aboutsummaryrefslogtreecommitdiffstats
path: root/lib/assoc_array.c
blob: 03dd576e67730fb2870c44512f55c1c0c39b77f3 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
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
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
1340
1341
1342
1343
1344
1345
1346
1347
1348
1349
1350
1351
1352
1353
1354
1355
1356
1357
1358
1359
1360
1361
1362
1363
1364
1365
1366
1367
1368
1369
1370
1371
1372
1373
1374
1375
1376
1377
1378
1379
1380
1381
1382
1383
1384
1385
1386
1387
1388
1389
1390
1391
1392
1393
1394
1395
1396
1397
1398
1399
1400
1401
1402
1403
1404
1405
1406
1407
1408
1409
1410
1411
1412
1413
1414
1415
1416
1417
1418
1419
1420
1421
1422
1423
1424
1425
1426
1427
1428
1429
1430
1431
1432
1433
1434
1435
1436
1437
1438
1439
1440
1441
1442
1443
1444
1445
1446
1447
1448
1449
1450
1451
1452
1453
1454
1455
1456
1457
1458
1459
1460
1461
1462
1463
1464
1465
1466
1467
1468
1469
1470
1471
1472
1473
1474
1475
1476
1477
1478
1479
1480
1481
1482
1483
1484
1485
1486
1487
1488
1489
1490
1491
1492
1493
1494
1495
1496
1497
1498
1499
1500
1501
1502
1503
1504
1505
1506
1507
1508
1509
1510
1511
1512
1513
1514
1515
1516
1517
1518
1519
1520
1521
1522
1523
1524
1525
1526
1527
1528
1529
1530
1531
1532
1533
1534
1535
1536
1537
1538
1539
1540
1541
1542
1543
1544
1545
1546
1547
1548
1549
1550
1551
1552
1553
1554
1555
1556
1557
1558
1559
1560
1561
1562
1563
1564
1565
1566
1567
1568
1569
1570
1571
1572
1573
1574
1575
1576
1577
1578
1579
1580
1581
1582
1583
1584
1585
1586
1587
1588
1589
1590
1591
1592
1593
1594
1595
1596
1597
1598
1599
1600
1601
1602
1603
1604
1605
1606
1607
1608
1609
1610
1611
1612
1613
1614
1615
1616
1617
1618
1619
1620
1621
1622
1623
1624
1625
1626
1627
1628
1629
1630
1631
1632
1633
1634
1635
1636
1637
1638
1639
1640
1641
1642
1643
1644
1645
1646
1647
1648
1649
1650
1651
1652
1653
1654
1655
1656
1657
1658
1659
1660
1661
1662
1663
1664
1665
1666
1667
1668
1669
1670
1671
1672
1673
1674
1675
1676
1677
1678
1679
1680
1681
1682
1683
1684
1685
1686
1687
1688
1689
1690
1691
1692
1693
1694
1695
1696
1697
1698
1699
1700
1701
1702
1703
1704
1705
1706
1707
1708
1709
1710
1711
1712
1713
1714
1715
1716
1717
1718
1719
1720
1721
1722
1723
1724
1725
1726
1727
1728
1729
1730
1731
1732
1733
1734
1735
1736
1737
1738
1739
1740
1741
1742
1743
1744
1745
1746
1747
1748
1749
/* Generic associative array implementation.
 *
 * See Documentation/assoc_array.txt for information.
 *
 * Copyright (C) 2013 Red Hat, Inc. All Rights Reserved.
 * Written by David Howells (dhowells@redhat.com)
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public Licence
 * as published by the Free Software Foundation; either version
 * 2 of the Licence, or (at your option) any later version.
 */
//#define DEBUG
#include <linux/rcupdate.h>
#include <linux/slab.h>
#include <linux/err.h>
#include <linux/assoc_array_priv.h>

/*
 * Iterate over an associative array.  The caller must hold the RCU read lock
 * or better.
 */
static int assoc_array_subtree_iterate(const struct assoc_array_ptr *root,
				       const struct assoc_array_ptr *stop,
				       int (*iterator)(const void *leaf,
						       void *iterator_data),
				       void *iterator_data)
{
	const struct assoc_array_shortcut *shortcut;
	const struct assoc_array_node *node;
	const struct assoc_array_ptr *cursor, *ptr, *parent;
	unsigned long has_meta;
	int slot, ret;

	cursor = root;

begin_node:
	if (assoc_array_ptr_is_shortcut(cursor)) {
		/* Descend through a shortcut */
		shortcut = assoc_array_ptr_to_shortcut(cursor);
		smp_read_barrier_depends();
		cursor = ACCESS_ONCE(shortcut->next_node);
	}

	node = assoc_array_ptr_to_node(cursor);
	smp_read_barrier_depends();
	slot = 0;

	/* We perform two passes of each node.
	 *
	 * The first pass does all the leaves in this node.  This means we
	 * don't miss any leaves if the node is split up by insertion whilst
	 * we're iterating over the branches rooted here (we may, however, see
	 * some leaves twice).
	 */
	has_meta = 0;
	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
		ptr = ACCESS_ONCE(node->slots[slot]);
		has_meta |= (unsigned long)ptr;
		if (ptr && assoc_array_ptr_is_leaf(ptr)) {
			/* We need a barrier between the read of the pointer
			 * and dereferencing the pointer - but only if we are
			 * actually going to dereference it.
			 */
			smp_read_barrier_depends();

			/* Invoke the callback */
			ret = iterator(assoc_array_ptr_to_leaf(ptr),
				       iterator_data);
			if (ret)
				return ret;
		}
	}

	/* The second pass attends to all the metadata pointers.  If we follow
	 * one of these we may find that we don't come back here, but rather go
	 * back to a replacement node with the leaves in a different layout.
	 *
	 * We are guaranteed to make progress, however, as the slot number for
	 * a particular portion of the key space cannot change - and we
	 * continue at the back pointer + 1.
	 */
	if (!(has_meta & ASSOC_ARRAY_PTR_META_TYPE))
		goto finished_node;
	slot = 0;

continue_node:
	node = assoc_array_ptr_to_node(cursor);
	smp_read_barrier_depends();

	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
		ptr = ACCESS_ONCE(node->slots[slot]);
		if (assoc_array_ptr_is_meta(ptr)) {
			cursor = ptr;
			goto begin_node;
		}
	}

finished_node:
	/* Move up to the parent (may need to skip back over a shortcut) */
	parent = ACCESS_ONCE(node->back_pointer);
	slot = node->parent_slot;
	if (parent == stop)
		return 0;

	if (assoc_array_ptr_is_shortcut(parent)) {
		shortcut = assoc_array_ptr_to_shortcut(parent);
		smp_read_barrier_depends();
		cursor = parent;
		parent = ACCESS_ONCE(shortcut->back_pointer);
		slot = shortcut->parent_slot;
		if (parent == stop)
			return 0;
	}

	/* Ascend to next slot in parent node */
	cursor = parent;
	slot++;
	goto continue_node;
}

/**
 * assoc_array_iterate - Pass all objects in the array to a callback
 * @array: The array to iterate over.
 * @iterator: The callback function.
 * @iterator_data: Private data for the callback function.
 *
 * Iterate over all the objects in an associative array.  Each one will be
 * presented to the iterator function.
 *
 * If the array is being modified concurrently with the iteration then it is
 * possible that some objects in the array will be passed to the iterator
 * callback more than once - though every object should be passed at least
 * once.  If this is undesirable then the caller must lock against modification
 * for the duration of this function.
 *
 * The function will return 0 if no objects were in the array or else it will
 * return the result of the last iterator function called.  Iteration stops
 * immediately if any call to the iteration function results in a non-zero
 * return.
 *
 * The caller should hold the RCU read lock or better if concurrent
 * modification is possible.
 */
int assoc_array_iterate(const struct assoc_array *array,
			int (*iterator)(const void *object,
					void *iterator_data),
			void *iterator_data)
{
	struct assoc_array_ptr *root = ACCESS_ONCE(array->root);

	if (!root)
		return 0;
	return assoc_array_subtree_iterate(root, NULL, iterator, iterator_data);
}

enum assoc_array_walk_status {
	assoc_array_walk_tree_empty,
	assoc_array_walk_found_terminal_node,
	assoc_array_walk_found_wrong_shortcut,
};

struct assoc_array_walk_result {
	struct {
		struct assoc_array_node	*node;	/* Node in which leaf might be found */
		int		level;
		int		slot;
	} terminal_node;
	struct {
		struct assoc_array_shortcut *shortcut;
		int		level;
		int		sc_level;
		unsigned long	sc_segments;
		unsigned long	dissimilarity;
	} wrong_shortcut;
};

