lib/list_sort.c

   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  */
  14 static 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  */
  43 static 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  */
  97 void 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 }
 142 EXPORT_SYMBOL(list_sort);
 143
 144 #ifdef CONFIG_TEST_LIST_SORT
 145
 146 #include <linux/random.h>
 147
 148 struct debug_el {
 149         struct list_head list;
 150         int value;
 151         unsigned serial;
 152 };
 153
 154 static int cmp(void *priv, struct list_head *a, struct list_head *b)
 155 {
 156         return container_of(a, struct debug_el, list)->value
 157              - container_of(b, struct debug_el, list)->value;
 158 }
 159
 160 /*
 161  * The pattern of set bits in the list length determines which cases
 162  * are hit in list_sort().
 163  */
 164 #define TEST_LIST_LEN (512+128+2) /* not including head */
 165
 166 static int __init list_sort_test(void)
 167 {
 168         int i, count = 1, err = -EINVAL;
 169         struct debug_el *el;
 170         struct list_head *cur, *tmp;
 171         LIST_HEAD(head);
 172
 173         printk(KERN_DEBUG "testing list_sort()\n");
 174
 175         for (i = 0; i < TEST_LIST_LEN; i++) {
 176                 el = kmalloc(sizeof(*el), GFP_KERNEL);
 177                 if (!el) {
 178                         printk(KERN_ERR "cancel list_sort() testing - cannot "
 179                                         "allocate memory\n");
 180                         goto exit;
 181                 }
 182                  /* force some equivalencies */
 183                 el->value = random32() % (TEST_LIST_LEN/3);
 184                 el->serial = i;
 185                 list_add_tail(&el->list, &head);
 186         }
 187
 188         list_sort(NULL, &head, cmp);
 189
 190         for (cur = head.next; cur->next != &head; cur = cur->next) {
 191                 struct debug_el *el1;
 192                 int cmp_result;
 193
 194                 if (cur->next->prev != cur) {
 195                         printk(KERN_ERR "list_sort() returned "
 196                                         "a corrupted list!\n");
 197                         goto exit;
 198                 }
 199
 200                 cmp_result = cmp(NULL, cur, cur->next);
 201                 if (cmp_result > 0) {
 202                         printk(KERN_ERR "list_sort() failed to sort!\n");
 203                         goto exit;
 204                 }
 205
 206                 el = container_of(cur, struct debug_el, list);
 207                 el1 = container_of(cur->next, struct debug_el, list);
 208                 if (cmp_result == 0 && el->serial >= el1->serial) {
 209                         printk(KERN_ERR "list_sort() failed to preserve order "
 210                                         "of equivalent elements!\n");
 211                         goto exit;
 212                 }
 213                 count++;
 214         }
 215
 216         if (count != TEST_LIST_LEN) {
 217                 printk(KERN_ERR "list_sort() returned list of "
 218                                 "different length!\n");
 219                 goto exit;
 220         }
 221
 222         err = 0;
 223 exit:
 224         list_for_each_safe(cur, tmp, &head) {
 225                 list_del(cur);
 226                 kfree(container_of(cur, struct debug_el, list));
 227         }
 228         return err;
 229 }
 230 module_init(list_sort_test);
 231 #endif /* CONFIG_TEST_LIST_SORT */