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1
2#define pr_fmt(fmt) "list_sort_test: " fmt
3
4#include <linux/kernel.h>
5#include <linux/bug.h>
6#include <linux/compiler.h>
7#include <linux/export.h>
8#include <linux/string.h>
9#include <linux/list_sort.h>
10#include <linux/list.h>
11
12#define MAX_LIST_LENGTH_BITS 20
13
14/*
15 * Returns a list organized in an intermediate format suited
16 * to chaining of merge() calls: null-terminated, no reserved or
17 * sentinel head node, "prev" links not maintained.
18 */
19static struct list_head *merge(void *priv,
20 int (*cmp)(void *priv, struct list_head *a,
21 struct list_head *b),
22 struct list_head *a, struct list_head *b)
23{
24 struct list_head head, *tail = &head;
25
26 while (a && b) {
27 /* if equal, take 'a' -- important for sort stability */
28 if ((*cmp)(priv, a, b) <= 0) {
29 tail->next = a;
30 a = a->next;
31 } else {
32 tail->next = b;
33 b = b->next;
34 }
35 tail = tail->next;
36 }
37 tail->next = a?:b;
38 return head.next;
39}
40
41/*
42 * Combine final list merge with restoration of standard doubly-linked
43 * list structure. This approach duplicates code from merge(), but
44 * runs faster than the tidier alternatives of either a separate final
45 * prev-link restoration pass, or maintaining the prev links
46 * throughout.
47 */
48static void merge_and_restore_back_links(void *priv,
49 int (*cmp)(void *priv, struct list_head *a,
50 struct list_head *b),
51 struct list_head *head,
52 struct list_head *a, struct list_head *b)
53{
54 struct list_head *tail = head;
55 u8 count = 0;
56
57 while (a && b) {
58 /* if equal, take 'a' -- important for sort stability */
59 if ((*cmp)(priv, a, b) <= 0) {
60 tail->next = a;
61 a->prev = tail;
62 a = a->next;
63 } else {
64 tail->next = b;
65 b->prev = tail;
66 b = b->next;
67 }
68 tail = tail->next;
69 }
70 tail->next = a ? : b;
71
72 do {
73 /*
74 * In worst cases this loop may run many iterations.
75 * Continue callbacks to the client even though no
76 * element comparison is needed, so the client's cmp()
77 * routine can invoke cond_resched() periodically.
78 */
79 if (unlikely(!(++count)))
80 (*cmp)(priv, tail->next, tail->next);
81
82 tail->next->prev = tail;
83 tail = tail->next;
84 } while (tail->next);
85
86 tail->next = head;
87 head->prev = tail;
88}
89
90/**
91 * list_sort - sort a list
92 * @priv: private data, opaque to list_sort(), passed to @cmp
93 * @head: the list to sort
94 * @cmp: the elements comparison function
95 *
96 * This function implements "merge sort", which has O(nlog(n))
97 * complexity.
98 *
99 * The comparison function @cmp must return a negative value if @a
100 * should sort before @b, and a positive value if @a should sort after
101 * @b. If @a and @b are equivalent, and their original relative
102 * ordering is to be preserved, @cmp must return 0.
103 */
104void list_sort(void *priv, struct list_head *head,
105 int (*cmp)(void *priv, struct list_head *a,
106 struct list_head *b))
107{
108 struct list_head *part[MAX_LIST_LENGTH_BITS+1]; /* sorted partial lists
109 -- last slot is a sentinel */
110 int lev; /* index into part[] */
111 int max_lev = 0;
112 struct list_head *list;
113
114 if (list_empty(head))
115 return;
116
117 memset(part, 0, sizeof(part));
118
119 head->prev->next = NULL;
120 list = head->next;
121
122 while (list) {
123 struct list_head *cur = list;
124 list = list->next;
125 cur->next = NULL;
126
127 for (lev = 0; part[lev]; lev++) {
128 cur = merge(priv, cmp, part[lev], cur);
129 part[lev] = NULL;
130 }
131 if (lev > max_lev) {
132 if (unlikely(lev >= ARRAY_SIZE(part)-1)) {
133 printk_once(KERN_DEBUG "list too long for efficiency\n");
134 lev--;
135 }
136 max_lev = lev;
137 }
138 part[lev] = cur;
139 }
140
141 for (lev = 0; lev < max_lev; lev++)
142 if (part[lev])
143 list = merge(priv, cmp, part[lev], list);
144
145 merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);
146}
147EXPORT_SYMBOL(list_sort);
148
149#ifdef CONFIG_TEST_LIST_SORT
150
151#include <linux/slab.h>
152#include <linux/random.h>
153
154/*
155 * The pattern of set bits in the list length determines which cases
156 * are hit in list_sort().
