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, tail);
  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, 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)) complexity.
  90  * The list is sorted in ascending order.
  91  *
  92  * The comparison function @cmp is supposed to return a negative value if @a is
  93  * less than @b, and a positive value if @a is greater than @b. If @a and @b
  94  * are equivalent, then it does not matter what this function returns.
  95  */
  96 void list_sort(void *priv, struct list_head *head,
  97                 int (*cmp)(void *priv, struct list_head *a,
  98                         struct list_head *b))
  99 {
 100         struct list_head *part[MAX_LIST_LENGTH_BITS+1]; /* sorted partial lists
 101                                                 -- last slot is a sentinel */
 102         int lev;  /* index into part[] */
 103         int max_lev = 0;
 104         struct list_head *list;
 105
 106         if (list_empty(head))
 107                 return;
 108
 109         memset(part, 0, sizeof(part));
 110
 111         head->prev->next = NULL;
 112         list = head->next;
 113
 114         while (list) {
 115                 struct list_head *cur = list;
 116                 list = list->next;
 117                 cur->next = NULL;
 118
 119                 for (lev = 0; part[lev]; lev++) {
 120                         cur = merge(priv, cmp, part[lev], cur);
 121                         part[lev] = NULL;
 122                 }
 123                 if (lev > max_lev) {
 124                         if (unlikely(lev >= ARRAY_SIZE(part)-1)) {
 125                                 printk_once(KERN_DEBUG "list passed to"
 126                                         " list_sort() too long for"
 127                                         " efficiency\n");
 128                                 lev--;
 129                         }
 130                         max_lev = lev;
 131                 }
 132                 part[lev] = cur;
 133         }
 134
 135         for (lev = 0; lev < max_lev; lev++)
 136                 if (part[lev])
 137                         list = merge(priv, cmp, part[lev], list);
 138
 139         merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);
 140 }
 141 EXPORT_SYMBOL(list_sort);
 142
 143 #ifdef DEBUG_LIST_SORT
 144 struct debug_el {
 145         struct list_head l_h;
 146         int value;
 147         unsigned serial;
 148 };
 149
 150 static int cmp(void *priv, struct list_head *a, struct list_head *b)
 151 {
 152         return container_of(a, struct debug_el, l_h)->value
 153              - container_of(b, struct debug_el, l_h)->value;
 154 }
 155
 156 /*
 157  * The pattern of set bits in the list length determines which cases
 158  * are hit in list_sort().
 159  */
 160 #define LIST_SORT_TEST_LENGTH (512+128+2) /* not including head */
 161
 162 static int __init list_sort_test(void)
 163 {
 164         int i, r = 1, count;
 165         struct list_head *head = kmalloc(sizeof(*head), GFP_KERNEL);
 166         struct list_head *cur;
 167
 168         printk(KERN_WARNING "testing list_sort()\n");
 169
 170         cur = head;
 171         for (i = 0; i < LIST_SORT_TEST_LENGTH; i++) {
 172                 struct debug_el *el = kmalloc(sizeof(*el), GFP_KERNEL);
 173                 BUG_ON(!el);
 174                  /* force some equivalencies */
 175                 el->value = (r = (r * 725861) % 6599) % (LIST_SORT_TEST_LENGTH/3);
 176                 el->serial = i;
 177
 178                 el->l_h.prev = cur;
 179                 cur->next = &el->l_h;
 180                 cur = cur->next;
 181         }
 182         head->prev = cur;
 183
 184         list_sort(NULL, head, cmp);
 185
 186         count = 1;
 187         for (cur = head->next; cur->next != head; cur = cur->next) {
 188                 struct debug_el *el = container_of(cur, struct debug_el, l_h);
 189                 int cmp_result = cmp(NULL, cur, cur->next);
 190                 if (cur->next->prev != cur) {
 191                         printk(KERN_EMERG "list_sort() returned "
 192                                                 "a corrupted list!\n");
 193                         return 1;
 194                 } else if (cmp_result > 0) {
 195                         printk(KERN_EMERG "list_sort() failed to sort!\n");
 196                         return 1;
 197                 } else if (cmp_result == 0 &&
 198                                 el->serial >= container_of(cur->next,
 199                                         struct debug_el, l_h)->serial) {
 200                         printk(KERN_EMERG "list_sort() failed to preserve order"
 201                                                  " of equivalent elements!\n");
 202                         return 1;
 203                 }
 204                 kfree(cur->prev);
 205                 count++;
 206         }
 207         kfree(cur);
 208         if (count != LIST_SORT_TEST_LENGTH) {
 209                 printk(KERN_EMERG "list_sort() returned list of"
 210                                                 "different length!\n");
 211                 return 1;
 212         }
 213         return 0;
 214 }
 215 module_init(list_sort_test);
 216 #endif