summaryrefslogtreecommitdiffstats
path: root/src/backend/storage/lmgr/deadlock.c
blob: 67733c0d1a76c0bb71f7a1eccff8a3f8fa571cbd (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
/*-------------------------------------------------------------------------
 *
 * deadlock.c
 *	  POSTGRES deadlock detection code
 *
 * See src/backend/storage/lmgr/README for a description of the deadlock
 * detection and resolution algorithms.
 *
 *
 * Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
 * Portions Copyright (c) 1994, Regents of the University of California
 *
 *
 * IDENTIFICATION
 *	  src/backend/storage/lmgr/deadlock.c
 *
 *	Interface:
 *
 *	DeadLockCheck()
 *	DeadLockReport()
 *	RememberSimpleDeadLock()
 *	InitDeadLockChecking()
 *
 *-------------------------------------------------------------------------
 */
#include "postgres.h"

#include "miscadmin.h"
#include "pg_trace.h"
#include "pgstat.h"
#include "storage/lmgr.h"
#include "storage/proc.h"
#include "utils/memutils.h"


/*
 * One edge in the waits-for graph.
 *
 * waiter and blocker may or may not be members of a lock group, but if either
 * is, it will be the leader rather than any other member of the lock group.
 * The group leaders act as representatives of the whole group even though
 * those particular processes need not be waiting at all.  There will be at
 * least one member of the waiter's lock group on the wait queue for the given
 * lock, maybe more.
 */
typedef struct
{
	PGPROC	   *waiter;			/* the leader of the waiting lock group */
	PGPROC	   *blocker;		/* the leader of the group it is waiting for */
	LOCK	   *lock;			/* the lock being waited for */
	int			pred;			/* workspace for TopoSort */
	int			link;			/* workspace for TopoSort */
} EDGE;

/* One potential reordering of a lock's wait queue */
typedef struct
{
	LOCK	   *lock;			/* the lock whose wait queue is described */
	PGPROC	  **procs;			/* array of PGPROC *'s in new wait order */
	int			nProcs;
} WAIT_ORDER;

/*
 * Information saved about each edge in a detected deadlock cycle.  This
 * is used to print a diagnostic message upon failure.
 *
 * Note: because we want to examine this info after releasing the lock
 * manager's partition locks, we can't just store LOCK and PGPROC pointers;
 * we must extract out all the info we want to be able to print.
 */
typedef struct
{
	LOCKTAG		locktag;		/* ID of awaited lock object */
	LOCKMODE	lockmode;		/* type of lock we're waiting for */
	int			pid;			/* PID of blocked backend */
} DEADLOCK_INFO;


static bool DeadLockCheckRecurse(PGPROC *proc);
static int	TestConfiguration(PGPROC *startProc);
static bool FindLockCycle(PGPROC *checkProc,
						  EDGE *softEdges, int *nSoftEdges);
static bool FindLockCycleRecurse(PGPROC *checkProc, int depth,
								 EDGE *softEdges, int *nSoftEdges);
static bool FindLockCycleRecurseMember(PGPROC *checkProc,
									   PGPROC *checkProcLeader,
									   int depth, EDGE *softEdges, int *nSoftEdges);
static bool ExpandConstraints(EDGE *constraints, int nConstraints);
static bool TopoSort(LOCK *lock, EDGE *constraints, int nConstraints,
					 PGPROC **ordering);

#ifdef DEBUG_DEADLOCK
static void PrintLockQueue(LOCK *lock, const char *info);
#endif


/*
 * Working space for the deadlock detector
 */

/* Workspace for FindLockCycle */
static PGPROC **visitedProcs;	/* Array of visited procs */
static int	nVisitedProcs;

/* Workspace for TopoSort */
static PGPROC **topoProcs;		/* Array of not-yet-output procs */
static int *beforeConstraints;	/* Counts of remaining before-constraints */
static int *afterConstraints;	/* List head for after-constraints */

/* Output area for ExpandConstraints */
static WAIT_ORDER *waitOrders;	/* Array of proposed queue rearrangements */
static int	nWaitOrders;
static PGPROC **waitOrderProcs; /* Space for waitOrders queue contents */

/* Current list of constraints being considered */
static EDGE *curConstraints;
static int	nCurConstraints;
static int	maxCurConstraints;

/* Storage space for results from FindLockCycle */
static EDGE *possibleConstraints;
static int	nPossibleConstraints;
static int	maxPossibleConstraints;
static DEADLOCK_INFO *deadlockDetails;
static int	nDeadlockDetails;

/* PGPROC pointer of any blocking autovacuum worker found */
static PGPROC *blocking_autovacuum_proc = NULL;


/*
 * InitDeadLockChecking -- initialize deadlock checker during backend startup
 *
 * This does per-backend initialization of the deadlock checker; primarily,
 * allocation of working memory for DeadLockCheck.  We do this per-backend
 * since there's no percentage in making the kernel do copy-on-write
 * inheritance of workspace from the postmaster.  We want to allocate the
 * space at startup because (a) the deadlock checker might be invoked when
 * there's no free memory left, and (b) the checker is normally run inside a
 * signal handler, which is a very dangerous place to invoke palloc from.
 */
void
InitDeadLockChecking(void)
{
	MemoryContext oldcxt;

