner died? */
	if (!pi_state->owner)
		newtid |= FUTEX_OWNER_DIED;

	err = futex_get_value_locked(&uval, uaddr);
	if (err)
		goto handle_err;

	for (;;) {
		newval = (uval & FUTEX_OWNER_DIED) | newtid;

		err = futex_cmpxchg_value_locked(&curval, uaddr, uval, newval);
		if (err)
			goto handle_err;

		if (curval == uval)
			break;
		uval = curval;
	}

	/*
	 * We fixed up user space. Now we need to fix the pi_state
	 * itself.
	 */
	pi_state_update_owner(pi_state, newowner);

	return argowner == current;

	/*
	 * In order to reschedule or handle a page fault, we need to drop the
	 * locks here. In the case of a fault, this gives the other task
	 * (either the highest priority waiter itself or the task which stole
	 * the rtmutex) the chance to try the fixup of the pi_state. So once we
	 * are back from handling the fault we need to check the pi_state after
	 * reacquiring the locks and before trying to do another fixup. When
	 * the fixup has been done already we simply return.
	 *
	 * Note: we hold both hb->lock and pi_mutex->wait_lock. We can safely
	 * drop hb->lock since the caller owns the hb -> futex_q relation.
	 * Dropping the pi_mutex->wait_lock requires the state revalidate.
	 */
handle_err:
	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
	spin_unlock(q->lock_ptr);

	switch (err) {
	case -EFAULT:
		err = fault_in_user_writeable(uaddr);
		break;

	case -EAGAIN:
		cond_resched();
		err = 0;
		break;

	default:
		WARN_ON_ONCE(1);
		break;
	}

	spin_lock(q->lock_ptr);
	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

	/*
	 * Check if someone else fixed it for us:
	 */
	if (pi_state->owner != oldowner)
		return argowner == current;

	/* Retry if err was -EAGAIN or the fault in succeeded */
	if (!err)
		goto retry;

	/*
	 * fault_in_user_writeable() failed so user state is immutable. At
	 * best we can make the kernel state consistent but user state will
	 * be most likely hosed and any subsequent unlock operation will be
	 * rejected due to PI futex rule [10].
	 *
	 * Ensure that the rtmutex owner is also the pi_state owner despite
	 * the user space value claiming something different. There is no
	 * point in unlocking the rtmutex if current is the owner as it
	 * would need to wait until the next waiter has taken the rtmutex
	 * to guarantee consistent state. Keep it simple. Userspace asked
	 * for this wreckaged state.
	 *
	 * The rtmutex has an owner - either current or some other
	 * task. See the EAGAIN loop above.
	 */
	pi_state_update_owner(pi_state, rt_mutex_owner(&pi_state->pi_mutex));

	return err;
}

static int fixup_pi_state_owner(u32 __user *uaddr, struct futex_q *q,
				struct task_struct *argowner)
{
	struct futex_pi_state *pi_state = q->pi_state;
	int ret;

	lockdep_assert_held(q->lock_ptr);

	raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);
	ret = __fixup_pi_state_owner(uaddr, q, argowner);
	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
	return ret;
}

/**
 * fixup_pi_owner() - Post lock pi_state and corner case management
 * @uaddr:	user address of the futex
 * @q:		futex_q (contains pi_state and access to the rt_mutex)
 * @locked:	if the attempt to take the rt_mutex succeeded (1) or not (0)
 *
 * After attempting to lock an rt_mutex, this function is called to cleanup
 * the pi_state owner as well as handle race conditions that may allow us to
 * acquire the lock. Must be called with the hb lock held.
 *
 * Return:
 *  -  1 - success, lock taken;
 *  -  0 - success, lock not taken;
 *  - <0 - on error (-EFAULT)
 */
int fixup_pi_owner(u32 __user *uaddr, struct futex_q *q, int locked)
{
	if (locked) {
		/*
		 * Got the lock. We might not be the anticipated owner if we
		 * did a lock-steal - fix up the PI-state in that case:
		 *
		 * Speculative pi_state->owner read (we don't hold wait_lock);
		 * since we own the lock pi_state->owner == current is the
		 * stable state, anything else needs more attention.
		 */
		if (q->pi_state->owner != current)
			return fixup_pi_state_owner(uaddr, q, current);
		return 1;
	}

