linux/lib/list_sort.c
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   1#include <linux/kernel.h>
   2#include <linux/module.h>
   3#include <linux/list_sort.h>
   4#include <linux/slab.h>
   5#include <linux/list.h>
   6
   7#define MAX_LIST_LENGTH_BITS 20
   8
   9/*
  10 * Returns a list organized in an intermediate format suited
  11 * to chaining of merge() calls: null-terminated, no reserved or
  12 * sentinel head node, "prev" links not maintained.
  13 */
  14static struct list_head *merge(void *priv,
  15                                int (*cmp)(void *priv, struct list_head *a,
  16                                        struct list_head *b),
  17                                struct list_head *a, struct list_head *b)
  18{
  19        struct list_head head, *tail = &head;
  20
  21        while (a && b) {
  22                /* if equal, take 'a' -- important for sort stability */
  23                if ((*cmp)(priv, a, b) <= 0) {
  24                        tail->next = a;
  25                        a = a->next;
  26                } else {
  27                        tail->next = b;
  28                        b = b->next;
  29                }
  30                tail = tail->next;
  31        }
  32        tail->next = a?:b;
  33        return head.next;
  34}
  35
  36/*
  37 * Combine final list merge with restoration of standard doubly-linked
  38 * list structure.  This approach duplicates code from merge(), but
  39 * runs faster than the tidier alternatives of either a separate final
  40 * prev-link restoration pass, or maintaining the prev links
  41 * throughout.
  42 */
  43static void merge_and_restore_back_links(void *priv,
  44                                int (*cmp)(void *priv, struct list_head *a,
  45                                        struct list_head *b),
  46                                struct list_head *head,
  47                                struct list_head *a, struct list_head *b)
  48{
  49        struct list_head *tail = head;
  50
  51        while (a && b) {
  52                /* if equal, take 'a' -- important for sort stability */
  53                if ((*cmp)(priv, a, b) <= 0) {
  54                        tail->next = a;
  55                        a->prev = tail;
  56                        a = a->next;
  57                } else {
  58                        tail->next = b;
  59                        b->prev = tail;
  60                        b = b->next;
  61                }
  62                tail = tail->next;
  63        }
  64        tail->next = a ? : b;
  65
  66        do {
  67                /*
  68                 * In worst cases this loop may run many iterations.
  69                 * Continue callbacks to the client even though no
  70                 * element comparison is needed, so the client's cmp()
  71                 * routine can invoke cond_resched() periodically.
  72                 */
  73                (*cmp)(priv, tail->next, tail->next);
  74
  75                tail->next->prev = tail;
  76                tail = tail->next;
  77        } while (tail->next);
  78
  79        tail->next = head;
  80        head->prev = tail;
  81}
  82
  83/**
  84 * list_sort - sort a list
  85 * @priv: private data, opaque to list_sort(), passed to @cmp
  86 * @head: the list to sort
  87 * @cmp: the elements comparison function
  88 *
  89 * This function implements "merge sort", which has O(nlog(n))
  90 * complexity.
  91 *
  92 * The comparison function @cmp must return a negative value if @a
  93 * should sort before @b, and a positive value if @a should sort after
  94 * @b. If @a and @b are equivalent, and their original relative
  95 * ordering is to be preserved, @cmp must return 0.
  96 */
  97void list_sort(void *priv, struct list_head *head,
  98                int (*cmp)(void *priv, struct list_head *a,
  99                        struct list_head *b))
 100{
 101        struct list_head *part[MAX_LIST_LENGTH_BITS+1]; /* sorted partial lists
 102                                                -- last slot is a sentinel */
 103        int lev;  /* index into part[] */
 104        int max_lev = 0;
 105        struct list_head *list;
 106
 107        if (list_empty(head))
 108                return;
 109
 110        memset(part, 0, sizeof(part));
 111
 112        head->prev->next = NULL;
 113        list = head->next;
 114
 115        while (list) {
 116                struct list_head *cur = list;
 117                list = list->next;
 118                cur->next = NULL;
 119
 120                for (lev = 0; part[lev]; lev++) {
 121                        cur = merge(priv, cmp, part[lev], cur);
 122                        part[lev] = NULL;
 123                }
 124                if (lev > max_lev) {
 125                        if (unlikely(lev >= ARRAY_SIZE(part)-1)) {
 126                                printk_once(KERN_DEBUG "list passed to"
 127                                        " list_sort() too long for"
 128                                        " efficiency\n");
 129                                lev--;
 130                        }
 131                        max_lev = lev;
 132                }
 133                part[lev] = cur;
 134        }
 135
 136        for (lev = 0; lev < max_lev; lev++)
 137                if (part[lev])
 138                        list = merge(priv, cmp, part[lev], list);
 139
 140        merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);
 141}
 142EXPORT_SYMBOL(list_sort);
 143
 144#ifdef CONFIG_TEST_LIST_SORT
 145
 146#include <linux/random.h>
 147
 148/*
 149 * The pattern of set bits in the list length determines which cases
 150 * are hit in list_sort().