	/* Make sure allocations are permanent */
	oldcxt = MemoryContextSwitchTo(TopMemoryContext);

	/*
	 * FindLockCycle needs at most MaxBackends entries in visitedProcs[] and
	 * deadlockDetails[].
	 */
	visitedProcs = (PGPROC **) palloc(MaxBackends * sizeof(PGPROC *));
	deadlockDetails = (DEADLOCK_INFO *) palloc(MaxBackends * sizeof(DEADLOCK_INFO));

	/*
	 * TopoSort needs to consider at most MaxBackends wait-queue entries, and
	 * it needn't run concurrently with FindLockCycle.
	 */
	topoProcs = visitedProcs;	/* re-use this space */
	beforeConstraints = (int *) palloc(MaxBackends * sizeof(int));
	afterConstraints = (int *) palloc(MaxBackends * sizeof(int));

	/*
	 * We need to consider rearranging at most MaxBackends/2 wait queues
	 * (since it takes at least two waiters in a queue to create a soft edge),
	 * and the expanded form of the wait queues can't involve more than
	 * MaxBackends total waiters.
	 */
	waitOrders = (WAIT_ORDER *)
		palloc((MaxBackends / 2) * sizeof(WAIT_ORDER));
	waitOrderProcs = (PGPROC **) palloc(MaxBackends * sizeof(PGPROC *));

	/*
	 * Allow at most MaxBackends distinct constraints in a configuration. (Is
	 * this enough?  In practice it seems it should be, but I don't quite see
	 * how to prove it.  If we run out, we might fail to find a workable wait
	 * queue rearrangement even though one exists.)  NOTE that this number
	 * limits the maximum recursion depth of DeadLockCheckRecurse. Making it
	 * really big might potentially allow a stack-overflow problem.
	 */
	maxCurConstraints = MaxBackends;
	curConstraints = (EDGE *) palloc(maxCurConstraints * sizeof(EDGE));

	/*
	 * Allow up to 3*MaxBackends constraints to be saved without having to
	 * re-run TestConfiguration.  (This is probably more than enough, but we
	 * can survive if we run low on space by doing excess runs of
	 * TestConfiguration to re-compute constraint lists each time needed.) The
	 * last MaxBackends entries in possibleConstraints[] are reserved as
	 * output workspace for FindLockCycle.
	 */
	maxPossibleConstraints = MaxBackends * 4;
	possibleConstraints =
		(EDGE *) palloc(maxPossibleConstraints * sizeof(EDGE));

	MemoryContextSwitchTo(oldcxt);
}

/*
 * DeadLockCheck -- Checks for deadlocks for a given process
 *
 * This code looks for deadlocks involving the given process.  If any
 * are found, it tries to rearrange lock wait queues to resolve the
 * deadlock.  If resolution is impossible, return DS_HARD_DEADLOCK ---
 * the caller is then expected to abort the given proc's transaction.
 *
 * Caller must already have locked all partitions of the lock tables.
 *
 * On failure, deadlock details are recorded in deadlockDetails[] for
 * subsequent printing by DeadLockReport().  That activity is separate
 * because (a) we don't want to do it while holding all those LWLocks,
 * and (b) we are typically invoked inside a signal handler.
 */
DeadLockState
DeadLockCheck(PGPROC *proc)
{
	int			i,
				j;

	/* Initialize to "no constraints" */
	nCurConstraints = 0;
	nPossibleConstraints = 0;
	nWaitOrders = 0;

	/* Initialize to not blocked by an autovacuum worker */
	blocking_autovacuum_proc = NULL;

	/* Search for deadlocks and possible fixes */
	if (DeadLockCheckRecurse(proc))
	{
		/*
		 * Call FindLockCycle one more time, to record the correct
		 * deadlockDetails[] for the basic state with no rearrangements.
		 */
		int			nSoftEdges;

		TRACE_POSTGRESQL_DEADLOCK_FOUND();

		nWaitOrders = 0;
		if (!FindLockCycle(proc, possibleConstraints, &nSoftEdges))
			elog(FATAL, "deadlock seems to have disappeared");

		return DS_HARD_DEADLOCK;	/* cannot find a non-deadlocked state */
	}

	/* Apply any needed rearrangements of wait queues */
	for (i = 0; i < nWaitOrders; i++)
	{
		LOCK	   *lock = waitOrders[i].lock;
		PGPROC	  **procs = waitOrders[i].procs;
		int			nProcs = waitOrders[i].nProcs;
		PROC_QUEUE *waitQueue = &(lock->waitProcs);

		Assert(nProcs == waitQueue->size);

