lib/list_sort.c

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