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- // SPDX-License-Identifier: GPL-2.0
- #include <linux/kernel.h>
- #include <linux/bug.h>
- #include <linux/compiler.h>
- #include <linux/export.h>
- #include <linux/string.h>
- #include <linux/list_sort.h>
- #include <linux/list.h>
- #define MAX_LIST_LENGTH_BITS 20
- /*
- * Returns a list organized in an intermediate format suited
- * to chaining of merge() calls: null-terminated, no reserved or
- * sentinel head node, "prev" links not maintained.
- */
- static struct list_head *merge(void *priv,
- int (*cmp)(void *priv, struct list_head *a,
- struct list_head *b),
- struct list_head *a, struct list_head *b)
- {
- struct list_head head, *tail = &head;
- while (a && b) {
- /* if equal, take 'a' -- important for sort stability */
- if ((*cmp)(priv, a, b) <= 0) {
- tail->next = a;
- a = a->next;
- } else {
- tail->next = b;
- b = b->next;
- }
- tail = tail->next;
- }
- tail->next = a?:b;
- return head.next;
- }
- /*
- * Combine final list merge with restoration of standard doubly-linked
- * list structure. This approach duplicates code from merge(), but
- * runs faster than the tidier alternatives of either a separate final
- * prev-link restoration pass, or maintaining the prev links
- * throughout.
- */
- static void merge_and_restore_back_links(void *priv,
- int (*cmp)(void *priv, struct list_head *a,
- struct list_head *b),
- struct list_head *head,
- struct list_head *a, struct list_head *b)
- {
- struct list_head *tail = head;
- u8 count = 0;
- while (a && b) {
- /* if equal, take 'a' -- important for sort stability */
- if ((*cmp)(priv, a, b) <= 0) {
- tail->next = a;
- a->prev = tail;
- a = a->next;
- } else {
- tail->next = b;
- b->prev = tail;
- b = b->next;
- }
- tail = tail->next;
- }
- tail->next = a ? : b;
- do {
- /*
- * In worst cases this loop may run many iterations.
- * Continue callbacks to the client even though no
- * element comparison is needed, so the client's cmp()
- * routine can invoke cond_resched() periodically.
- */
- if (unlikely(!(++count)))
- (*cmp)(priv, tail->next, tail->next);
- tail->next->prev = tail;
- tail = tail->next;
- } while (tail->next);
- tail->next = head;
- head->prev = tail;
- }
- /**
- * list_sort - sort a list
- * @priv: private data, opaque to list_sort(), passed to @cmp
- * @head: the list to sort
- * @cmp: the elements comparison function
- *
- * This function implements "merge sort", which has O(nlog(n))
- * complexity.
- *
- * The comparison function @cmp must return a negative value if @a
- * should sort before @b, and a positive value if @a should sort after
- * @b. If @a and @b are equivalent, and their original relative
- * ordering is to be preserved, @cmp must return 0.
- */
- void list_sort(void *priv, struct list_head *head,
- int (*cmp)(void *priv, struct list_head *a,
- struct list_head *b))
- {
- struct list_head *part[MAX_LIST_LENGTH_BITS+1]; /* sorted partial lists
- -- last slot is a sentinel */
- int lev; /* index into part[] */
- int max_lev = 0;
- struct list_head *list;
- if (list_empty(head))
- return;
- memset(part, 0, sizeof(part));
- head->prev->next = NULL;
- list = head->next;
- while (list) {
- struct list_head *cur = list;
- list = list->next;
- cur->next = NULL;
- for (lev = 0; part[lev]; lev++) {
- cur = merge(priv, cmp, part[lev], cur);
- part[lev] = NULL;
- }
- if (lev > max_lev) {
- if (unlikely(lev >= ARRAY_SIZE(part)-1)) {
- printk_once(KERN_DEBUG "list too long for efficiency\n");
- lev--;
- }
- max_lev = lev;
- }
- part[lev] = cur;
- }
- for (lev = 0; lev < max_lev; lev++)
- if (part[lev])
- list = merge(priv, cmp, part[lev], list);
- merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);
- }
- EXPORT_SYMBOL(list_sort);
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