#ifdef DEBUG_DEADLOCK
		PrintLockQueue(lock, "DeadLockCheck:");
#endif

		/* Reset the queue and re-add procs in the desired order */
		ProcQueueInit(waitQueue);
		for (j = 0; j < nProcs; j++)
		{
			SHMQueueInsertBefore(&(waitQueue->links), &(procs[j]->links));
			waitQueue->size++;
		}

#ifdef DEBUG_DEADLOCK
		PrintLockQueue(lock, "rearranged to:");
#endif

		/* See if any waiters for the lock can be woken up now */
		ProcLockWakeup(GetLocksMethodTable(lock), lock);
	}

	/* Return code tells caller if we had to escape a deadlock or not */
	if (nWaitOrders > 0)
		return DS_SOFT_DEADLOCK;
	else if (blocking_autovacuum_proc != NULL)
		return DS_BLOCKED_BY_AUTOVACUUM;
	else
		return DS_NO_DEADLOCK;
}

/*
 * Return the PGPROC of the autovacuum that's blocking a process.
 *
 * We reset the saved pointer as soon as we pass it back.
 */
PGPROC *
GetBlockingAutoVacuumPgproc(void)
{
	PGPROC	   *ptr;

	ptr = blocking_autovacuum_proc;
	blocking_autovacuum_proc = NULL;

	return ptr;
}

/*
 * DeadLockCheckRecurse -- recursively search for valid orderings
 *
 * curConstraints[] holds the current set of constraints being considered
 * by an outer level of recursion.  Add to this each possible solution
 * constraint for any cycle detected at this level.
 *
 * Returns true if no solution exists.  Returns false if a deadlock-free
 * state is attainable, in which case waitOrders[] shows the required
 * rearrangements of lock wait queues (if any).
 */
static bool
DeadLockCheckRecurse(PGPROC *proc)
{
	int			nEdges;
	int			oldPossibleConstraints;
	bool		savedList;
	int			i;

	nEdges = TestConfiguration(proc);
	if (nEdges < 0)
		return true;			/* hard deadlock --- no solution */
	if (nEdges == 0)
		return false;			/* good configuration found */
	if (nCurConstraints >= maxCurConstraints)
		return true;			/* out of room for active constraints? */
	oldPossibleConstraints = nPossibleConstraints;
	if (nPossibleConstraints + nEdges + MaxBackends <= maxPossibleConstraints)
	{
		/* We can save the edge list in possibleConstraints[] */
		nPossibleConstraints += nEdges;
		savedList = true;
	}
	else
	{
		/* Not room; will need to regenerate the edges on-the-fly */
		savedList = false;
	}

	/*
	 * Try each available soft edge as an addition to the configuration.
	 */
	for (i = 0; i < nEdges; i++)
	{
		if (!savedList && i > 0)
		{
			/* Regenerate the list of possible added constraints */
			if (nEdges != TestConfiguration(proc))
				elog(FATAL, "inconsistent results during deadlock check");
		}
		curConstraints[nCurConstraints] =
			possibleConstraints[oldPossibleConstraints + i];
		nCurConstraints++;
		if (!DeadLockCheckRecurse(proc))
			return false;		/* found a valid solution! */
		/* give up on that added constraint, try again */
		nCurConstraints--;
	}
	nPossibleConstraints = oldPossibleConstraints;
	return true;				/* no solution found */
}


/*--------------------
 * Test a configuration (current set of constraints) for validity.
 *
 * Returns:
 *		0: the configuration is good (no deadlocks)
 *	   -1: the configuration has a hard deadlock or is not self-consistent
 *		>0: the configuration has one or more soft deadlocks
 *
 * In the soft-deadlock case, one of the soft cycles is chosen arbitrarily
 * and a list of its soft edges is returned beginning at
 * possibleConstraints+nPossibleConstraints.  The return value is the
 * number of soft edges.
 *--------------------
 */
static int
TestConfiguration(PGPROC *startProc)
{
	int			softFound = 0;
	EDGE	   *softEdges = possibleConstraints + nPossibleConstraints;
	int			nSoftEdges;
	int			i;

	/*
	 * Make sure we have room for FindLockCycle's output.
	 */
	if (nPossibleConstraints + MaxBackends > maxPossibleConstraints)
		return -1;

	/*
	 * Expand current constraint set into wait orderings.  Fail if the
	 * constraint set is not self-consistent.
	 */
	if (!ExpandConstraints(curConstraints, nCurConstraints))
		return -1;

	/*
	 * Check for cycles involving startProc or any of the procs mentioned in
	 * constraints.  We check startProc last because if it has a soft cycle
	 * still to be dealt with, we want to deal with that first.
	 */
	for (i = 0; i < nCurConstraints; i++)
	{
		if (FindLockCycle(curConstraints[i].waiter, softEdges, &nSoftEdges))
		{
			if (nSoftEdges == 0)
				return -1;		/* hard deadlock detected */
			softFound = nSoftEdges;
		}
		if (FindLockCycle(curConstraints[i].blocker, softEdges, &nSoftEdges))
		{
			if (nSoftEdges == 0)
				return -1;		/* hard deadlock detected */
			softFound = nSoftEdges;
		}
	}
	if (FindLockCycle(startProc, softEdges, &nSoftEdges))
	{
		if (nSoftEdges == 0)
			return -1;			/* hard deadlock detected */
		softFound = nSoftEdges;
	}
	return softFound;
}