 151 */
 152#define TEST_LIST_LEN (512+128+2) /* not including head */
 153
 154#define TEST_POISON1 0xDEADBEEF
 155#define TEST_POISON2 0xA324354C
 156
 157struct debug_el {
 158        unsigned int poison1;
 159        struct list_head list;
 160        unsigned int poison2;
 161        int value;
 162        unsigned serial;
 163};
 164
 165/* Array, containing pointers to all elements in the test list */
 166static struct debug_el **elts __initdata;
 167
 168static int __init check(struct debug_el *ela, struct debug_el *elb)
 169{
 170        if (ela->serial >= TEST_LIST_LEN) {
 171                printk(KERN_ERR "list_sort_test: error: incorrect serial %d\n",
 172                                ela->serial);
 173                return -EINVAL;
 174        }
 175        if (elb->serial >= TEST_LIST_LEN) {
 176                printk(KERN_ERR "list_sort_test: error: incorrect serial %d\n",
 177                                elb->serial);
 178                return -EINVAL;
 179        }
 180        if (elts[ela->serial] != ela || elts[elb->serial] != elb) {
 181                printk(KERN_ERR "list_sort_test: error: phantom element\n");
 182                return -EINVAL;
 183        }
 184        if (ela->poison1 != TEST_POISON1 || ela->poison2 != TEST_POISON2) {
 185                printk(KERN_ERR "list_sort_test: error: bad poison: %#x/%#x\n",
 186                                ela->poison1, ela->poison2);
 187                return -EINVAL;
 188        }
 189        if (elb->poison1 != TEST_POISON1 || elb->poison2 != TEST_POISON2) {
 190                printk(KERN_ERR "list_sort_test: error: bad poison: %#x/%#x\n",
 191                                elb->poison1, elb->poison2);
 192                return -EINVAL;
 193        }
 194        return 0;
 195}
 196
 197static int __init cmp(void *priv, struct list_head *a, struct list_head *b)
 198{
 199        struct debug_el *ela, *elb;
 200
 201        ela = container_of(a, struct debug_el, list);
 202        elb = container_of(b, struct debug_el, list);
 203
 204        check(ela, elb);
 205        return ela->value - elb->value;
 206}
 207
 208static int __init list_sort_test(void)
 209{
 210        int i, count = 1, err = -EINVAL;
 211        struct debug_el *el;
 212        struct list_head *cur, *tmp;
 213        LIST_HEAD(head);
 214
 215        printk(KERN_DEBUG "list_sort_test: start testing list_sort()\n");
 216
 217        elts = kmalloc(sizeof(void *) * TEST_LIST_LEN, GFP_KERNEL);
 218        if (!elts) {
 219                printk(KERN_ERR "list_sort_test: error: cannot allocate "
 220                                "memory\n");
 221                goto exit;
 222        }
 223
 224        for (i = 0; i < TEST_LIST_LEN; i++) {
 225                el = kmalloc(sizeof(*el), GFP_KERNEL);
 226                if (!el) {
 227                        printk(KERN_ERR "list_sort_test: error: cannot "
 228                                        "allocate memory\n");
 229                        goto exit;
 230                }
 231                 /* force some equivalencies */
 232                el->value = prandom_u32() % (TEST_LIST_LEN / 3);
 233                el->serial = i;
 234                el->poison1 = TEST_POISON1;
 235                el->poison2 = TEST_POISON2;
 236                elts[i] = el;
 237                list_add_tail(&el->list, &head);
 238        }
 239
 240        list_sort(NULL, &head, cmp);
 241
 242        for (cur = head.next; cur->next != &head; cur = cur->next) {
 243                struct debug_el *el1;
 244                int cmp_result;
 245
 246                if (cur->next->prev != cur) {
 247                        printk(KERN_ERR "list_sort_test: error: list is "
 248                                        "corrupted\n");
 249                        goto exit;
 250                }
 251
 252                cmp_result = cmp(NULL, cur, cur->next);
 253                if (cmp_result > 0) {
 254                        printk(KERN_ERR "list_sort_test: error: list is not "
 255                                        "sorted\n");
 256                        goto exit;
 257                }
 258
 259                el = container_of(cur, struct debug_el, list);
 260                el1 = container_of(cur->next, struct debug_el, list);
 261                if (cmp_result == 0 && el->serial >= el1->serial) {
 262                        printk(KERN_ERR "list_sort_test: error: order of "
 263                                        "equivalent elements not preserved\n");
 264                        goto exit;
 265                }
 266
 267                if (check(el, el1)) {
 268                        printk(KERN_ERR "list_sort_test: error: element check "
 269                                        "failed\n");
 270                        goto exit;
 271                }
 272                count++;
 273        }
 274
 275        if (count != TEST_LIST_LEN) {
 276                printk(KERN_ERR "list_sort_test: error: bad list length %d",
 277                                count);
 278                goto exit;
 279        }
 280
 281        err = 0;
 282exit:
 283        kfree(elts);
 284        list_for_each_safe(cur, tmp, &head) {
 285                list_del(cur);
 286                kfree(container_of(cur, struct debug_el, list));
 287        }
 288        return err;
 289}
 290module_init(list_sort_test);
 291#endif /* CONFIG_TEST_LIST_SORT */
 292
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