/*
 * Navigate through the internal tree looking for the closest node to the key.
 */
static enum assoc_array_walk_status
assoc_array_walk(const struct assoc_array *array,
		 const struct assoc_array_ops *ops,
		 const void *index_key,
		 struct assoc_array_walk_result *result)
{
	struct assoc_array_shortcut *shortcut;
	struct assoc_array_node *node;
	struct assoc_array_ptr *cursor, *ptr;
	unsigned long sc_segments, dissimilarity;
	unsigned long segments;
	int level, sc_level, next_sc_level;
	int slot;

	pr_devel("-->%s()\n", __func__);

	cursor = ACCESS_ONCE(array->root);
	if (!cursor)
		return assoc_array_walk_tree_empty;

	level = 0;

	/* Use segments from the key for the new leaf to navigate through the
	 * internal tree, skipping through nodes and shortcuts that are on
	 * route to the destination.  Eventually we'll come to a slot that is
	 * either empty or contains a leaf at which point we've found a node in
	 * which the leaf we're looking for might be found or into which it
	 * should be inserted.
	 */
jumped:
	segments = ops->get_key_chunk(index_key, level);
	pr_devel("segments[%d]: %lx\n", level, segments);

	if (assoc_array_ptr_is_shortcut(cursor))
		goto follow_shortcut;

consider_node:
	node = assoc_array_ptr_to_node(cursor);
	smp_read_barrier_depends();

	slot = segments >> (level & ASSOC_ARRAY_KEY_CHUNK_MASK);
	slot &= ASSOC_ARRAY_FAN_MASK;
	ptr = ACCESS_ONCE(node->slots[slot]);

	pr_devel("consider slot %x [ix=%d type=%lu]\n",
		 slot, level, (unsigned long)ptr & 3);

	if (!assoc_array_ptr_is_meta(ptr)) {
		/* The node doesn't have a node/shortcut pointer in the slot
		 * corresponding to the index key that we have to follow.
		 */
		result->terminal_node.node = node;
		result->terminal_node.level = level;
		result->terminal_node.slot = slot;
		pr_devel("<--%s() = terminal_node\n", __func__);
		return assoc_array_walk_found_terminal_node;
	}

	if (assoc_array_ptr_is_node(ptr)) {
		/* There is a pointer to a node in the slot corresponding to
		 * this index key segment, so we need to follow it.
		 */
		cursor = ptr;
		level += ASSOC_ARRAY_LEVEL_STEP;
		if ((level & ASSOC_ARRAY_KEY_CHUNK_MASK) != 0)
			goto consider_node;
		goto jumped;
	}

	/* There is a shortcut in the slot corresponding to the index key
	 * segment.  We follow the shortcut if its partial index key matches
	 * this leaf's.  Otherwise we need to split the shortcut.
	 */
	cursor = ptr;
follow_shortcut:
	shortcut = assoc_array_ptr_to_shortcut(cursor);
	smp_read_barrier_depends();
	pr_devel("shortcut to %d\n", shortcut->skip_to_level);
	sc_level = level + ASSOC_ARRAY_LEVEL_STEP;
	BUG_ON(sc_level > shortcut->skip_to_level);

	do {
		/* Check the leaf against the shortcut's index key a word at a
		 * time, trimming the final word (the shortcut stores the index
		 * key completely from the root to the shortcut's target).
		 */
		if ((sc_level & ASSOC_ARRAY_KEY_CHUNK_MASK) == 0)
			segments = ops->get_key_chunk(index_key, sc_level);

		sc_segments = shortcut->index_key[sc_level >> ASSOC_ARRAY_KEY_CHUNK_SHIFT];
		dissimilarity = segments ^ sc_segments;

		if (round_up(sc_level, ASSOC_ARRAY_KEY_CHUNK_SIZE) > shortcut->skip_to_level) {
			/* Trim segments that are beyond the shortcut */
			int shift = shortcut->skip_to_level & ASSOC_ARRAY_KEY_CHUNK_MASK;
			dissimilarity &= ~(ULONG_MAX << shift);
			next_sc_level = shortcut->skip_to_level;
		} else {
			next_sc_level = sc_level + ASSOC_ARRAY_KEY_CHUNK_SIZE;
			next_sc_level = round_down(next_sc_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
		}

		if (dissimilarity != 0) {
			/* This shortcut points elsewhere */
			result->wrong_shortcut.shortcut = shortcut;
			result->wrong_shortcut.level = level;
			result->wrong_shortcut.sc_level = sc_level;
			result->wrong_shortcut.sc_segments = sc_segments;
			result->wrong_shortcut.dissimilarity = dissimilarity;
			return assoc_array_walk_found_wrong_shortcut;
		}

		sc_level = next_sc_level;
	} while (sc_level < shortcut->skip_to_level);

	/* The shortcut matches the leaf's index to this point. */
	cursor = ACCESS_ONCE(shortcut->next_node);
	if (((level ^ sc_level) & ~ASSOC_ARRAY_KEY_CHUNK_MASK) != 0) {
		level = sc_level;
		goto jumped;
	} else {
		level = sc_level;
		goto consider_node;
	}
}

/**
 * assoc_array_find - Find an object by index key
 * @array: The associative array to search.
 * @ops: The operations to use.
 * @index_key: The key to the object.
 *
 * Find an object in an associative array by walking through the internal tree
 * to the node that should contain the object and then searching the leaves
 * there.  NULL is returned if the requested object was not found in the array.
 *
 * The caller must hold the RCU read lock or better.
 */
void *assoc_array_find(const struct assoc_array *array,
		       const struct assoc_array_ops *ops,
		       const void *index_key)
{
	struct assoc_array_walk_result result;
	const struct assoc_array_node *node;
	const struct assoc_array_ptr *ptr;
	const void *leaf;
	int slot;

	if (assoc_array_walk(array, ops, index_key, &result) !=
	    assoc_array_walk_found_terminal_node)
		return NULL;

	node = result.terminal_node.node;
	smp_read_barrier_depends();

	/* If the target key is available to us, it's has to be pointed to by
	 * the terminal node.
	 */
	for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
		ptr = ACCESS_ONCE(node->slots[slot]);
		if (ptr && assoc_array_ptr_is_leaf(ptr)) {
			/* We need a barrier between the read of the pointer
			 * and dereferencing the pointer - but only if we are
			 * actually going to dereference it.
			 */
			leaf = assoc_array_ptr_to_leaf(ptr);
			smp_read_barrier_depends();
			if (ops->compare_object(leaf, index_key))
				return (void *)leaf;
		}
	}

	return NULL;
}

/*
 * Destructively iterate over an associative array.  The caller must prevent
 * other simultaneous accesses.
 */
static void assoc_array_destroy_subtree(struct assoc_array_ptr *root,
					const struct assoc_array_ops *ops)
{
	struct assoc_array_shortcut *shortcut;
	struct assoc_array_node *node;
	struct assoc_array_ptr *cursor, *parent = NULL;
	int slot = -1;

	pr_devel("-->%s()\n", __func__);

	cursor = root;
	if (!cursor) {
		pr_devel("empty\n");
		return;
	}

move_to_meta:
	if (assoc_array_ptr_is_shortcut(cursor)) {
		/* Descend through a shortcut */
		pr_devel("[%d] shortcut\n", slot);
		BUG_ON(!assoc_array_ptr_is_shortcut(cursor));
		shortcut = assoc_array_ptr_to_shortcut(cursor);
		BUG_ON(shortcut->back_pointer != parent);
		BUG_ON(slot != -1 && shortcut->parent_slot != slot);
		parent = cursor;
		cursor = shortcut->next_node;
		slot = -1;
		BUG_ON(!assoc_array_ptr_is_node(cursor));
	}

	pr_devel("[%d] node\n", slot);
	node = assoc_array_ptr_to_node(cursor);
	BUG_ON(node->back_pointer != parent);
	BUG_ON(slot != -1 && node->parent_slot != slot);
	slot = 0;

continue_node:
	pr_devel("Node %p [back=%p]\n", node, node->back_pointer);
	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
		struct assoc_array_ptr *ptr = node->slots[slot];
		if (!ptr)
			continue;
		if (assoc_array_ptr_is_meta(ptr)) {
			parent = cursor;
			cursor = ptr;
			goto move_to_meta;
		}

		if (ops) {
			pr_devel("[%d] free leaf\n", slot);
			ops->free_object(assoc_array_ptr_to_leaf(ptr));
		}
	}

	parent = node->back_pointer;
	slot = node->parent_slot;
	pr_devel("free node\n");
	kfree(node);
	if (!parent)
		return; /* Done */