157 */
158#define TEST_LIST_LEN (512+128+2) /* not including head */
159
160#define TEST_POISON1 0xDEADBEEF
161#define TEST_POISON2 0xA324354C
162
163struct debug_el {
164 unsigned int poison1;
165 struct list_head list;
166 unsigned int poison2;
167 int value;
168 unsigned serial;
169};
170
171/* Array, containing pointers to all elements in the test list */
172static struct debug_el **elts __initdata;
173
174static int __init check(struct debug_el *ela, struct debug_el *elb)
175{
176 if (ela->serial >= TEST_LIST_LEN) {
177 pr_err("error: incorrect serial %d\n", ela->serial);
178 return -EINVAL;
179 }
180 if (elb->serial >= TEST_LIST_LEN) {
181 pr_err("error: incorrect serial %d\n", elb->serial);
182 return -EINVAL;
183 }
184 if (elts[ela->serial] != ela || elts[elb->serial] != elb) {
185 pr_err("error: phantom element\n");
186 return -EINVAL;
187 }
188 if (ela->poison1 != TEST_POISON1 || ela->poison2 != TEST_POISON2) {
189 pr_err("error: bad poison: %#x/%#x\n",
190 ela->poison1, ela->poison2);
191 return -EINVAL;
192 }
193 if (elb->poison1 != TEST_POISON1 || elb->poison2 != TEST_POISON2) {
194 pr_err("error: bad poison: %#x/%#x\n",
195 elb->poison1, elb->poison2);
196 return -EINVAL;
197 }
198 return 0;
199}
200
201static int __init cmp(void *priv, struct list_head *a, struct list_head *b)
202{
203 struct debug_el *ela, *elb;
204
205 ela = container_of(a, struct debug_el, list);
206 elb = container_of(b, struct debug_el, list);
207
208 check(ela, elb);
209 return ela->value - elb->value;
210}
211
212static int __init list_sort_test(void)
213{
214 int i, count = 1, err = -ENOMEM;
215 struct debug_el *el;
216 struct list_head *cur;
217 LIST_HEAD(head);
218
219 pr_debug("start testing list_sort()\n");
220
221 elts = kcalloc(TEST_LIST_LEN, sizeof(*elts), GFP_KERNEL);
222 if (!elts) {
223 pr_err("error: cannot allocate memory\n");
224 return err;
225 }
226
227 for (i = 0; i < TEST_LIST_LEN; i++) {
228 el = kmalloc(sizeof(*el), GFP_KERNEL);
229 if (!el) {
230 pr_err("error: cannot allocate memory\n");
231 goto exit;
232 }
233 /* force some equivalencies */
234 el->value = prandom_u32() % (TEST_LIST_LEN / 3);
235 el->serial = i;
236 el->poison1 = TEST_POISON1;
237 el->poison2 = TEST_POISON2;
238 elts[i] = el;
239 list_add_tail(&el->list, &head);
240 }
241
242 list_sort(NULL, &head, cmp);
243
244 err = -EINVAL;
245 for (cur = head.next; cur->next != &head; cur = cur->next) {
246 struct debug_el *el1;
247 int cmp_result;
248
249 if (cur->next->prev != cur) {
250 pr_err("error: list is corrupted\n");
251 goto exit;
252 }
253
254 cmp_result = cmp(NULL, cur, cur->next);
255 if (cmp_result > 0) {
256 pr_err("error: list is not sorted\n");
257 goto exit;
258 }
259
260 el = container_of(cur, struct debug_el, list);
261 el1 = container_of(cur->next, struct debug_el, list);
262 if (cmp_result == 0 && el->serial >= el1->serial) {
263 pr_err("error: order of equivalent elements not "
264 "preserved\n");
265 goto exit;
266 }
267
268 if (check(el, el1)) {
269 pr_err("error: element check failed\n");
270 goto exit;
271 }
272 count++;
273 }
274 if (head.prev != cur) {
275 pr_err("error: list is corrupted\n");
276 goto exit;
277 }
278
279
280 if (count != TEST_LIST_LEN) {
281 pr_err("error: bad list length %d", count);
282 goto exit;
283 }
284
285 err = 0;
286exit:
287 for (i = 0; i < TEST_LIST_LEN; i++)
288 kfree(elts[i]);
289 kfree(elts);
290 return err;
291}
292late_initcall(list_sort_test);
293#endif /* CONFIG_TEST_LIST_SORT */
1// SPDX-License-Identifier: GPL-2.0
2#include <linux/kernel.h>
3#include <linux/bug.h>
4#include <linux/compiler.h>
5#include <linux/export.h>
6#include <linux/string.h>
7#include <linux/list_sort.h>
8#include <linux/list.h>
9
10typedef int __attribute__((nonnull(2,3))) (*cmp_func)(void *,
11 struct list_head const *, struct list_head const *);
12
13/*
14 * Returns a list organized in an intermediate format suited
15 * to chaining of merge() calls: null-terminated, no reserved or
16 * sentinel head node, "prev" links not maintained.
