lib/list_sort: simplify and remove MAX_LIST_LENGTH_BITS

struct list_head;

__attribute__((nonnull(2,3)))

void list_sort(void *priv, struct list_head *head,

	       int (*cmp)(void *priv, struct list_head *a,

			  struct list_head *b));

#include <linux/list_sort.h>

#include <linux/list.h>

#define MAX_LIST_LENGTH_BITS 20

typedef int __attribute__((nonnull(2,3))) (*cmp_func)(void *,

		struct list_head const *, struct list_head const *);

/*

 * 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),

__attribute__((nonnull(2,3,4)))

static struct list_head *merge(void *priv, cmp_func cmp,

				struct list_head *a, struct list_head *b)

{

	struct list_head head, *tail = &head;

	struct list_head *head, **tail = &head;

	while (a && b) {

	for (;;) {

		/* if equal, take 'a' -- important for sort stability */

		if ((*cmp)(priv, a, b) <= 0) {

			tail->next = a;

		if (cmp(priv, a, b) <= 0) {

			*tail = a;

			tail = &a->next;

			a = a->next;

			if (!a) {

				*tail = b;

				break;

			}

		} else {

			tail->next = b;

			*tail = b;

			tail = &b->next;

			b = b->next;

			if (!b) {

				*tail = a;

				break;

			}

		}

		tail = tail->next;

	}

	tail->next = a?:b;

	return head.next;

	return head;

}

/*

 * 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,

__attribute__((nonnull(2,3,4,5)))

static void merge_final(void *priv, cmp_func cmp, struct list_head *head,

			struct list_head *a, struct list_head *b)

{

	struct list_head *tail = head;

	u8 count = 0;

	while (a && b) {

	for (;;) {

		/* if equal, take 'a' -- important for sort stability */

		if ((*cmp)(priv, a, b) <= 0) {

		if (cmp(priv, a, b) <= 0) {

			tail->next = a;

			a->prev = tail;

			tail = a;

			a = a->next;

			if (!a)

				break;

		} else {

			tail->next = b;

			b->prev = tail;

			tail = b;

			b = b->next;

			if (!b) {

				b = a;

				break;

			}

		}

		tail = tail->next;

	}

	tail->next = a ? : b;

	/* Finish linking remainder of list b on to tail */

	tail->next = b;

	do {

		/*

		 * In worst cases this loop may run many iterations.

		 * If the merge is highly unbalanced (e.g. the input is

		 * already sorted), 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);

		if (unlikely(!++count))

			cmp(priv, b, b);

		b->prev = tail;

		tail = b;

		b = b->next;

	} while (b);

	/* And the final links to make a circular doubly-linked list */

	tail->next = head;

	head->prev = tail;

}

 * @head: the list to sort

 * @cmp: the elements comparison function

 *

 * This function implements "merge sort", which has O(nlog(n))

 * complexity.

 * This function implements a bottom-up merge sort, which has O(nlog(n))

 * complexity.  We use depth-first order to take advantage of cacheing.

 * (E.g. when we get to the fourth element, we immediately merge the

 * first two 2-element lists.)

 *

 * 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.

 * The comparison funtion @cmp must return > 0 if @a should sort after

 * @b ("@a > @b" if you want an ascending sort), and <= 0 if @a should

 * sort before @b *or* their original order should be preserved.  It is

 * always called with the element that came first in the input in @a,

 * and list_sort is a stable sort, so it is not necessary to distinguish

 * the @a < @b and @a == @b cases.

 *

 * This is compatible with two styles of @cmp function:

 * - The traditional style which returns <0 / =0 / >0, or

 * - Returning a boolean 0/1.

 * The latter offers a chance to save a few cycles in the comparison

 * (which is used by e.g. plug_ctx_cmp() in block/blk-mq.c).

 *

 * A good way to write a multi-word comparison is

 *	if (a->high != b->high)

 *		return a->high > b->high;

 *	if (a->middle != b->middle)

 *		return a->middle > b->middle;

 *	return a->low > b->low;

 */

__attribute__((nonnull(2,3)))

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;

	struct list_head *list = head->next, *pending = NULL;

	size_t count = 0;	/* Count of pending */

	if (list_empty(head))

	if (list == head->prev)	/* Zero or one elements */

		return;

	memset(part, 0, sizeof(part));

	/* Convert to a null-terminated singly-linked list. */

	head->prev->next = NULL;

	list = head->next;

	while (list) {

	/*

	 * Data structure invariants:

	 * - All lists are singly linked and null-terminated; prev

	 *   pointers are not maintained.

	 * - pending is a prev-linked "list of lists" of sorted

	 *   sublists awaiting further merging.

	 * - Each of the sorted sublists is power-of-two in size,

	 *   corresponding to bits set in "count".

	 * - Sublists are sorted by size and age, smallest & newest at front.

	 */

	do {

		size_t bits;

		struct list_head *cur = list;

		/* Extract the head of "list" as a single-element list "cur" */

		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;

		/* Do merges corresponding to set lsbits in count */

		for (bits = count; bits & 1; bits >>= 1) {

			cur = merge(priv, (cmp_func)cmp, pending, cur);

			pending = pending->prev;  /* Untouched by merge() */

		}

		part[lev] = cur;

		/* And place the result at the head of "pending" */

		cur->prev = pending;

		pending = cur;

		count++;

	} while (list->next);

	/* Now merge together last element with all pending lists */

	while (pending->prev) {

		list = merge(priv, (cmp_func)cmp, pending, list);

		pending = pending->prev;

	}

	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);

	/* The final merge, rebuilding prev links */

	merge_final(priv, (cmp_func)cmp, head, pending, list);

}

EXPORT_SYMBOL(list_sort);