	/* Move back up to the parent (may need to free a shortcut on
	 * the way up) */
	if (assoc_array_ptr_is_shortcut(parent)) {
		shortcut = assoc_array_ptr_to_shortcut(parent);
		BUG_ON(shortcut->next_node != cursor);
		cursor = parent;
		parent = shortcut->back_pointer;
		slot = shortcut->parent_slot;
		pr_devel("free shortcut\n");
		kfree(shortcut);
		if (!parent)
			return;

		BUG_ON(!assoc_array_ptr_is_node(parent));
	}

	/* Ascend to next slot in parent node */
	pr_devel("ascend to %p[%d]\n", parent, slot);
	cursor = parent;
	node = assoc_array_ptr_to_node(cursor);
	slot++;
	goto continue_node;
}

/**
 * assoc_array_destroy - Destroy an associative array
 * @array: The array to destroy.
 * @ops: The operations to use.
 *
 * Discard all metadata and free all objects in an associative array.  The
 * array will be empty and ready to use again upon completion.  This function
 * cannot fail.
 *
 * The caller must prevent all other accesses whilst this takes place as no
 * attempt is made to adjust pointers gracefully to permit RCU readlock-holding
 * accesses to continue.  On the other hand, no memory allocation is required.
 */
void assoc_array_destroy(struct assoc_array *array,
			 const struct assoc_array_ops *ops)
{
	assoc_array_destroy_subtree(array->root, ops);
	array->root = NULL;
}

/*
 * Handle insertion into an empty tree.
 */
static bool assoc_array_insert_in_empty_tree(struct assoc_array_edit *edit)
{
	struct assoc_array_node *new_n0;

	pr_devel("-->%s()\n", __func__);

	new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
	if (!new_n0)
		return false;

	edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
	edit->leaf_p = &new_n0->slots[0];
	edit->adjust_count_on = new_n0;
	edit->set[0].ptr = &edit->array->root;
	edit->set[0].to = assoc_array_node_to_ptr(new_n0);

	pr_devel("<--%s() = ok [no root]\n", __func__);
	return true;
}

/*
 * Handle insertion into a terminal node.
 */
static bool assoc_array_insert_into_terminal_node(struct assoc_array_edit *edit,
						  const struct assoc_array_ops *ops,
						  const void *index_key,
						  struct assoc_array_walk_result *result)
{
	struct assoc_array_shortcut *shortcut, *new_s0;
	struct assoc_array_node *node, *new_n0, *new_n1, *side;
	struct assoc_array_ptr *ptr;
	unsigned long dissimilarity, base_seg, blank;
	size_t keylen;
	bool have_meta;
	int level, diff;
	int slot, next_slot, free_slot, i, j;

	node	= result->terminal_node.node;
	level	= result->terminal_node.level;
	edit->segment_cache[ASSOC_ARRAY_FAN_OUT] = result->terminal_node.slot;

	pr_devel("-->%s()\n", __func__);

	/* We arrived at a node which doesn't have an onward node or shortcut
	 * pointer that we have to follow.  This means that (a) the leaf we
	 * want must go here (either by insertion or replacement) or (b) we
	 * need to split this node and insert in one of the fragments.
	 */
	free_slot = -1;

	/* Firstly, we have to check the leaves in this node to see if there's
	 * a matching one we should replace in place.
	 */
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		ptr = node->slots[i];
		if (!ptr) {
			free_slot = i;
			continue;
		}
		if (ops->compare_object(assoc_array_ptr_to_leaf(ptr), index_key)) {
			pr_devel("replace in slot %d\n", i);
			edit->leaf_p = &node->slots[i];
			edit->dead_leaf = node->slots[i];
			pr_devel("<--%s() = ok [replace]\n", __func__);
			return true;
		}
	}

	/* If there is a free slot in this node then we can just insert the
	 * leaf here.
	 */
	if (free_slot >= 0) {
		pr_devel("insert in free slot %d\n", free_slot);
		edit->leaf_p = &node->slots[free_slot];
		edit->adjust_count_on = node;
		pr_devel("<--%s() = ok [insert]\n", __func__);
		return true;
	}

	/* The node has no spare slots - so we're either going to have to split
	 * it or insert another node before it.
	 *
	 * Whatever, we're going to need at least two new nodes - so allocate
	 * those now.  We may also need a new shortcut, but we deal with that
	 * when we need it.
	 */
	new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
	if (!new_n0)
		return false;
	edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
	new_n1 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
	if (!new_n1)
		return false;
	edit->new_meta[1] = assoc_array_node_to_ptr(new_n1);

	/* We need to find out how similar the leaves are. */
	pr_devel("no spare slots\n");
	have_meta = false;
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		ptr = node->slots[i];
		if (assoc_array_ptr_is_meta(ptr)) {
			edit->segment_cache[i] = 0xff;
			have_meta = true;
			continue;
		}
		base_seg = ops->get_object_key_chunk(
			assoc_array_ptr_to_leaf(ptr), level);
		base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
		edit->segment_cache[i] = base_seg & ASSOC_ARRAY_FAN_MASK;
	}

	if (have_meta) {
		pr_devel("have meta\n");
		goto split_node;
	}

	/* The node contains only leaves */
	dissimilarity = 0;
	base_seg = edit->segment_cache[0];
	for (i = 1; i < ASSOC_ARRAY_FAN_OUT; i++)
		dissimilarity |= edit->segment_cache[i] ^ base_seg;

	pr_devel("only leaves; dissimilarity=%lx\n", dissimilarity);

	if ((dissimilarity & ASSOC_ARRAY_FAN_MASK) == 0) {
		/* The old leaves all cluster in the same slot.  We will need
		 * to insert a shortcut if the new node wants to cluster with them.
		 */
		if ((edit->segment_cache[ASSOC_ARRAY_FAN_OUT] ^ base_seg) == 0)
			goto all_leaves_cluster_together;

		/* Otherwise we can just insert a new node ahead of the old
		 * one.
		 */
		goto present_leaves_cluster_but_not_new_leaf;
	}

split_node:
	pr_devel("split node\n");

	/* We need to split the current node; we know that the node doesn't
	 * simply contain a full set of leaves that cluster together (it
	 * contains meta pointers and/or non-clustering leaves).
	 *
	 * We need to expel at least two leaves out of a set consisting of the
	 * leaves in the node and the new leaf.
	 *
	 * We need a new node (n0) to replace the current one and a new node to
	 * take the expelled nodes (n1).
	 */
	edit->set[0].to = assoc_array_node_to_ptr(new_n0);
	new_n0->back_pointer = node->back_pointer;
	new_n0->parent_slot = node->parent_slot;
	new_n1->back_pointer = assoc_array_node_to_ptr(new_n0);
	new_n1->parent_slot = -1; /* Need to calculate this */

do_split_node:
	pr_devel("do_split_node\n");

	new_n0->nr_leaves_on_branch = node->nr_leaves_on_branch;
	new_n1->nr_leaves_on_branch = 0;