/*
 * FindLockCycle -- basic check for deadlock cycles
 *
 * Scan outward from the given proc to see if there is a cycle in the
 * waits-for graph that includes this proc.  Return true if a cycle
 * is found, else false.  If a cycle is found, we return a list of
 * the "soft edges", if any, included in the cycle.  These edges could
 * potentially be eliminated by rearranging wait queues.  We also fill
 * deadlockDetails[] with information about the detected cycle; this info
 * is not used by the deadlock algorithm itself, only to print a useful
 * message after failing.
 *
 * Since we need to be able to check hypothetical configurations that would
 * exist after wait queue rearrangement, the routine pays attention to the
 * table of hypothetical queue orders in waitOrders[].  These orders will
 * be believed in preference to the actual ordering seen in the locktable.
 */
static bool
FindLockCycle(PGPROC *checkProc,
			  EDGE *softEdges,	/* output argument */
			  int *nSoftEdges)	/* output argument */
{
	nVisitedProcs = 0;
	nDeadlockDetails = 0;
	*nSoftEdges = 0;
	return FindLockCycleRecurse(checkProc, 0, softEdges, nSoftEdges);
}

static bool
FindLockCycleRecurse(PGPROC *checkProc,
					 int depth,
					 EDGE *softEdges,	/* output argument */
					 int *nSoftEdges)	/* output argument */
{
	int			i;
	dlist_iter	iter;

	/*
	 * If this process is a lock group member, check the leader instead. (Note
	 * that we might be the leader, in which case this is a no-op.)
	 */
	if (checkProc->lockGroupLeader != NULL)
		checkProc = checkProc->lockGroupLeader;

	/*
	 * Have we already seen this proc?
	 */
	for (i = 0; i < nVisitedProcs; i++)
	{
		if (visitedProcs[i] == checkProc)
		{
			/* If we return to starting point, we have a deadlock cycle */
			if (i == 0)
			{
				/*
				 * record total length of cycle --- outer levels will now fill
				 * deadlockDetails[]
				 */
				Assert(depth <= MaxBackends);
				nDeadlockDetails = depth;

				return true;
			}

			/*
			 * Otherwise, we have a cycle but it does not include the start
			 * point, so say "no deadlock".
			 */
			return false;
		}
	}
	/* Mark proc as seen */
	Assert(nVisitedProcs < MaxBackends);
	visitedProcs[nVisitedProcs++] = checkProc;

	/*
	 * If the process is waiting, there is an outgoing waits-for edge to each
	 * process that blocks it.
	 */
	if (checkProc->links.next != NULL && checkProc->waitLock != NULL &&
		FindLockCycleRecurseMember(checkProc, checkProc, depth, softEdges,
								   nSoftEdges))
		return true;

	/*
	 * If the process is not waiting, there could still be outgoing waits-for
	 * edges if it is part of a lock group, because other members of the lock
	 * group might be waiting even though this process is not.  (Given lock
	 * groups {A1, A2} and {B1, B2}, if A1 waits for B1 and B2 waits for A2,
	 * that is a deadlock even neither of B1 and A2 are waiting for anything.)
	 */
	dlist_foreach(iter, &checkProc->lockGroupMembers)
	{
		PGPROC	   *memberProc;

		memberProc = dlist_container(PGPROC, lockGroupLink, iter.cur);

		if (memberProc->links.next != NULL && memberProc->waitLock != NULL &&
			memberProc != checkProc &&
			FindLockCycleRecurseMember(memberProc, checkProc, depth, softEdges,
									   nSoftEdges))
			return true;
	}

	return false;
}

static bool
FindLockCycleRecurseMember(PGPROC *checkProc,
						   PGPROC *checkProcLeader,
						   int depth,
						   EDGE *softEdges, /* output argument */
						   int *nSoftEdges) /* output argument */
{
	PGPROC	   *proc;
	LOCK	   *lock = checkProc->waitLock;
	PROCLOCK   *proclock;
	SHM_QUEUE  *procLocks;
	LockMethod	lockMethodTable;
	PROC_QUEUE *waitQueue;
	int			queue_size;
	int			conflictMask;
	int			i;
	int			numLockModes,
				lm;

	/*
	 * The relation extension or page lock can never participate in actual
	 * deadlock cycle.  See Asserts in LockAcquireExtended.  So, there is no
	 * advantage in checking wait edges from them.
	 */
	if (LOCK_LOCKTAG(*lock) == LOCKTAG_RELATION_EXTEND ||
		(LOCK_LOCKTAG(*lock) == LOCKTAG_PAGE))
		return false;

	lockMethodTable = GetLocksMethodTable(lock);
	numLockModes = lockMethodTable->numLockModes;
	conflictMask = lockMethodTable->conflictTab[checkProc->waitLockMode];