	/*
	 * If we didn't get the lock; check if anybody stole it from us. In
	 * that case, we need to fix up the uval to point to them instead of
	 * us, otherwise bad things happen. [10]
	 *
	 * Another speculative read; pi_state->owner == current is unstable
	 * but needs our attention.
	 */
	if (q->pi_state->owner == current)
		return fixup_pi_state_owner(uaddr, q, NULL);

	/*
	 * Paranoia check. If we did not take the lock, then we should not be
	 * the owner of the rt_mutex. Warn and establish consistent state.
	 */
	if (WARN_ON_ONCE(rt_mutex_owner(&q->pi_state->pi_mutex) == current))
		return fixup_pi_state_owner(uaddr, q, current);

	return 0;
}

/*
 * Userspace tried a 0 -> TID atomic transition of the futex value
 * and failed. The kernel side here does the whole locking operation:
 * if there are waiters then it will block as a consequence of relying
 * on rt-mutexes, it does PI, etc. (Due to races the kernel might see
 * a 0 value of the futex too.).
 *
 * Also serves as futex trylock_pi()'ing, and due semantics.
 */
int futex_lock_pi(u32 __user *uaddr, unsigned int flags, ktime_t *time, int trylock)
{
	struct hrtimer_sleeper timeout, *to;
	struct task_struct *exiting = NULL;
	struct rt_mutex_waiter rt_waiter;
	struct futex_hash_bucket *hb;
	struct futex_q q = futex_q_init;
	int res, ret;

	if (!IS_ENABLED(CONFIG_FUTEX_PI))
		return -ENOSYS;

	if (refill_pi_state_cache())
		return -ENOMEM;

	to = futex_setup_timer(time, &timeout, flags, 0);

retry:
	ret = get_futex_key(uaddr, flags, &q.key, FUTEX_WRITE);
	if (unlikely(ret != 0))
		goto out;

retry_private:
	hb = futex_q_lock(&q);

	ret = futex_lock_pi_atomic(uaddr, hb, &q.key, &q.pi_state, current,
				   &exiting, 0);
	if (unlikely(ret)) {
		/*
		 * Atomic work succeeded and we got the lock,
		 * or failed. Either way, we do _not_ block.
		 */
		switch (ret) {
		case 1:
			/* We got the lock. */
			ret = 0;
			goto out_unlock_put_key;
		case -EFAULT:
			goto uaddr_faulted;
		case -EBUSY:
		case -EAGAIN:
			/*
			 * Two reasons for this:
			 * - EBUSY: Task is exiting and we just wait for the
			 *   exit to complete.
			 * - EAGAIN: The user space value changed.
			 */
			futex_q_unlock(hb);
			/*
			 * Handle the case where the owner is in the middle of
			 * exiting. Wait for the exit to complete otherwise
			 * this task might loop forever, aka. live lock.
			 */
			wait_for_owner_exiting(ret, exiting);
			cond_resched();
			goto retry;
		default:
			goto out_unlock_put_key;
		}
	}

	WARN_ON(!q.pi_state);

	/*
	 * Only actually queue now that the atomic ops are done:
	 */
	__futex_queue(&q, hb);

	if (trylock) {
		ret = rt_mutex_futex_trylock(&q.pi_state->pi_mutex);
		/* Fixup the trylock return value: */
		ret = ret ? 0 : -EWOULDBLOCK;
		goto no_block;
	}

	/*
	 * Must be done before we enqueue the waiter, here is unfortunately
	 * under the hb lock, but that *should* work because it does nothing.
	 */
	rt_mutex_pre_schedule();

	rt_mutex_init_waiter(&rt_waiter);