	/* Begin by finding two matching leaves.  There have to be at least two
	 * that match - even if there are meta pointers - because any leaf that
	 * would match a slot with a meta pointer in it must be somewhere
	 * behind that meta pointer and cannot be here.  Further, given N
	 * remaining leaf slots, we now have N+1 leaves to go in them.
	 */
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		slot = edit->segment_cache[i];
		if (slot != 0xff)
			for (j = i + 1; j < ASSOC_ARRAY_FAN_OUT + 1; j++)
				if (edit->segment_cache[j] == slot)
					goto found_slot_for_multiple_occupancy;
	}
found_slot_for_multiple_occupancy:
	pr_devel("same slot: %x %x [%02x]\n", i, j, slot);
	BUG_ON(i >= ASSOC_ARRAY_FAN_OUT);
	BUG_ON(j >= ASSOC_ARRAY_FAN_OUT + 1);
	BUG_ON(slot >= ASSOC_ARRAY_FAN_OUT);

	new_n1->parent_slot = slot;

	/* Metadata pointers cannot change slot */
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++)
		if (assoc_array_ptr_is_meta(node->slots[i]))
			new_n0->slots[i] = node->slots[i];
		else
			new_n0->slots[i] = NULL;
	BUG_ON(new_n0->slots[slot] != NULL);
	new_n0->slots[slot] = assoc_array_node_to_ptr(new_n1);

	/* Filter the leaf pointers between the new nodes */
	free_slot = -1;
	next_slot = 0;
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		if (assoc_array_ptr_is_meta(node->slots[i]))
			continue;
		if (edit->segment_cache[i] == slot) {
			new_n1->slots[next_slot++] = node->slots[i];
			new_n1->nr_leaves_on_branch++;
		} else {
			do {
				free_slot++;
			} while (new_n0->slots[free_slot] != NULL);
			new_n0->slots[free_slot] = node->slots[i];
		}
	}

	pr_devel("filtered: f=%x n=%x\n", free_slot, next_slot);

	if (edit->segment_cache[ASSOC_ARRAY_FAN_OUT] != slot) {
		do {
			free_slot++;
		} while (new_n0->slots[free_slot] != NULL);
		edit->leaf_p = &new_n0->slots[free_slot];
		edit->adjust_count_on = new_n0;
	} else {
		edit->leaf_p = &new_n1->slots[next_slot++];
		edit->adjust_count_on = new_n1;
	}

	BUG_ON(next_slot <= 1);

	edit->set_backpointers_to = assoc_array_node_to_ptr(new_n0);
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		if (edit->segment_cache[i] == 0xff) {
			ptr = node->slots[i];
			BUG_ON(assoc_array_ptr_is_leaf(ptr));
			if (assoc_array_ptr_is_node(ptr)) {
				side = assoc_array_ptr_to_node(ptr);
				edit->set_backpointers[i] = &side->back_pointer;
			} else {
				shortcut = assoc_array_ptr_to_shortcut(ptr);
				edit->set_backpointers[i] = &shortcut->back_pointer;
			}
		}
	}

	ptr = node->back_pointer;
	if (!ptr)
		edit->set[0].ptr = &edit->array->root;
	else if (assoc_array_ptr_is_node(ptr))
		edit->set[0].ptr = &assoc_array_ptr_to_node(ptr)->slots[node->parent_slot];
	else
		edit->set[0].ptr = &assoc_array_ptr_to_shortcut(ptr)->next_node;
	edit->excised_meta[0] = assoc_array_node_to_ptr(node);
	pr_devel("<--%s() = ok [split node]\n", __func__);
	return true;

present_leaves_cluster_but_not_new_leaf:
	/* All the old leaves cluster in the same slot, but the new leaf wants
	 * to go into a different slot, so we create a new node to hold the new
	 * leaf and a pointer to a new node holding all the old leaves.
	 */
	pr_devel("present leaves cluster but not new leaf\n");

	new_n0->back_pointer = node->back_pointer;
	new_n0->parent_slot = node->parent_slot;
	new_n0->nr_leaves_on_branch = node->nr_leaves_on_branch;
	new_n1->back_pointer = assoc_array_node_to_ptr(new_n0);
	new_n1->parent_slot = edit->segment_cache[0];
	new_n1->nr_leaves_on_branch = node->nr_leaves_on_branch;
	edit->adjust_count_on = new_n0;

	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++)
		new_n1->slots[i] = node->slots[i];

	new_n0->slots[edit->segment_cache[0]] = assoc_array_node_to_ptr(new_n0);
	edit->leaf_p = &new_n0->slots[edit->segment_cache[ASSOC_ARRAY_FAN_OUT]];

	edit->set[0].ptr = &assoc_array_ptr_to_node(node->back_pointer)->slots[node->parent_slot];
	edit->set[0].to = assoc_array_node_to_ptr(new_n0);
	edit->excised_meta[0] = assoc_array_node_to_ptr(node);
	pr_devel("<--%s() = ok [insert node before]\n", __func__);
	return true;

all_leaves_cluster_together:
	/* All the leaves, new and old, want to cluster together in this node
	 * in the same slot, so we have to replace this node with a shortcut to
	 * skip over the identical parts of the key and then place a pair of
	 * nodes, one inside the other, at the end of the shortcut and
	 * distribute the keys between them.
	 *
	 * Firstly we need to work out where the leaves start diverging as a
	 * bit position into their keys so that we know how big the shortcut
	 * needs to be.
	 *
	 * We only need to make a single pass of N of the N+1 leaves because if
	 * any keys differ between themselves at bit X then at least one of
	 * them must also differ with the base key at bit X or before.
	 */
	pr_devel("all leaves cluster together\n");
	diff = INT_MAX;
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		int x = ops->diff_objects(assoc_array_ptr_to_leaf(node->slots[i]),
					  index_key);
		if (x < diff) {
			BUG_ON(x < 0);
			diff = x;
		}
	}
	BUG_ON(diff == INT_MAX);
	BUG_ON(diff < level + ASSOC_ARRAY_LEVEL_STEP);

	keylen = round_up(diff, ASSOC_ARRAY_KEY_CHUNK_SIZE);
	keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;

	new_s0 = kzalloc(sizeof(struct assoc_array_shortcut) +
			 keylen * sizeof(unsigned long), GFP_KERNEL);
	if (!new_s0)
		return false;
	edit->new_meta[2] = assoc_array_shortcut_to_ptr(new_s0);

	edit->set[0].to = assoc_array_shortcut_to_ptr(new_s0);
	new_s0->back_pointer = node->back_pointer;
	new_s0->parent_slot = node->parent_slot;
	new_s0->next_node = assoc_array_node_to_ptr(new_n0);
	new_n0->back_pointer = assoc_array_shortcut_to_ptr(new_s0);
	new_n0->parent_slot = 0;
	new_n1->back_pointer = assoc_array_node_to_ptr(new_n0);
	new_n1->parent_slot = -1; /* Need to calculate this */

	new_s0->skip_to_level = level = diff & ~ASSOC_ARRAY_LEVEL_STEP_MASK;
	pr_devel("skip_to_level = %d [diff %d]\n", level, diff);
	BUG_ON(level <= 0);

	for (i = 0; i < keylen; i++)
		new_s0->index_key[i] =
			ops->get_key_chunk(index_key, i * ASSOC_ARRAY_KEY_CHUNK_SIZE);

	blank = ULONG_MAX << (level & ASSOC_ARRAY_KEY_CHUNK_MASK);
	pr_devel("blank off [%zu] %d: %lx\n", keylen - 1, level, blank);
	new_s0->index_key[keylen - 1] &= ~blank;

	/* This now reduces to a node splitting exercise for which we'll need
	 * to regenerate the disparity table.
	 */
	for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
		ptr = node->slots[i];
		base_seg = ops->get_object_key_chunk(assoc_array_ptr_to_leaf(ptr),
						     level);
		base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
		edit->segment_cache[i] = base_seg & ASSOC_ARRAY_FAN_MASK;
	}

	base_seg = ops->get_key_chunk(index_key, level);
	base_seg >>= level & ASSOC_ARRAY_KEY_CHUNK_MASK;
	edit->segment_cache[ASSOC_ARRAY_FAN_OUT] = base_seg & ASSOC_ARRAY_FAN_MASK;
	goto do_split_node;
}

