lib/list_sort.c

   1 #include <linux/kernel.h>
   2 #include <linux/bug.h>
   3 #include <linux/compiler.h>
   4 #include <linux/export.h>
   5 #include <linux/string.h>
   6 #include <linux/list_sort.h>
   7 #include <linux/list.h>
   8
   9 #define MAX_LIST_LENGTH_BITS 20
  10
  11 /*
  12  * Returns a list organized in an intermediate format suited
  13  * to chaining of merge() calls: null-terminated, no reserved or
  14  * sentinel head node, "prev" links not maintained.
  15  */
  16 static struct list_head *merge(void *priv,
  17                                 int (*cmp)(void *priv, struct list_head *a,
  18                                         struct list_head *b),
  19                                 struct list_head *a, struct list_head *b)
  20 {
  21         struct list_head head, *tail = &head;
  22
  23         while (a && b) {
  24                 /* if equal, take 'a' -- important for sort stability */
  25                 if ((*cmp)(priv, a, b) <= 0) {
  26                         tail->next = a;
  27                         a = a->next;
  28                 } else {
  29                         tail->next = b;
  30                         b = b->next;
  31                 }
  32                 tail = tail->next;
  33         }
  34         tail->next = a?:b;
  35         return head.next;
  36 }
  37
  38 /*
  39  * Combine final list merge with restoration of standard doubly-linked
  40  * list structure.  This approach duplicates code from merge(), but
  41  * runs faster than the tidier alternatives of either a separate final
  42  * prev-link restoration pass, or maintaining the prev links
  43  * throughout.
  44  */
  45 static void merge_and_restore_back_links(void *priv,
  46                                 int (*cmp)(void *priv, struct list_head *a,
  47                                         struct list_head *b),
  48                                 struct list_head *head,
  49                                 struct list_head *a, struct list_head *b)
  50 {
  51         struct list_head *tail = head;
  52         u8 count = 0;
  53
  54         while (a && b) {
  55                 /* if equal, take 'a' -- important for sort stability */
  56                 if ((*cmp)(priv, a, b) <= 0) {
  57                         tail->next = a;
  58                         a->prev = tail;
  59                         a = a->next;
  60                 } else {
  61                         tail->next = b;
  62                         b->prev = tail;
  63                         b = b->next;
  64                 }
  65                 tail = tail->next;
  66         }
  67         tail->next = a ? : b;
  68
  69         do {
  70                 /*
  71                  * In worst cases this loop may run many iterations.
  72                  * Continue callbacks to the client even though no
  73                  * element comparison is needed, so the client's cmp()
  74                  * routine can invoke cond_resched() periodically.
  75                  */
  76                 if (unlikely(!(++count)))
  77                         (*cmp)(priv, tail->next, tail->next);
  78
  79                 tail->next->prev = tail;
  80                 tail = tail->next;
  81         } while (tail->next);
  82
  83         tail->next = head;
  84         head->prev = tail;
  85 }
  86
  87 /**
  88  * list_sort - sort a list
  89  * @priv: private data, opaque to list_sort(), passed to @cmp
  90  * @head: the list to sort
  91  * @cmp: the elements comparison function
  92  *
  93  * This function implements "merge sort", which has O(nlog(n))
  94  * complexity.
  95  *
  96  * The comparison function @cmp must return a negative value if @a
  97  * should sort before @b, and a positive value if @a should sort after
  98  * @b. If @a and @b are equivalent, and their original relative
  99  * ordering is to be preserved, @cmp must return 0.
 100  */
 101 void list_sort(void *priv, struct list_head *head,
 102                 int (*cmp)(void *priv, struct list_head *a,
 103                         struct list_head *b))
 104 {
 105         struct list_head *part[MAX_LIST_LENGTH_BITS+1]; /* sorted partial lists
 106                                                 -- last slot is a sentinel */
 107         int lev;  /* index into part[] */
 108         int max_lev = 0;
 109         struct list_head *list;
 110
 111         if (list_empty(head))
 112                 return;
 113
 114         memset(part, 0, sizeof(part));
 115
 116         head->prev->next = NULL;
 117         list = head->next;
 118
 119         while (list) {
 120                 struct list_head *cur = list;
 121                 list = list->next;
 122                 cur->next = NULL;
 123
 124                 for (lev = 0; part[lev]; lev++) {
 125                         cur = merge(priv, cmp, part[lev], cur);
 126                         part[lev] = NULL;
 127                 }
 128                 if (lev > max_lev) {
 129                         if (unlikely(lev >= ARRAY_SIZE(part)-1)) {
 130                                 printk_once(KERN_DEBUG "list too long for efficiency\n");
 131                                 lev--;
 132                         }
 133                         max_lev = lev;
 134                 }
 135                 part[lev] = cur;
 136         }
 137
 138         for (lev = 0; lev < max_lev; lev++)
 139                 if (part[lev])
 140                         list = merge(priv, cmp, part[lev], list);
 141
 142         merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);
 143 }
 144 EXPORT_SYMBOL(list_sort);