	/*
	 * On PREEMPT_RT, when hb->lock becomes an rt_mutex, we must not
	 * hold it while doing rt_mutex_start_proxy(), because then it will
	 * include hb->lock in the blocking chain, even through we'll not in
	 * fact hold it while blocking. This will lead it to report -EDEADLK
	 * and BUG when futex_unlock_pi() interleaves with this.
	 *
	 * Therefore acquire wait_lock while holding hb->lock, but drop the
	 * latter before calling __rt_mutex_start_proxy_lock(). This
	 * interleaves with futex_unlock_pi() -- which does a similar lock
	 * handoff -- such that the latter can observe the futex_q::pi_state
	 * before __rt_mutex_start_proxy_lock() is done.
	 */
	raw_spin_lock_irq(&q.pi_state->pi_mutex.wait_lock);
	spin_unlock(q.lock_ptr);
	/*
	 * __rt_mutex_start_proxy_lock() unconditionally enqueues the @rt_waiter
	 * such that futex_unlock_pi() is guaranteed to observe the waiter when
	 * it sees the futex_q::pi_state.
	 */
	ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
	raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);

	if (ret) {
		if (ret == 1)
			ret = 0;
		goto cleanup;
	}

	if (unlikely(to))
		hrtimer_sleeper_start_expires(to, HRTIMER_MODE_ABS);

	ret = rt_mutex_wait_proxy_lock(&q.pi_state->pi_mutex, to, &rt_waiter);

cleanup:
	/*
	 * If we failed to acquire the lock (deadlock/signal/timeout), we must
	 * must unwind the above, however we canont lock hb->lock because
	 * rt_mutex already has a waiter enqueued and hb->lock can itself try
	 * and enqueue an rt_waiter through rtlock.
	 *
	 * Doing the cleanup without holding hb->lock can cause inconsistent
	 * state between hb and pi_state, but only in the direction of not
	 * seeing a waiter that is leaving.
	 *
	 * See futex_unlock_pi(), it deals with this inconsistency.
	 *
	 * There be dragons here, since we must deal with the inconsistency on
	 * the way out (here), it is impossible to detect/warn about the race
	 * the other way around (missing an incoming waiter).
	 *
	 * What could possibly go wrong...
	 */
	if (ret && !rt_mutex_cleanup_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter))
		ret = 0;

	/*
	 * Now that the rt_waiter has been dequeued, it is safe to use
	 * spinlock/rtlock (which might enqueue its own rt_waiter) and fix up
	 * the
	 */
	spin_lock(q.lock_ptr);
	/*
	 * Waiter is unqueued.
	 */
	rt_mutex_post_schedule();
no_block:
	/*
	 * Fixup the pi_state owner and possibly acquire the lock if we
	 * haven't already.
	 */
	res = fixup_pi_owner(uaddr, &q, !ret);
	/*
	 * If fixup_pi_owner() returned an error, propagate that.  If it acquired
	 * the lock, clear our -ETIMEDOUT or -EINTR.
	 */
	if (res)
		ret = (res < 0) ? res : 0;

	futex_unqueue_pi(&q);
	spin_unlock(q.lock_ptr);
	goto out;

out_unlock_put_key:
	futex_q_unlock(hb);

out:
	if (to) {
		hrtimer_cancel(&to->timer);
		destroy_hrtimer_on_stack(&to->timer);
	}
	return ret != -EINTR ? ret : -ERESTARTNOINTR;

uaddr_faulted:
	futex_q_unlock(hb);

	ret = fault_in_user_writeable(uaddr);
	if (ret)
		goto out;

	if (!(flags & FLAGS_SHARED))
		goto retry_private;

	goto retry;
}

/*
 * Userspace attempted a TID -> 0 atomic transition, and failed.
 * This is the in-kernel slowpath: we look up the PI state (if any),
 * and do the rt-mutex unlock.
 */
int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
{
	u32 curval, uval, vpid = task_pid_vnr(current);
	union futex_key key = FUTEX_KEY_INIT;
	struct futex_hash_bucket *hb;
	struct futex_q *top_waiter;
	int ret;

	if (!IS_ENABLED(CONFIG_FUTEX_PI))
		return -ENOSYS;

retry:
	if (get_user(uval, uaddr))
		return -EFAULT;
	/*
	 * We release only a lock we actually own:
	 */
	if ((uval & FUTEX_TID_MASK) != vpid)
		return -EPERM;

	ret = get_futex_key(uaddr, flags, &key, FUTEX_WRITE);
	if (ret)
		return ret;

	hb = futex_hash(&key);
	spin_lock(&hb->lock);