/*
 * Handle insertion into the middle of a shortcut.
 */
static bool assoc_array_insert_mid_shortcut(struct assoc_array_edit *edit,
					    const struct assoc_array_ops *ops,
					    struct assoc_array_walk_result *result)
{
	struct assoc_array_shortcut *shortcut, *new_s0, *new_s1;
	struct assoc_array_node *node, *new_n0, *side;
	unsigned long sc_segments, dissimilarity, blank;
	size_t keylen;
	int level, sc_level, diff;
	int sc_slot;

	shortcut	= result->wrong_shortcut.shortcut;
	level		= result->wrong_shortcut.level;
	sc_level	= result->wrong_shortcut.sc_level;
	sc_segments	= result->wrong_shortcut.sc_segments;
	dissimilarity	= result->wrong_shortcut.dissimilarity;

	pr_devel("-->%s(ix=%d dis=%lx scix=%d)\n",
		 __func__, level, dissimilarity, sc_level);

	/* We need to split a shortcut and insert a node between the two
	 * pieces.  Zero-length pieces will be dispensed with entirely.
	 *
	 * First of all, we need to find out in which level the first
	 * difference was.
	 */
	diff = __ffs(dissimilarity);
	diff &= ~ASSOC_ARRAY_LEVEL_STEP_MASK;
	diff += sc_level & ~ASSOC_ARRAY_KEY_CHUNK_MASK;
	pr_devel("diff=%d\n", diff);

	if (!shortcut->back_pointer) {
		edit->set[0].ptr = &edit->array->root;
	} else if (assoc_array_ptr_is_node(shortcut->back_pointer)) {
		node = assoc_array_ptr_to_node(shortcut->back_pointer);
		edit->set[0].ptr = &node->slots[shortcut->parent_slot];
	} else {
		BUG();
	}

	edit->excised_meta[0] = assoc_array_shortcut_to_ptr(shortcut);

	/* Create a new node now since we're going to need it anyway */
	new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
	if (!new_n0)
		return false;
	edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);
	edit->adjust_count_on = new_n0;

	/* Insert a new shortcut before the new node if this segment isn't of
	 * zero length - otherwise we just connect the new node directly to the
	 * parent.
	 */
	level += ASSOC_ARRAY_LEVEL_STEP;
	if (diff > level) {
		pr_devel("pre-shortcut %d...%d\n", level, diff);
		keylen = round_up(diff, ASSOC_ARRAY_KEY_CHUNK_SIZE);
		keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;

		new_s0 = kzalloc(sizeof(struct assoc_array_shortcut) +
				 keylen * sizeof(unsigned long), GFP_KERNEL);
		if (!new_s0)
			return false;
		edit->new_meta[1] = assoc_array_shortcut_to_ptr(new_s0);
		edit->set[0].to = assoc_array_shortcut_to_ptr(new_s0);
		new_s0->back_pointer = shortcut->back_pointer;
		new_s0->parent_slot = shortcut->parent_slot;
		new_s0->next_node = assoc_array_node_to_ptr(new_n0);
		new_s0->skip_to_level = diff;

		new_n0->back_pointer = assoc_array_shortcut_to_ptr(new_s0);
		new_n0->parent_slot = 0;

		memcpy(new_s0->index_key, shortcut->index_key,
		       keylen * sizeof(unsigned long));

		blank = ULONG_MAX << (diff & ASSOC_ARRAY_KEY_CHUNK_MASK);
		pr_devel("blank off [%zu] %d: %lx\n", keylen - 1, diff, blank);
		new_s0->index_key[keylen - 1] &= ~blank;
	} else {
		pr_devel("no pre-shortcut\n");
		edit->set[0].to = assoc_array_node_to_ptr(new_n0);
		new_n0->back_pointer = shortcut->back_pointer;
		new_n0->parent_slot = shortcut->parent_slot;
	}

	side = assoc_array_ptr_to_node(shortcut->next_node);
	new_n0->nr_leaves_on_branch = side->nr_leaves_on_branch;

	/* We need to know which slot in the new node is going to take a
	 * metadata pointer.
	 */
	sc_slot = sc_segments >> (diff & ASSOC_ARRAY_KEY_CHUNK_MASK);
	sc_slot &= ASSOC_ARRAY_FAN_MASK;

	pr_devel("new slot %lx >> %d -> %d\n",
		 sc_segments, diff & ASSOC_ARRAY_KEY_CHUNK_MASK, sc_slot);

	/* Determine whether we need to follow the new node with a replacement
	 * for the current shortcut.  We could in theory reuse the current
	 * shortcut if its parent slot number doesn't change - but that's a
	 * 1-in-16 chance so not worth expending the code upon.
	 */
	level = diff + ASSOC_ARRAY_LEVEL_STEP;
	if (level < shortcut->skip_to_level) {
		pr_devel("post-shortcut %d...%d\n", level, shortcut->skip_to_level);
		keylen = round_up(shortcut->skip_to_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
		keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;

		new_s1 = kzalloc(sizeof(struct assoc_array_shortcut) +
				 keylen * sizeof(unsigned long), GFP_KERNEL);
		if (!new_s1)
			return false;
		edit->new_meta[2] = assoc_array_shortcut_to_ptr(new_s1);

		new_s1->back_pointer = assoc_array_node_to_ptr(new_n0);
		new_s1->parent_slot = sc_slot;
		new_s1->next_node = shortcut->next_node;
		new_s1->skip_to_level = shortcut->skip_to_level;

		new_n0->slots[sc_slot] = assoc_array_shortcut_to_ptr(new_s1);

		memcpy(new_s1->index_key, shortcut->index_key,
		       keylen * sizeof(unsigned long));

		edit->set[1].ptr = &side->back_pointer;
		edit->set[1].to = assoc_array_shortcut_to_ptr(new_s1);
	} else {
		pr_devel("no post-shortcut\n");

		/* We don't have to replace the pointed-to node as long as we
		 * use memory barriers to make sure the parent slot number is
		 * changed before the back pointer (the parent slot number is
		 * irrelevant to the old parent shortcut).
		 */
		new_n0->slots[sc_slot] = shortcut->next_node;
		edit->set_parent_slot[0].p = &side->parent_slot;
		edit->set_parent_slot[0].to = sc_slot;
		edit->set[1].ptr = &side->back_pointer;
		edit->set[1].to = assoc_array_node_to_ptr(new_n0);
	}

	/* Install the new leaf in a spare slot in the new node. */
	if (sc_slot == 0)
		edit->leaf_p = &new_n0->slots[1];
	else
		edit->leaf_p = &new_n0->slots[0];

	pr_devel("<--%s() = ok [split shortcut]\n", __func__);
	return edit;
}

/**
 * assoc_array_insert - Script insertion of an object into an associative array
 * @array: The array to insert into.
 * @ops: The operations to use.
 * @index_key: The key to insert at.
 * @object: The object to insert.
 *
 * Precalculate and preallocate a script for the insertion or replacement of an
 * object in an associative array.  This results in an edit script that can
 * either be applied or cancelled.
 *
 * The function returns a pointer to an edit script or -ENOMEM.
 *
 * The caller should lock against other modifications and must continue to hold
 * the lock until assoc_array_apply_edit() has been called.
 *
 * Accesses to the tree may take place concurrently with this function,
 * provided they hold the RCU read lock.
 */
struct assoc_array_edit *assoc_array_insert(struct assoc_array *array,
					    const struct assoc_array_ops *ops,
					    const void *index_key,
					    void *object)
{
	struct assoc_array_walk_result result;
	struct assoc_array_edit *edit;

	pr_devel("-->%s()\n", __func__);

	/* The leaf pointer we're given must not have the bottom bit set as we
	 * use those for type-marking the pointer.  NULL pointers are also not
	 * allowed as they indicate an empty slot but we have to allow them
	 * here as they can be updated later.
	 */
	BUG_ON(assoc_array_ptr_is_meta(object));

	edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
	if (!edit)
		return ERR_PTR(-ENOMEM);
	edit->array = array;
	edit->ops = ops;
	edit->leaf = assoc_array_leaf_to_ptr(object);
	edit->adjust_count_by = 1;

	switch (assoc_array_walk(array, ops, index_key, &result)) {
	case assoc_array_walk_tree_empty:
		/* Allocate a root node if there isn't one yet */
		if (!assoc_array_insert_in_empty_tree(edit))
			goto enomem;
		return edit;

	case assoc_array_walk_found_terminal_node:
		/* We found a node that doesn't have a node/shortcut pointer in
		 * the slot corresponding to the index key that we have to
		 * follow.
		 */
		if (!assoc_array_insert_into_terminal_node(edit, ops, index_key,
							   &result))
			goto enomem;
		return edit;

	case assoc_array_walk_found_wrong_shortcut:
		/* We found a shortcut that didn't match our key in a slot we
		 * needed to follow.
		 */
		if (!assoc_array_insert_mid_shortcut(edit, ops, &result))
			goto enomem;
		return edit;
	}

enomem:
	/* Clean up after an out of memory error */
	pr_devel("enomem\n");
	assoc_array_cancel_edit(edit);
	return ERR_PTR(-ENOMEM);
}

