#include <linux/kernel.h>
  #include <linux/module.h>
  #include <linux/list_sort.h>
  #include <linux/slab.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;
  
  	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.
  		 */

  		(*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 passed to"
  					" list_sort() 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 DEBUG_LIST_SORT
  struct debug_el {
  	struct list_head l_h;
  	int value;
  	unsigned serial;
  };

  static int cmp(void *priv, struct list_head *a, struct list_head *b)
  {
  	return container_of(a, struct debug_el, l_h)->value
  	     - container_of(b, struct debug_el, l_h)->value;

  }

  /*
   * The pattern of set bits in the list length determines which cases
   * are hit in list_sort().
   */
  #define LIST_SORT_TEST_LENGTH (512+128+2) /* not including head */
  
  static int __init list_sort_test(void)
  {
  	int i, r = 1, count;
  	struct list_head *head = kmalloc(sizeof(*head), GFP_KERNEL);
  	struct list_head *cur;
  
  	printk(KERN_WARNING "testing list_sort()
  ");
  
  	cur = head;
  	for (i = 0; i < LIST_SORT_TEST_LENGTH; i++) {
  		struct debug_el *el = kmalloc(sizeof(*el), GFP_KERNEL);
  		BUG_ON(!el);
  		 /* force some equivalencies */
  		el->value = (r = (r * 725861) % 6599) % (LIST_SORT_TEST_LENGTH/3);
  		el->serial = i;
  
  		el->l_h.prev = cur;
  		cur->next = &el->l_h;
  		cur = cur->next;
  	}
  	head->prev = cur;
  
  	list_sort(NULL, head, cmp);
  
  	count = 1;
  	for (cur = head->next; cur->next != head; cur = cur->next) {
  		struct debug_el *el = container_of(cur, struct debug_el, l_h);
  		int cmp_result = cmp(NULL, cur, cur->next);
  		if (cur->next->prev != cur) {
  			printk(KERN_EMERG "list_sort() returned "
  						"a corrupted list!
  ");
  			return 1;
  		} else if (cmp_result > 0) {
  			printk(KERN_EMERG "list_sort() failed to sort!
  ");
  			return 1;
  		} else if (cmp_result == 0 &&
  				el->serial >= container_of(cur->next,
  					struct debug_el, l_h)->serial) {
  			printk(KERN_EMERG "list_sort() failed to preserve order"
  						 " of equivalent elements!
  ");
  			return 1;
  		}
  		kfree(cur->prev);
  		count++;
  	}
  	kfree(cur);
  	if (count != LIST_SORT_TEST_LENGTH) {
  		printk(KERN_EMERG "list_sort() returned list of"
  						"different length!
  ");
  		return 1;
  	}
  	return 0;
  }
  module_init(list_sort_test);
  #endif