	/*
	 * Scan for procs that already hold conflicting locks.  These are "hard"
	 * edges in the waits-for graph.
	 */
	procLocks = &(lock->procLocks);

	proclock = (PROCLOCK *) SHMQueueNext(procLocks, procLocks,
										 offsetof(PROCLOCK, lockLink));

	while (proclock)
	{
		PGPROC	   *leader;

		proc = proclock->tag.myProc;
		leader = proc->lockGroupLeader == NULL ? proc : proc->lockGroupLeader;

		/* A proc never blocks itself or any other lock group member */
		if (leader != checkProcLeader)
		{
			for (lm = 1; lm <= numLockModes; lm++)
			{
				if ((proclock->holdMask & LOCKBIT_ON(lm)) &&
					(conflictMask & LOCKBIT_ON(lm)))
				{
					/* This proc hard-blocks checkProc */
					if (FindLockCycleRecurse(proc, depth + 1,
											 softEdges, nSoftEdges))
					{
						/* fill deadlockDetails[] */
						DEADLOCK_INFO *info = &deadlockDetails[depth];

						info->locktag = lock->tag;
						info->lockmode = checkProc->waitLockMode;
						info->pid = checkProc->pid;

						return true;
					}

					/*
					 * No deadlock here, but see if this proc is an autovacuum
					 * that is directly hard-blocking our own proc.  If so,
					 * report it so that the caller can send a cancel signal
					 * to it, if appropriate.  If there's more than one such
					 * proc, it's indeterminate which one will be reported.
					 *
					 * We don't touch autovacuums that are indirectly blocking
					 * us; it's up to the direct blockee to take action.  This
					 * rule simplifies understanding the behavior and ensures
					 * that an autovacuum won't be canceled with less than
					 * deadlock_timeout grace period.
					 *
					 * Note we read statusFlags without any locking.  This is
					 * OK only for checking the PROC_IS_AUTOVACUUM flag,
					 * because that flag is set at process start and never
					 * reset.  There is logic elsewhere to avoid canceling an
					 * autovacuum that is working to prevent XID wraparound
					 * problems (which needs to read a different statusFlags
					 * bit), but we don't do that here to avoid grabbing
					 * ProcArrayLock.
					 */
					if (checkProc == MyProc &&
						proc->statusFlags & PROC_IS_AUTOVACUUM)
						blocking_autovacuum_proc = proc;

					/* We're done looking at this proclock */
					break;
				}
			}
		}

		proclock = (PROCLOCK *) SHMQueueNext(procLocks, &proclock->lockLink,
											 offsetof(PROCLOCK, lockLink));
	}

	/*
	 * Scan for procs that are ahead of this one in the lock's wait queue.
	 * Those that have conflicting requests soft-block this one.  This must be
	 * done after the hard-block search, since if another proc both hard- and
	 * soft-blocks this one, we want to call it a hard edge.
	 *
	 * If there is a proposed re-ordering of the lock's wait order, use that
	 * rather than the current wait order.
	 */
	for (i = 0; i < nWaitOrders; i++)
	{
		if (waitOrders[i].lock == lock)
			break;
	}

	if (i < nWaitOrders)
	{
		/* Use the given hypothetical wait queue order */
		PGPROC	  **procs = waitOrders[i].procs;

		queue_size = waitOrders[i].nProcs;

		for (i = 0; i < queue_size; i++)
		{
			PGPROC	   *leader;

			proc = procs[i];
			leader = proc->lockGroupLeader == NULL ? proc :
				proc->lockGroupLeader;

			/*
			 * TopoSort will always return an ordering with group members
			 * adjacent to each other in the wait queue (see comments
			 * therein). So, as soon as we reach a process in the same lock
			 * group as checkProc, we know we've found all the conflicts that
			 * precede any member of the lock group lead by checkProcLeader.
			 */
			if (leader == checkProcLeader)
				break;

			/* Is there a conflict with this guy's request? */
			if ((LOCKBIT_ON(proc->waitLockMode) & conflictMask) != 0)
			{
				/* This proc soft-blocks checkProc */
				if (FindLockCycleRecurse(proc, depth + 1,
										 softEdges, nSoftEdges))
				{
					/* fill deadlockDetails[] */
					DEADLOCK_INFO *info = &deadlockDetails[depth];

					info->locktag = lock->tag;
					info->lockmode = checkProc->waitLockMode;
					info->pid = checkProc->pid;

					/*
					 * Add this edge to the list of soft edges in the cycle
					 */
					Assert(*nSoftEdges < MaxBackends);
					softEdges[*nSoftEdges].waiter = checkProcLeader;
					softEdges[*nSoftEdges].blocker = leader;
					softEdges[*nSoftEdges].lock = lock;
					(*nSoftEdges)++;
					return true;
				}
			}
		}
	}
	else
	{
		PGPROC	   *lastGroupMember = NULL;

		/* Use the true lock wait queue order */
		waitQueue = &(lock->waitProcs);