/**
 * assoc_array_insert_set_object - Set the new object pointer in an edit script
 * @edit: The edit script to modify.
 * @object: The object pointer to set.
 *
 * Change the object to be inserted in an edit script.  The object pointed to
 * by the old object is not freed.  This must be done prior to applying the
 * script.
 */
void assoc_array_insert_set_object(struct assoc_array_edit *edit, void *object)
{
	BUG_ON(!object);
	edit->leaf = assoc_array_leaf_to_ptr(object);
}

struct assoc_array_delete_collapse_context {
	struct assoc_array_node	*node;
	const void		*skip_leaf;
	int			slot;
};

/*
 * Subtree collapse to node iterator.
 */
static int assoc_array_delete_collapse_iterator(const void *leaf,
						void *iterator_data)
{
	struct assoc_array_delete_collapse_context *collapse = iterator_data;

	if (leaf == collapse->skip_leaf)
		return 0;

	BUG_ON(collapse->slot >= ASSOC_ARRAY_FAN_OUT);

	collapse->node->slots[collapse->slot++] = assoc_array_leaf_to_ptr(leaf);
	return 0;
}

/**
 * assoc_array_delete - Script deletion of an object from an associative array
 * @array: The array to search.
 * @ops: The operations to use.
 * @index_key: The key to the object.
 *
 * Precalculate and preallocate a script for the deletion of an object from an
 * associative array.  This results in an edit script that can either be
 * applied or cancelled.
 *
 * The function returns a pointer to an edit script if the object was found,
 * NULL if the object was not found or -ENOMEM.
 *
 * The caller should lock against other modifications and must continue to hold
 * the lock until assoc_array_apply_edit() has been called.
 *
 * Accesses to the tree may take place concurrently with this function,
 * provided they hold the RCU read lock.
 */
struct assoc_array_edit *assoc_array_delete(struct assoc_array *array,
					    const struct assoc_array_ops *ops,
					    const void *index_key)
{
	struct assoc_array_delete_collapse_context collapse;
	struct assoc_array_walk_result result;
	struct assoc_array_node *node, *new_n0;
	struct assoc_array_edit *edit;
	struct assoc_array_ptr *ptr;
	bool has_meta;
	int slot, i;

	pr_devel("-->%s()\n", __func__);

	edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
	if (!edit)
		return ERR_PTR(-ENOMEM);
	edit->array = array;
	edit->ops = ops;
	edit->adjust_count_by = -1;

	switch (assoc_array_walk(array, ops, index_key, &result)) {
	case assoc_array_walk_found_terminal_node:
		/* We found a node that should contain the leaf we've been
		 * asked to remove - *if* it's in the tree.
		 */
		pr_devel("terminal_node\n");
		node = result.terminal_node.node;

		for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
			ptr = node->slots[slot];
			if (ptr &&
			    assoc_array_ptr_is_leaf(ptr) &&
			    ops->compare_object(assoc_array_ptr_to_leaf(ptr),
						index_key))
				goto found_leaf;
		}
	case assoc_array_walk_tree_empty:
	case assoc_array_walk_found_wrong_shortcut:
	default:
		assoc_array_cancel_edit(edit);
		pr_devel("not found\n");
		return NULL;
	}

found_leaf:
	BUG_ON(array->nr_leaves_on_tree <= 0);

	/* In the simplest form of deletion we just clear the slot and release
	 * the leaf after a suitable interval.
	 */
	edit->dead_leaf = node->slots[slot];
	edit->set[0].ptr = &node->slots[slot];
	edit->set[0].to = NULL;
	edit->adjust_count_on = node;

	/* If that concludes erasure of the last leaf, then delete the entire
	 * internal array.
	 */
	if (array->nr_leaves_on_tree == 1) {
		edit->set[1].ptr = &array->root;
		edit->set[1].to = NULL;
		edit->adjust_count_on = NULL;
		edit->excised_subtree = array->root;
		pr_devel("all gone\n");
		return edit;
	}

	/* However, we'd also like to clear up some metadata blocks if we
	 * possibly can.
	 *
	 * We go for a simple algorithm of: if this node has FAN_OUT or fewer
	 * leaves in it, then attempt to collapse it - and attempt to
	 * recursively collapse up the tree.
	 *
	 * We could also try and collapse in partially filled subtrees to take
	 * up space in this node.
	 */
	if (node->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT + 1) {
		struct assoc_array_node *parent, *grandparent;
		struct assoc_array_ptr *ptr;

		/* First of all, we need to know if this node has metadata so
		 * that we don't try collapsing if all the leaves are already
		 * here.
		 */
		has_meta = false;
		for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
			ptr = node->slots[i];
			if (assoc_array_ptr_is_meta(ptr)) {
				has_meta = true;
				break;
			}
		}

		pr_devel("leaves: %ld [m=%d]\n",
			 node->nr_leaves_on_branch - 1, has_meta);

		/* Look further up the tree to see if we can collapse this node
		 * into a more proximal node too.
		 */
		parent = node;
	collapse_up:
		pr_devel("collapse subtree: %ld\n", parent->nr_leaves_on_branch);

		ptr = parent->back_pointer;
		if (!ptr)
			goto do_collapse;
		if (assoc_array_ptr_is_shortcut(ptr)) {
			struct assoc_array_shortcut *s = assoc_array_ptr_to_shortcut(ptr);
			ptr = s->back_pointer;
			if (!ptr)
				goto do_collapse;
		}

		grandparent = assoc_array_ptr_to_node(ptr);
		if (grandparent->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT + 1) {
			parent = grandparent;
			goto collapse_up;
		}

	do_collapse:
		/* There's no point collapsing if the original node has no meta
		 * pointers to discard and if we didn't merge into one of that
		 * node's ancestry.
		 */
		if (has_meta || parent != node) {
			node = parent;

			/* Create a new node to collapse into */
			new_n0 = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
			if (!new_n0)
				goto enomem;
			edit->new_meta[0] = assoc_array_node_to_ptr(new_n0);

			new_n0->back_pointer = node->back_pointer;
			new_n0->parent_slot = node->parent_slot;
			new_n0->nr_leaves_on_branch = node->nr_leaves_on_branch;
			edit->adjust_count_on = new_n0;

			collapse.node = new_n0;
			collapse.skip_leaf = assoc_array_ptr_to_leaf(edit->dead_leaf);
			collapse.slot = 0;
			assoc_array_subtree_iterate(assoc_array_node_to_ptr(node),
						    node->back_pointer,
						    assoc_array_delete_collapse_iterator,
						    &collapse);
			pr_devel("collapsed %d,%lu\n", collapse.slot, new_n0->nr_leaves_on_branch);
			BUG_ON(collapse.slot != new_n0->nr_leaves_on_branch - 1);

			if (!node->back_pointer) {
				edit->set[1].ptr = &array->root;
			} else if (assoc_array_ptr_is_leaf(node->back_pointer)) {
				BUG();
			} else if (assoc_array_ptr_is_node(node->back_pointer)) {
				struct assoc_array_node *p =
					assoc_array_ptr_to_node(node->back_pointer);
				edit->set[1].ptr = &p->slots[node->parent_slot];
			} else if (assoc_array_ptr_is_shortcut(node->back_pointer)) {
				struct assoc_array_shortcut *s =
					assoc_array_ptr_to_shortcut(node->back_pointer);
				edit->set[1].ptr = &s->next_node;
			}
			edit->set[1].to = assoc_array_node_to_ptr(new_n0);
			edit->excised_subtree = assoc_array_node_to_ptr(node);
		}
	}

	return edit;

enomem:
	/* Clean up after an out of memory error */
	pr_devel("enomem\n");
	assoc_array_cancel_edit(edit);
	return ERR_PTR(-ENOMEM);
}