	/*
	 * Check waiters first. We do not trust user space values at
	 * all and we at least want to know if user space fiddled
	 * with the futex value instead of blindly unlocking.
	 */
	top_waiter = futex_top_waiter(hb, &key);
	if (top_waiter) {
		struct futex_pi_state *pi_state = top_waiter->pi_state;
		struct rt_mutex_waiter *rt_waiter;

		ret = -EINVAL;
		if (!pi_state)
			goto out_unlock;

		/*
		 * If current does not own the pi_state then the futex is
		 * inconsistent and user space fiddled with the futex value.
		 */
		if (pi_state->owner != current)
			goto out_unlock;

		/*
		 * By taking wait_lock while still holding hb->lock, we ensure
		 * there is no point where we hold neither; and thereby
		 * wake_futex_pi() must observe any new waiters.
		 *
		 * Since the cleanup: case in futex_lock_pi() removes the
		 * rt_waiter without holding hb->lock, it is possible for
		 * wake_futex_pi() to not find a waiter while the above does,
		 * in this case the waiter is on the way out and it can be
		 * ignored.
		 *
		 * In particular; this forces __rt_mutex_start_proxy() to
		 * complete such that we're guaranteed to observe the
		 * rt_waiter.
		 */
		raw_spin_lock_irq(&pi_state->pi_mutex.wait_lock);

		/*
		 * Futex vs rt_mutex waiter state -- if there are no rt_mutex
		 * waiters even though futex thinks there are, then the waiter
		 * is leaving and the uncontended path is safe to take.
		 */
		rt_waiter = rt_mutex_top_waiter(&pi_state->pi_mutex);
		if (!rt_waiter) {
			raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
			goto do_uncontended;
		}

		get_pi_state(pi_state);
		spin_unlock(&hb->lock);

		/* drops pi_state->pi_mutex.wait_lock */
		ret = wake_futex_pi(uaddr, uval, pi_state, rt_waiter);

		put_pi_state(pi_state);

		/*
		 * Success, we're done! No tricky corner cases.
		 */
		if (!ret)
			return ret;
		/*
		 * The atomic access to the futex value generated a
		 * pagefault, so retry the user-access and the wakeup:
		 */
		if (ret == -EFAULT)
			goto pi_faulted;
		/*
		 * A unconditional UNLOCK_PI op raced against a waiter
		 * setting the FUTEX_WAITERS bit. Try again.
		 */
		if (ret == -EAGAIN)
			goto pi_retry;
		/*
		 * wake_futex_pi has detected invalid state. Tell user
		 * space.
		 */
		return ret;
	}

do_uncontended:
	/*
	 * We have no kernel internal state, i.e. no waiters in the
	 * kernel. Waiters which are about to queue themselves are stuck
	 * on hb->lock. So we can safely ignore them. We do neither
	 * preserve the WAITERS bit not the OWNER_DIED one. We are the
	 * owner.
	 */
	if ((ret = futex_cmpxchg_value_locked(&curval, uaddr, uval, 0))) {
		spin_unlock(&hb->lock);
		switch (ret) {
		case -EFAULT:
			goto pi_faulted;

		case -EAGAIN:
			goto pi_retry;

		default:
			WARN_ON_ONCE(1);
			return ret;
		}
	}

	/*
	 * If uval has changed, let user space handle it.
	 */
	ret = (curval == uval) ? 0 : -EAGAIN;

out_unlock:
	spin_unlock(&hb->lock);
	return ret;

pi_retry:
	cond_resched();
	goto retry;

pi_faulted:

	ret = fault_in_user_writeable(uaddr);
	if (!ret)
		goto retry;

	return ret;
}