		/*
		 * Find the last member of the lock group that is present in the wait
		 * queue.  Anything after this is not a soft lock conflict. If group
		 * locking is not in use, then we know immediately which process we're
		 * looking for, but otherwise we've got to search the wait queue to
		 * find the last process actually present.
		 */
		if (checkProc->lockGroupLeader == NULL)
			lastGroupMember = checkProc;
		else
		{
			proc = (PGPROC *) waitQueue->links.next;
			queue_size = waitQueue->size;
			while (queue_size-- > 0)
			{
				if (proc->lockGroupLeader == checkProcLeader)
					lastGroupMember = proc;
				proc = (PGPROC *) proc->links.next;
			}
			Assert(lastGroupMember != NULL);
		}

		/*
		 * OK, now rescan (or scan) the queue to identify the soft conflicts.
		 */
		queue_size = waitQueue->size;
		proc = (PGPROC *) waitQueue->links.next;
		while (queue_size-- > 0)
		{
			PGPROC	   *leader;

			leader = proc->lockGroupLeader == NULL ? proc :
				proc->lockGroupLeader;

			/* Done when we reach the target proc */
			if (proc == lastGroupMember)
				break;

			/* Is there a conflict with this guy's request? */
			if ((LOCKBIT_ON(proc->waitLockMode) & conflictMask) != 0 &&
				leader != checkProcLeader)
			{
				/* This proc soft-blocks checkProc */
				if (FindLockCycleRecurse(proc, depth + 1,
										 softEdges, nSoftEdges))
				{
					/* fill deadlockDetails[] */
					DEADLOCK_INFO *info = &deadlockDetails[depth];

					info->locktag = lock->tag;
					info->lockmode = checkProc->waitLockMode;
					info->pid = checkProc->pid;

					/*
					 * Add this edge to the list of soft edges in the cycle
					 */
					Assert(*nSoftEdges < MaxBackends);
					softEdges[*nSoftEdges].waiter = checkProcLeader;
					softEdges[*nSoftEdges].blocker = leader;
					softEdges[*nSoftEdges].lock = lock;
					(*nSoftEdges)++;
					return true;
				}
			}

			proc = (PGPROC *) proc->links.next;
		}
	}

	/*
	 * No conflict detected here.
	 */
	return false;
}


/*
 * ExpandConstraints -- expand a list of constraints into a set of
 *		specific new orderings for affected wait queues
 *
 * Input is a list of soft edges to be reversed.  The output is a list
 * of nWaitOrders WAIT_ORDER structs in waitOrders[], with PGPROC array
 * workspace in waitOrderProcs[].
 *
 * Returns true if able to build an ordering that satisfies all the
 * constraints, false if not (there are contradictory constraints).
 */
static bool
ExpandConstraints(EDGE *constraints,
				  int nConstraints)
{
	int			nWaitOrderProcs = 0;
	int			i,
				j;

	nWaitOrders = 0;

	/*
	 * Scan constraint list backwards.  This is because the last-added
	 * constraint is the only one that could fail, and so we want to test it
	 * for inconsistency first.
	 */
	for (i = nConstraints; --i >= 0;)
	{
		LOCK	   *lock = constraints[i].lock;

		/* Did we already make a list for this lock? */
		for (j = nWaitOrders; --j >= 0;)
		{
			if (waitOrders[j].lock == lock)
				break;
		}
		if (j >= 0)
			continue;
		/* No, so allocate a new list */
		waitOrders[nWaitOrders].lock = lock;
		waitOrders[nWaitOrders].procs = waitOrderProcs + nWaitOrderProcs;
		waitOrders[nWaitOrders].nProcs = lock->waitProcs.size;
		nWaitOrderProcs += lock->waitProcs.size;
		Assert(nWaitOrderProcs <= MaxBackends);

		/*
		 * Do the topo sort.  TopoSort need not examine constraints after this
		 * one, since they must be for different locks.
		 */
		if (!TopoSort(lock, constraints, i + 1,
					  waitOrders[nWaitOrders].procs))
			return false;
		nWaitOrders++;
	}
	return true;
}


/*
 * TopoSort -- topological sort of a wait queue
 *
 * Generate a re-ordering of a lock's wait queue that satisfies given
 * constraints about certain procs preceding others.  (Each such constraint
 * is a fact of a partial ordering.)  Minimize rearrangement of the queue
 * not needed to achieve the partial ordering.
 *
 * This is a lot simpler and slower than, for example, the topological sort
 * algorithm shown in Knuth's Volume 1.  However, Knuth's method doesn't
 * try to minimize the damage to the existing order.  In practice we are
 * not likely to be working with more than a few constraints, so the apparent
 * slowness of the algorithm won't really matter.
 *
 * The initial queue ordering is taken directly from the lock's wait queue.
 * The output is an array of PGPROC pointers, of length equal to the lock's
 * wait queue length (the caller is responsible for providing this space).
 * The partial order is specified by an array of EDGE structs.  Each EDGE
 * is one that we need to reverse, therefore the "waiter" must appear before
 * the "blocker" in the output array.  The EDGE array may well contain
 * edges associated with other locks; these should be ignored.
 *
 * Returns true if able to build an ordering that satisfies all the
 * constraints, false if not (there are contradictory constraints).
 */
static bool
TopoSort(LOCK *lock,
		 EDGE *constraints,
		 int nConstraints,
		 PGPROC **ordering)		/* output argument */
{
	PROC_QUEUE *waitQueue = &(lock->waitProcs);
	int			queue_size = waitQueue->size;
	PGPROC	   *proc;
	int			i,
				j,
				jj,
				k,
				kk,
				last;

