#define pr_fmt(fmt) "list_sort_test: " fmt

  #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
  ");

  				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);

  #ifdef CONFIG_TEST_LIST_SORT

  #include <linux/slab.h>

  #include <linux/random.h>

  /*
   * The pattern of set bits in the list length determines which cases
   * are hit in list_sort().
   */
  #define TEST_LIST_LEN (512+128+2) /* not including head */
  
  #define TEST_POISON1 0xDEADBEEF
  #define TEST_POISON2 0xA324354C

  struct debug_el {

  	unsigned int poison1;

  	struct list_head list;

  	unsigned int poison2;

  	int value;
  	unsigned serial;
  };

  /* Array, containing pointers to all elements in the test list */
  static struct debug_el **elts __initdata;
  
  static int __init check(struct debug_el *ela, struct debug_el *elb)

  {

  	if (ela->serial >= TEST_LIST_LEN) {

  		pr_err("error: incorrect serial %d
  ", ela->serial);

  		return -EINVAL;
  	}
  	if (elb->serial >= TEST_LIST_LEN) {

  		pr_err("error: incorrect serial %d
  ", elb->serial);

  		return -EINVAL;
  	}
  	if (elts[ela->serial] != ela || elts[elb->serial] != elb) {

  		pr_err("error: phantom element
  ");

  		return -EINVAL;
  	}
  	if (ela->poison1 != TEST_POISON1 || ela->poison2 != TEST_POISON2) {

  		pr_err("error: bad poison: %#x/%#x
  ",
  			ela->poison1, ela->poison2);

  		return -EINVAL;
  	}
  	if (elb->poison1 != TEST_POISON1 || elb->poison2 != TEST_POISON2) {

  		pr_err("error: bad poison: %#x/%#x
  ",
  			elb->poison1, elb->poison2);

  		return -EINVAL;
  	}
  	return 0;

  }

  static int __init cmp(void *priv, struct list_head *a, struct list_head *b)
  {
  	struct debug_el *ela, *elb;
  
  	ela = container_of(a, struct debug_el, list);
  	elb = container_of(b, struct debug_el, list);
  
  	check(ela, elb);
  	return ela->value - elb->value;
  }

  
  static int __init list_sort_test(void)
  {

  	int i, count = 1, err = -ENOMEM;

  	struct debug_el *el;

  	struct list_head *cur;

  	LIST_HEAD(head);

  	pr_debug("start testing list_sort()
  ");

  	elts = kcalloc(TEST_LIST_LEN, sizeof(*elts), GFP_KERNEL);

  	if (!elts) {

  		pr_err("error: cannot allocate memory
  ");

  		return err;

  	}

  	for (i = 0; i < TEST_LIST_LEN; i++) {

  		el = kmalloc(sizeof(*el), GFP_KERNEL);
  		if (!el) {

  			pr_err("error: cannot allocate memory
  ");

  			goto exit;
  		}

  		 /* force some equivalencies */

  		el->value = prandom_u32() % (TEST_LIST_LEN / 3);

  		el->serial = i;

  		el->poison1 = TEST_POISON1;
  		el->poison2 = TEST_POISON2;
  		elts[i] = el;

  		list_add_tail(&el->list, &head);

  	}

  	list_sort(NULL, &head, cmp);

  	err = -EINVAL;

  	for (cur = head.next; cur->next != &head; cur = cur->next) {
  		struct debug_el *el1;
  		int cmp_result;

  		if (cur->next->prev != cur) {

  			pr_err("error: list is corrupted
  ");

  			goto exit;
  		}
  
  		cmp_result = cmp(NULL, cur, cur->next);
  		if (cmp_result > 0) {

  			pr_err("error: list is not sorted
  ");

  			goto exit;
  		}
  
  		el = container_of(cur, struct debug_el, list);
  		el1 = container_of(cur->next, struct debug_el, list);
  		if (cmp_result == 0 && el->serial >= el1->serial) {

  			pr_err("error: order of equivalent elements not "
  				"preserved
  ");

  			goto exit;

  		}

  
  		if (check(el, el1)) {

  			pr_err("error: element check failed
  ");

  			goto exit;
  		}

  		count++;
  	}

  	if (head.prev != cur) {

  		pr_err("error: list is corrupted
  ");

  		goto exit;
  	}

  	if (count != TEST_LIST_LEN) {

  		pr_err("error: bad list length %d", count);

  		goto exit;
  	}
  
  	err = 0;
  exit:

  	for (i = 0; i < TEST_LIST_LEN; i++)
  		kfree(elts[i]);

  	kfree(elts);

  	return err;

  }

  late_initcall(list_sort_test);

  #endif /* CONFIG_TEST_LIST_SORT */