17 */
18__attribute__((nonnull(2,3,4)))
19static struct list_head *merge(void *priv, cmp_func cmp,
20 struct list_head *a, struct list_head *b)
21{
22 struct list_head *head, **tail = &head;
23
24 for (;;) {
25 /* if equal, take 'a' -- important for sort stability */
26 if (cmp(priv, a, b) <= 0) {
27 *tail = a;
28 tail = &a->next;
29 a = a->next;
30 if (!a) {
31 *tail = b;
32 break;
33 }
34 } else {
35 *tail = b;
36 tail = &b->next;
37 b = b->next;
38 if (!b) {
39 *tail = a;
40 break;
41 }
42 }
43 }
44 return head;
45}
46
47/*
48 * Combine final list merge with restoration of standard doubly-linked
49 * list structure. This approach duplicates code from merge(), but
50 * runs faster than the tidier alternatives of either a separate final
51 * prev-link restoration pass, or maintaining the prev links
52 * throughout.
53 */
54__attribute__((nonnull(2,3,4,5)))
55static void merge_final(void *priv, cmp_func cmp, struct list_head *head,
56 struct list_head *a, struct list_head *b)
57{
58 struct list_head *tail = head;
59 u8 count = 0;
60
61 for (;;) {
62 /* if equal, take 'a' -- important for sort stability */
63 if (cmp(priv, a, b) <= 0) {
64 tail->next = a;
65 a->prev = tail;
66 tail = a;
67 a = a->next;
68 if (!a)
69 break;
70 } else {
71 tail->next = b;
72 b->prev = tail;
73 tail = b;
74 b = b->next;
75 if (!b) {
76 b = a;
77 break;
78 }
79 }
80 }
81
82 /* Finish linking remainder of list b on to tail */
83 tail->next = b;
84 do {
85 /*
86 * If the merge is highly unbalanced (e.g. the input is
87 * already sorted), this loop may run many iterations.
88 * Continue callbacks to the client even though no
89 * element comparison is needed, so the client's cmp()
90 * routine can invoke cond_resched() periodically.
91 */
92 if (unlikely(!++count))
93 cmp(priv, b, b);
94 b->prev = tail;
95 tail = b;
96 b = b->next;
97 } while (b);
98
99 /* And the final links to make a circular doubly-linked list */
100 tail->next = head;
101 head->prev = tail;
102}
103
104/**
105 * list_sort - sort a list
106 * @priv: private data, opaque to list_sort(), passed to @cmp
107 * @head: the list to sort
108 * @cmp: the elements comparison function
109 *
110 * The comparison funtion @cmp must return > 0 if @a should sort after
111 * @b ("@a > @b" if you want an ascending sort), and <= 0 if @a should
112 * sort before @b *or* their original order should be preserved. It is
113 * always called with the element that came first in the input in @a,
114 * and list_sort is a stable sort, so it is not necessary to distinguish
115 * the @a < @b and @a == @b cases.
116 *
117 * This is compatible with two styles of @cmp function:
118 * - The traditional style which returns <0 / =0 / >0, or
119 * - Returning a boolean 0/1.
120 * The latter offers a chance to save a few cycles in the comparison
121 * (which is used by e.g. plug_ctx_cmp() in block/blk-mq.c).
122 *
123 * A good way to write a multi-word comparison is::
124 *
125 * if (a->high != b->high)
126 * return a->high > b->high;
127 * if (a->middle != b->middle)
128 * return a->middle > b->middle;
129 * return a->low > b->low;
130 *
131 *
132 * This mergesort is as eager as possible while always performing at least
133 * 2:1 balanced merges. Given two pending sublists of size 2^k, they are
134 * merged to a size-2^(k+1) list as soon as we have 2^k following elements.
135 *
136 * Thus, it will avoid cache thrashing as long as 3*2^k elements can
137 * fit into the cache. Not quite as good as a fully-eager bottom-up
138 * mergesort, but it does use 0.2*n fewer comparisons, so is faster in
139 * the common case that everything fits into L1.
140 *
141 *
142 * The merging is controlled by "count", the number of elements in the
143 * pending lists. This is beautiully simple code, but rather subtle.
144 *
145 * Each time we increment "count", we set one bit (bit k) and clear
146 * bits k-1 .. 0. Each time this happens (except the very first time
147 * for each bit, when count increments to 2^k), we merge two lists of
148 * size 2^k into one list of size 2^(k+1).