	/* First, fill topoProcs[] array with the procs in their current order */
	proc = (PGPROC *) waitQueue->links.next;
	for (i = 0; i < queue_size; i++)
	{
		topoProcs[i] = proc;
		proc = (PGPROC *) proc->links.next;
	}

	/*
	 * Scan the constraints, and for each proc in the array, generate a count
	 * of the number of constraints that say it must be before something else,
	 * plus a list of the constraints that say it must be after something
	 * else. The count for the j'th proc is stored in beforeConstraints[j],
	 * and the head of its list in afterConstraints[j].  Each constraint
	 * stores its list link in constraints[i].link (note any constraint will
	 * be in just one list). The array index for the before-proc of the i'th
	 * constraint is remembered in constraints[i].pred.
	 *
	 * Note that it's not necessarily the case that every constraint affects
	 * this particular wait queue.  Prior to group locking, a process could be
	 * waiting for at most one lock.  But a lock group can be waiting for
	 * zero, one, or multiple locks.  Since topoProcs[] is an array of the
	 * processes actually waiting, while constraints[] is an array of group
	 * leaders, we've got to scan through topoProcs[] for each constraint,
	 * checking whether both a waiter and a blocker for that group are
	 * present.  If so, the constraint is relevant to this wait queue; if not,
	 * it isn't.
	 */
	MemSet(beforeConstraints, 0, queue_size * sizeof(int));
	MemSet(afterConstraints, 0, queue_size * sizeof(int));
	for (i = 0; i < nConstraints; i++)
	{
		/*
		 * Find a representative process that is on the lock queue and part of
		 * the waiting lock group.  This may or may not be the leader, which
		 * may or may not be waiting at all.  If there are any other processes
		 * in the same lock group on the queue, set their number of
		 * beforeConstraints to -1 to indicate that they should be emitted
		 * with their groupmates rather than considered separately.
		 *
		 * In this loop and the similar one just below, it's critical that we
		 * consistently select the same representative member of any one lock
		 * group, so that all the constraints are associated with the same
		 * proc, and the -1's are only associated with not-representative
		 * members.  We select the last one in the topoProcs array.
		 */
		proc = constraints[i].waiter;
		Assert(proc != NULL);
		jj = -1;
		for (j = queue_size; --j >= 0;)
		{
			PGPROC	   *waiter = topoProcs[j];

			if (waiter == proc || waiter->lockGroupLeader == proc)
			{
				Assert(waiter->waitLock == lock);
				if (jj == -1)
					jj = j;
				else
				{
					Assert(beforeConstraints[j] <= 0);
					beforeConstraints[j] = -1;
				}
			}
		}

		/* If no matching waiter, constraint is not relevant to this lock. */
		if (jj < 0)
			continue;

		/*
		 * Similarly, find a representative process that is on the lock queue
		 * and waiting for the blocking lock group.  Again, this could be the
		 * leader but does not need to be.
		 */
		proc = constraints[i].blocker;
		Assert(proc != NULL);
		kk = -1;
		for (k = queue_size; --k >= 0;)
		{
			PGPROC	   *blocker = topoProcs[k];

			if (blocker == proc || blocker->lockGroupLeader == proc)
			{
				Assert(blocker->waitLock == lock);
				if (kk == -1)
					kk = k;
				else
				{
					Assert(beforeConstraints[k] <= 0);
					beforeConstraints[k] = -1;
				}
			}
		}

		/* If no matching blocker, constraint is not relevant to this lock. */
		if (kk < 0)
			continue;

		Assert(beforeConstraints[jj] >= 0);
		beforeConstraints[jj]++;	/* waiter must come before */
		/* add this constraint to list of after-constraints for blocker */
		constraints[i].pred = jj;
		constraints[i].link = afterConstraints[kk];
		afterConstraints[kk] = i + 1;
	}

	/*--------------------
	 * Now scan the topoProcs array backwards.  At each step, output the
	 * last proc that has no remaining before-constraints plus any other
	 * members of the same lock group; then decrease the beforeConstraints
	 * count of each of the procs it was constrained against.
	 * i = index of ordering[] entry we want to output this time
	 * j = search index for topoProcs[]
	 * k = temp for scanning constraint list for proc j
	 * last = last non-null index in topoProcs (avoid redundant searches)
	 *--------------------
	 */
	last = queue_size - 1;
	for (i = queue_size - 1; i >= 0;)
	{
		int			c;
		int			nmatches = 0;