/**
 * assoc_array_clear - Script deletion of all objects from an associative array
 * @array: The array to clear.
 * @ops: The operations to use.
 *
 * Precalculate and preallocate a script for the deletion of all the objects
 * from an associative array.  This results in an edit script that can either
 * be applied or cancelled.
 *
 * The function returns a pointer to an edit script if there are objects to be
 * deleted, NULL if there are no objects in the array or -ENOMEM.
 *
 * The caller should lock against other modifications and must continue to hold
 * the lock until assoc_array_apply_edit() has been called.
 *
 * Accesses to the tree may take place concurrently with this function,
 * provided they hold the RCU read lock.
 */
struct assoc_array_edit *assoc_array_clear(struct assoc_array *array,
					   const struct assoc_array_ops *ops)
{
	struct assoc_array_edit *edit;

	pr_devel("-->%s()\n", __func__);

	if (!array->root)
		return NULL;

	edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
	if (!edit)
		return ERR_PTR(-ENOMEM);
	edit->array = array;
	edit->ops = ops;
	edit->set[1].ptr = &array->root;
	edit->set[1].to = NULL;
	edit->excised_subtree = array->root;
	edit->ops_for_excised_subtree = ops;
	pr_devel("all gone\n");
	return edit;
}

/*
 * Handle the deferred destruction after an applied edit.
 */
static void assoc_array_rcu_cleanup(struct rcu_head *head)
{
	struct assoc_array_edit *edit =
		container_of(head, struct assoc_array_edit, rcu);
	int i;

	pr_devel("-->%s()\n", __func__);

	if (edit->dead_leaf)
		edit->ops->free_object(assoc_array_ptr_to_leaf(edit->dead_leaf));
	for (i = 0; i < ARRAY_SIZE(edit->excised_meta); i++)
		if (edit->excised_meta[i])
			kfree(assoc_array_ptr_to_node(edit->excised_meta[i]));

	if (edit->excised_subtree) {
		BUG_ON(assoc_array_ptr_is_leaf(edit->excised_subtree));
		if (assoc_array_ptr_is_node(edit->excised_subtree)) {
			struct assoc_array_node *n =
				assoc_array_ptr_to_node(edit->excised_subtree);
			n->back_pointer = NULL;
		} else {
			struct assoc_array_shortcut *s =
				assoc_array_ptr_to_shortcut(edit->excised_subtree);
			s->back_pointer = NULL;
		}
		assoc_array_destroy_subtree(edit->excised_subtree,
					    edit->ops_for_excised_subtree);
	}

	kfree(edit);
}

/**
 * assoc_array_apply_edit - Apply an edit script to an associative array
 * @edit: The script to apply.
 *
 * Apply an edit script to an associative array to effect an insertion,
 * deletion or clearance.  As the edit script includes preallocated memory,
 * this is guaranteed not to fail.
 *
 * The edit script, dead objects and dead metadata will be scheduled for
 * destruction after an RCU grace period to permit those doing read-only
 * accesses on the array to continue to do so under the RCU read lock whilst
 * the edit is taking place.
 */
void assoc_array_apply_edit(struct assoc_array_edit *edit)
{
	struct assoc_array_shortcut *shortcut;
	struct assoc_array_node *node;
	struct assoc_array_ptr *ptr;
	int i;

	pr_devel("-->%s()\n", __func__);

	smp_wmb();
	if (edit->leaf_p)
		*edit->leaf_p = edit->leaf;

	smp_wmb();
	for (i = 0; i < ARRAY_SIZE(edit->set_parent_slot); i++)
		if (edit->set_parent_slot[i].p)
			*edit->set_parent_slot[i].p = edit->set_parent_slot[i].to;

	smp_wmb();
	for (i = 0; i < ARRAY_SIZE(edit->set_backpointers); i++)
		if (edit->set_backpointers[i])
			*edit->set_backpointers[i] = edit->set_backpointers_to;

	smp_wmb();
	for (i = 0; i < ARRAY_SIZE(edit->set); i++)
		if (edit->set[i].ptr)
			*edit->set[i].ptr = edit->set[i].to;

	if (edit->array->root == NULL) {
		edit->array->nr_leaves_on_tree = 0;
	} else if (edit->adjust_count_on) {
		node = edit->adjust_count_on;
		for (;;) {
			node->nr_leaves_on_branch += edit->adjust_count_by;

			ptr = node->back_pointer;
			if (!ptr)
				break;
			if (assoc_array_ptr_is_shortcut(ptr)) {
				shortcut = assoc_array_ptr_to_shortcut(ptr);
				ptr = shortcut->back_pointer;
				if (!ptr)
					break;
			}
			BUG_ON(!assoc_array_ptr_is_node(ptr));
			node = assoc_array_ptr_to_node(ptr);
		}

		edit->array->nr_leaves_on_tree += edit->adjust_count_by;
	}

	call_rcu(&edit->rcu, assoc_array_rcu_cleanup);
}

/**
 * assoc_array_cancel_edit - Discard an edit script.
 * @edit: The script to discard.
 *
 * Free an edit script and all the preallocated data it holds without making
 * any changes to the associative array it was intended for.
 *
 * NOTE!  In the case of an insertion script, this does _not_ release the leaf
 * that was to be inserted.  That is left to the caller.
 */
void assoc_array_cancel_edit(struct assoc_array_edit *edit)
{
	struct assoc_array_ptr *ptr;
	int i;

	pr_devel("-->%s()\n", __func__);

	/* Clean up after an out of memory error */
	for (i = 0; i < ARRAY_SIZE(edit->new_meta); i++) {
		ptr = edit->new_meta[i];
		if (ptr) {
			if (assoc_array_ptr_is_node(ptr))
				kfree(assoc_array_ptr_to_node(ptr));
			else
				kfree(assoc_array_ptr_to_shortcut(ptr));
		}
	}
	kfree(edit);
}

/**
 * assoc_array_gc - Garbage collect an associative array.
 * @array: The array to clean.
 * @ops: The operations to use.
 * @iterator: A callback function to pass judgement on each object.
 * @iterator_data: Private data for the callback function.
 *
 * Collect garbage from an associative array and pack down the internal tree to
 * save memory.
 *
 * The iterator function is asked to pass judgement upon each object in the
 * array.  If it returns false, the object is discard and if it returns true,
 * the object is kept.  If it returns true, it must increment the object's
 * usage count (or whatever it needs to do to retain it) before returning.
 *
 * This function returns 0 if successful or -ENOMEM if out of memory.  In the
 * latter case, the array is not changed.
 *
 * The caller should lock against other modifications and must continue to hold
 * the lock until assoc_array_apply_edit() has been called.
 *
 * Accesses to the tree may take place concurrently with this function,
 * provided they hold the RCU read lock.
 */
int assoc_array_gc(struct assoc_array *array,
		   const struct assoc_array_ops *ops,
		   bool (*iterator)(void *object, void *iterator_data),
		   void *iterator_data)
{
	struct assoc_array_shortcut *shortcut, *new_s;
	struct assoc_array_node *node, *new_n;
	struct assoc_array_edit *edit;
	struct assoc_array_ptr *cursor, *ptr;
	struct assoc_array_ptr *new_root, *new_parent, **new_ptr_pp;
	unsigned long nr_leaves_on_tree;
	int keylen, slot, nr_free, next_slot, i;

	pr_devel("-->%s()\n", __func__);

	if (!array->root)
		return 0;

	edit = kzalloc(sizeof(struct assoc_array_edit), GFP_KERNEL);
	if (!edit)
		return -ENOMEM;
	edit->array = array;
	edit->ops = ops;
	edit->ops_for_excised_subtree = ops;
	edit->set[0].ptr = &array->root;
	edit->excised_subtree = array->root;

	new_root = new_parent = NULL;
	new_ptr_pp = &new_root;
	cursor = array->root;

descend:
	/* If this point is a shortcut, then we need to duplicate it and
	 * advance the target cursor.
	 */
	if (assoc_array_ptr_is_shortcut(cursor)) {
		shortcut = assoc_array_ptr_to_shortcut(cursor);
		keylen = round_up(shortcut->skip_to_level, ASSOC_ARRAY_KEY_CHUNK_SIZE);
		keylen >>= ASSOC_ARRAY_KEY_CHUNK_SHIFT;
		new_s = kmalloc(sizeof(struct assoc_array_shortcut) +
				keylen * sizeof(unsigned long), GFP_KERNEL);
		if (!new_s)
			goto enomem;
		pr_devel("dup shortcut %p -> %p\n", shortcut, new_s);
		memcpy(new_s, shortcut, (sizeof(struct assoc_array_shortcut) +
					 keylen * sizeof(unsigned long)));
		new_s->back_pointer = new_parent;
		new_s->parent_slot = shortcut->parent_slot;
		*new_ptr_pp = new_parent = assoc_array_shortcut_to_ptr(new_s);
		new_ptr_pp = &new_s->next_node;
		cursor = shortcut->next_node;
	}