149 *
150 * This merge happens exactly when the count reaches an odd multiple of
151 * 2^k, which is when we have 2^k elements pending in smaller lists,
152 * so it's safe to merge away two lists of size 2^k.
153 *
154 * After this happens twice, we have created two lists of size 2^(k+1),
155 * which will be merged into a list of size 2^(k+2) before we create
156 * a third list of size 2^(k+1), so there are never more than two pending.
157 *
158 * The number of pending lists of size 2^k is determined by the
159 * state of bit k of "count" plus two extra pieces of information:
160 *
161 * - The state of bit k-1 (when k == 0, consider bit -1 always set), and
162 * - Whether the higher-order bits are zero or non-zero (i.e.
163 * is count >= 2^(k+1)).
164 *
165 * There are six states we distinguish. "x" represents some arbitrary
166 * bits, and "y" represents some arbitrary non-zero bits:
167 * 0: 00x: 0 pending of size 2^k; x pending of sizes < 2^k
168 * 1: 01x: 0 pending of size 2^k; 2^(k-1) + x pending of sizes < 2^k
169 * 2: x10x: 0 pending of size 2^k; 2^k + x pending of sizes < 2^k
170 * 3: x11x: 1 pending of size 2^k; 2^(k-1) + x pending of sizes < 2^k
171 * 4: y00x: 1 pending of size 2^k; 2^k + x pending of sizes < 2^k
172 * 5: y01x: 2 pending of size 2^k; 2^(k-1) + x pending of sizes < 2^k
173 * (merge and loop back to state 2)
174 *
175 * We gain lists of size 2^k in the 2->3 and 4->5 transitions (because
176 * bit k-1 is set while the more significant bits are non-zero) and
177 * merge them away in the 5->2 transition. Note in particular that just
178 * before the 5->2 transition, all lower-order bits are 11 (state 3),
179 * so there is one list of each smaller size.
180 *
181 * When we reach the end of the input, we merge all the pending
182 * lists, from smallest to largest. If you work through cases 2 to
183 * 5 above, you can see that the number of elements we merge with a list
184 * of size 2^k varies from 2^(k-1) (cases 3 and 5 when x == 0) to
185 * 2^(k+1) - 1 (second merge of case 5 when x == 2^(k-1) - 1).
186 */
187__attribute__((nonnull(2,3)))
188void list_sort(void *priv, struct list_head *head,
189 int (*cmp)(void *priv, struct list_head *a,
190 struct list_head *b))
191{
192 struct list_head *list = head->next, *pending = NULL;
193 size_t count = 0; /* Count of pending */
194
195 if (list == head->prev) /* Zero or one elements */
196 return;
197
198 /* Convert to a null-terminated singly-linked list. */
199 head->prev->next = NULL;
200
201 /*
202 * Data structure invariants:
203 * - All lists are singly linked and null-terminated; prev
204 * pointers are not maintained.
205 * - pending is a prev-linked "list of lists" of sorted
206 * sublists awaiting further merging.
207 * - Each of the sorted sublists is power-of-two in size.
208 * - Sublists are sorted by size and age, smallest & newest at front.
209 * - There are zero to two sublists of each size.
210 * - A pair of pending sublists are merged as soon as the number
211 * of following pending elements equals their size (i.e.
212 * each time count reaches an odd multiple of that size).
213 * That ensures each later final merge will be at worst 2:1.
214 * - Each round consists of:
215 * - Merging the two sublists selected by the highest bit
216 * which flips when count is incremented, and
217 * - Adding an element from the input as a size-1 sublist.
218 */
219 do {
220 size_t bits;
221 struct list_head **tail = &pending;
222
223 /* Find the least-significant clear bit in count */
224 for (bits = count; bits & 1; bits >>= 1)
225 tail = &(*tail)->prev;
226 /* Do the indicated merge */
227 if (likely(bits)) {
228 struct list_head *a = *tail, *b = a->prev;
229
230 a = merge(priv, (cmp_func)cmp, b, a);
231 /* Install the merged result in place of the inputs */
232 a->prev = b->prev;
233 *tail = a;
234 }
235
236 /* Move one element from input list to pending */
237 list->prev = pending;
238 pending = list;
239 list = list->next;
240 pending->next = NULL;
241 count++;
242 } while (list);
243
244 /* End of input; merge together all the pending lists. */
245 list = pending;
246 pending = pending->prev;
247 for (;;) {
248 struct list_head *next = pending->prev;
249
250 if (!next)
251 break;
252 list = merge(priv, (cmp_func)cmp, pending, list);
253 pending = next;
254 }
255 /* The final merge, rebuilding prev links */
256 merge_final(priv, (cmp_func)cmp, head, pending, list);
257}
258EXPORT_SYMBOL(list_sort);