		/* Find next candidate to output */
		while (topoProcs[last] == NULL)
			last--;
		for (j = last; j >= 0; j--)
		{
			if (topoProcs[j] != NULL && beforeConstraints[j] == 0)
				break;
		}

		/* If no available candidate, topological sort fails */
		if (j < 0)
			return false;

		/*
		 * Output everything in the lock group.  There's no point in
		 * outputting an ordering where members of the same lock group are not
		 * consecutive on the wait queue: if some other waiter is between two
		 * requests that belong to the same group, then either it conflicts
		 * with both of them and is certainly not a solution; or it conflicts
		 * with at most one of them and is thus isomorphic to an ordering
		 * where the group members are consecutive.
		 */
		proc = topoProcs[j];
		if (proc->lockGroupLeader != NULL)
			proc = proc->lockGroupLeader;
		Assert(proc != NULL);
		for (c = 0; c <= last; ++c)
		{
			if (topoProcs[c] == proc || (topoProcs[c] != NULL &&
										 topoProcs[c]->lockGroupLeader == proc))
			{
				ordering[i - nmatches] = topoProcs[c];
				topoProcs[c] = NULL;
				++nmatches;
			}
		}
		Assert(nmatches > 0);
		i -= nmatches;

		/* Update beforeConstraints counts of its predecessors */
		for (k = afterConstraints[j]; k > 0; k = constraints[k - 1].link)
			beforeConstraints[constraints[k - 1].pred]--;
	}

	/* Done */
	return true;
}

#ifdef DEBUG_DEADLOCK
static void
PrintLockQueue(LOCK *lock, const char *info)
{
	PROC_QUEUE *waitQueue = &(lock->waitProcs);
	int			queue_size = waitQueue->size;
	PGPROC	   *proc;
	int			i;

	printf("%s lock %p queue ", info, lock);
	proc = (PGPROC *) waitQueue->links.next;
	for (i = 0; i < queue_size; i++)
	{
		printf(" %d", proc->pid);
		proc = (PGPROC *) proc->links.next;
	}
	printf("\n");
	fflush(stdout);
}
#endif

/*
 * Report a detected deadlock, with available details.
 */
void
DeadLockReport(void)
{
	StringInfoData clientbuf;	/* errdetail for client */
	StringInfoData logbuf;		/* errdetail for server log */
	StringInfoData locktagbuf;
	int			i;

	initStringInfo(&clientbuf);
	initStringInfo(&logbuf);
	initStringInfo(&locktagbuf);

	/* Generate the "waits for" lines sent to the client */
	for (i = 0; i < nDeadlockDetails; i++)
	{
		DEADLOCK_INFO *info = &deadlockDetails[i];
		int			nextpid;

		/* The last proc waits for the first one... */
		if (i < nDeadlockDetails - 1)
			nextpid = info[1].pid;
		else
			nextpid = deadlockDetails[0].pid;

		/* reset locktagbuf to hold next object description */
		resetStringInfo(&locktagbuf);

		DescribeLockTag(&locktagbuf, &info->locktag);

		if (i > 0)
			appendStringInfoChar(&clientbuf, '\n');

		appendStringInfo(&clientbuf,
						 _("Process %d waits for %s on %s; blocked by process %d."),
						 info->pid,
						 GetLockmodeName(info->locktag.locktag_lockmethodid,
										 info->lockmode),
						 locktagbuf.data,
						 nextpid);
	}

	/* Duplicate all the above for the server ... */
	appendBinaryStringInfo(&logbuf, clientbuf.data, clientbuf.len);

	/* ... and add info about query strings */
	for (i = 0; i < nDeadlockDetails; i++)
	{
		DEADLOCK_INFO *info = &deadlockDetails[i];

		appendStringInfoChar(&logbuf, '\n');

		appendStringInfo(&logbuf,
						 _("Process %d: %s"),
						 info->pid,
						 pgstat_get_backend_current_activity(info->pid, false));
	}

	pgstat_report_deadlock();

	ereport(ERROR,
			(errcode(ERRCODE_T_R_DEADLOCK_DETECTED),
			 errmsg("deadlock detected"),
			 errdetail_internal("%s", clientbuf.data),
			 errdetail_log("%s", logbuf.data),
			 errhint("See server log for query details.")));
}

/*
 * RememberSimpleDeadLock: set up info for DeadLockReport when ProcSleep
 * detects a trivial (two-way) deadlock.  proc1 wants to block for lockmode
 * on lock, but proc2 is already waiting and would be blocked by proc1.
 */
void
RememberSimpleDeadLock(PGPROC *proc1,
					   LOCKMODE lockmode,
					   LOCK *lock,
					   PGPROC *proc2)
{
	DEADLOCK_INFO *info = &deadlockDetails[0];

	info->locktag = lock->tag;
	info->lockmode = lockmode;
	info->pid = proc1->pid;
	info++;
	info->locktag = proc2->waitLock->tag;
	info->lockmode = proc2->waitLockMode;
	info->pid = proc2->pid;
	nDeadlockDetails = 2;
}