	/* Duplicate the node at this position */
	node = assoc_array_ptr_to_node(cursor);
	new_n = kzalloc(sizeof(struct assoc_array_node), GFP_KERNEL);
	if (!new_n)
		goto enomem;
	pr_devel("dup node %p -> %p\n", node, new_n);
	new_n->back_pointer = new_parent;
	new_n->parent_slot = node->parent_slot;
	*new_ptr_pp = new_parent = assoc_array_node_to_ptr(new_n);
	new_ptr_pp = NULL;
	slot = 0;

continue_node:
	/* Filter across any leaves and gc any subtrees */
	for (; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
		ptr = node->slots[slot];
		if (!ptr)
			continue;

		if (assoc_array_ptr_is_leaf(ptr)) {
			if (iterator(assoc_array_ptr_to_leaf(ptr),
				     iterator_data))
				/* The iterator will have done any reference
				 * counting on the object for us.
				 */
				new_n->slots[slot] = ptr;
			continue;
		}

		new_ptr_pp = &new_n->slots[slot];
		cursor = ptr;
		goto descend;
	}

	pr_devel("-- compress node %p --\n", new_n);

	/* Count up the number of empty slots in this node and work out the
	 * subtree leaf count.
	 */
	new_n->nr_leaves_on_branch = 0;
	nr_free = 0;
	for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
		ptr = new_n->slots[slot];
		if (!ptr)
			nr_free++;
		else if (assoc_array_ptr_is_leaf(ptr))
			new_n->nr_leaves_on_branch++;
	}
	pr_devel("free=%d, leaves=%lu\n", nr_free, new_n->nr_leaves_on_branch);

	/* See what we can fold in */
	next_slot = 0;
	for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++) {
		struct assoc_array_shortcut *s;
		struct assoc_array_node *child;

		ptr = new_n->slots[slot];
		if (!ptr || assoc_array_ptr_is_leaf(ptr))
			continue;

		s = NULL;
		if (assoc_array_ptr_is_shortcut(ptr)) {
			s = assoc_array_ptr_to_shortcut(ptr);
			ptr = s->next_node;
		}

		child = assoc_array_ptr_to_node(ptr);
		new_n->nr_leaves_on_branch += child->nr_leaves_on_branch;

		if (child->nr_leaves_on_branch <= nr_free + 1) {
			/* Fold the child node into this one */
			pr_devel("[%d] fold node %lu/%d [nx %d]\n",
				 slot, child->nr_leaves_on_branch, nr_free + 1,
				 next_slot);

			/* We would already have reaped an intervening shortcut
			 * on the way back up the tree.
			 */
			BUG_ON(s);

			new_n->slots[slot] = NULL;
			nr_free++;
			if (slot < next_slot)
				next_slot = slot;
			for (i = 0; i < ASSOC_ARRAY_FAN_OUT; i++) {
				struct assoc_array_ptr *p = child->slots[i];
				if (!p)
					continue;
				BUG_ON(assoc_array_ptr_is_meta(p));
				while (new_n->slots[next_slot])
					next_slot++;
				BUG_ON(next_slot >= ASSOC_ARRAY_FAN_OUT);
				new_n->slots[next_slot++] = p;
				nr_free--;
			}
			kfree(child);
		} else {
			pr_devel("[%d] retain node %lu/%d [nx %d]\n",
				 slot, child->nr_leaves_on_branch, nr_free + 1,
				 next_slot);
		}
	}

	pr_devel("after: %lu\n", new_n->nr_leaves_on_branch);

	nr_leaves_on_tree = new_n->nr_leaves_on_branch;

	/* Excise this node if it is singly occupied by a shortcut */
	if (nr_free == ASSOC_ARRAY_FAN_OUT - 1) {
		for (slot = 0; slot < ASSOC_ARRAY_FAN_OUT; slot++)
			if ((ptr = new_n->slots[slot]))
				break;

		if (assoc_array_ptr_is_meta(ptr) &&
		    assoc_array_ptr_is_shortcut(ptr)) {
			pr_devel("excise node %p with 1 shortcut\n", new_n);
			new_s = assoc_array_ptr_to_shortcut(ptr);
			new_parent = new_n->back_pointer;
			slot = new_n->parent_slot;
			kfree(new_n);
			if (!new_parent) {
				new_s->back_pointer = NULL;
				new_s->parent_slot = 0;
				new_root = ptr;
				goto gc_complete;
			}

			if (assoc_array_ptr_is_shortcut(new_parent)) {
				/* We can discard any preceding shortcut also */
				struct assoc_array_shortcut *s =
					assoc_array_ptr_to_shortcut(new_parent);

				pr_devel("excise preceding shortcut\n");

				new_parent = new_s->back_pointer = s->back_pointer;
				slot = new_s->parent_slot = s->parent_slot;
				kfree(s);
				if (!new_parent) {
					new_s->back_pointer = NULL;
					new_s->parent_slot = 0;
					new_root = ptr;
					goto gc_complete;
				}
			}

			new_s->back_pointer = new_parent;
			new_s->parent_slot = slot;
			new_n = assoc_array_ptr_to_node(new_parent);
			new_n->slots[slot] = ptr;
			goto ascend_old_tree;
		}
	}

	/* Excise any shortcuts we might encounter that point to nodes that
	 * only contain leaves.
	 */
	ptr = new_n->back_pointer;
	if (!ptr)
		goto gc_complete;

	if (assoc_array_ptr_is_shortcut(ptr)) {
		new_s = assoc_array_ptr_to_shortcut(ptr);
		new_parent = new_s->back_pointer;
		slot = new_s->parent_slot;

		if (new_n->nr_leaves_on_branch <= ASSOC_ARRAY_FAN_OUT) {
			struct assoc_array_node *n;

			pr_devel("excise shortcut\n");
			new_n->back_pointer = new_parent;
			new_n->parent_slot = slot;
			kfree(new_s);
			if (!new_parent) {
				new_root = assoc_array_node_to_ptr(new_n);
				goto gc_complete;
			}

			n = assoc_array_ptr_to_node(new_parent);
			n->slots[slot] = assoc_array_node_to_ptr(new_n);
		}
	} else {
		new_parent = ptr;
	}
	new_n = assoc_array_ptr_to_node(new_parent);

ascend_old_tree:
	ptr = node->back_pointer;
	if (assoc_array_ptr_is_shortcut(ptr)) {
		shortcut = assoc_array_ptr_to_shortcut(ptr);
		slot = shortcut->parent_slot;
		cursor = shortcut->back_pointer;
		if (!cursor)
			goto gc_complete;
	} else {
		slot = node->parent_slot;
		cursor = ptr;
	}
	BUG_ON(!cursor);
	node = assoc_array_ptr_to_node(cursor);
	slot++;
	goto continue_node;

gc_complete:
	edit->set[0].to = new_root;
	assoc_array_apply_edit(edit);
	array->nr_leaves_on_tree = nr_leaves_on_tree;
	return 0;

enomem:
	pr_devel("enomem\n");
	assoc_array_destroy_subtree(new_root, edit->ops);
	kfree(edit);
	return -ENOMEM;
}