1 files changed, 183 insertions, 69 deletions

diff --git a/lib/list_sort.c b/lib/list_sort.c
index 19d11e0..362c10f 100644
--- a/lib/list_sort.c
+++ b/lib/list_sort.c
@@ -4,99 +4,213 @@
 #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, tail);
+
+		tail->next->prev = tail;
+		tail = tail->next;
+	} while (tail->next);
+
+	tail->next = head;
+	head->prev = tail;
+}
+
 /**
  * list_sort - sort a list.
  * @priv: private data, passed to @cmp
  * @head: the list to sort
  * @cmp: the elements comparison function
  *
- * This function has been implemented by Mark J Roberts <mjr@znex.org>. It
- * implements "merge sort" which has O(nlog(n)) complexity. The list is sorted
- * in ascending order.
+ * This function implements "merge sort" which has O(nlog(n)) complexity.
+ * The list is sorted in ascending order.
  *
  * The comparison function @cmp is supposed to return a negative value if @a is
  * less than @b, and a positive value if @a is greater than @b. If @a and @b
  * are equivalent, then it does not matter what this function returns.
  */
 void list_sort(void *priv, struct list_head *head,
-	       int (*cmp)(void *priv, struct list_head *a,
-			  struct list_head *b))
+		int (*cmp)(void *priv, struct list_head *a,
+			struct list_head *b))
 {
-	struct list_head *p, *q, *e, *list, *tail, *oldhead;
-	int insize, nmerges, psize, qsize, i;
+	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;
-	list_del(head);
-	insize = 1;
-	for (;;) {
-		p = oldhead = list;
-		list = tail = NULL;
-		nmerges = 0;
-
-		while (p) {
-			nmerges++;
-			q = p;
-			psize = 0;
-			for (i = 0; i < insize; i++) {
-				psize++;
-				q = q->next == oldhead ? NULL : q->next;
-				if (!q)
-					break;
-			}
 
-			qsize = insize;
-			while (psize > 0 || (qsize > 0 && q)) {
-				if (!psize) {
-					e = q;
-					q = q->next;
-					qsize--;
-					if (q == oldhead)
-						q = NULL;
-				} else if (!qsize || !q) {
-					e = p;
-					p = p->next;
-					psize--;
-					if (p == oldhead)
-						p = NULL;
-				} else if (cmp(priv, p, q) <= 0) {
-					e = p;
-					p = p->next;
-					psize--;
-					if (p == oldhead)
-						p = NULL;
-				} else {
-					e = q;
-					q = q->next;
-					qsize--;
-					if (q == oldhead)
-						q = NULL;
-				}
-				if (tail)
-					tail->next = e;
-				else
-					list = e;
-				e->prev = tail;
-				tail = e;
+	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\n");
+				lev--;
 			}
-			p = q;
+			max_lev = lev;
 		}
+		part[lev] = cur;
+	}
 
-		tail->next = list;
-		list->prev = tail;
+	for (lev = 0; lev < max_lev; lev++)
+		if (part[lev])
+			list = merge(priv, cmp, part[lev], list);
 
-		if (nmerges <= 1)
-			break;
+	merge_and_restore_back_links(priv, cmp, head, part[max_lev], list);
+}
+EXPORT_SYMBOL(list_sort);
 
-		insize *= 2;
-	}
+#ifdef DEBUG_LIST_SORT
+struct debug_el {
+	struct list_head l_h;
+	int value;
+	unsigned serial;
+};
 
-	head->next = list;
-	head->prev = list->prev;
-	list->prev->next = head;
-	list->prev = head;
+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;
 }
 
-EXPORT_SYMBOL(list_sort);
+/*
+ * 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()\n");
+
+	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!\n");
+			return 1;
+		} else if (cmp_result > 0) {
+			printk(KERN_EMERG "list_sort() failed to sort!\n");
+			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!\n");
+			return 1;
+		}
+		kfree(cur->prev);
+		count++;
+	}
+	kfree(cur);
+	if (count != LIST_SORT_TEST_LENGTH) {
+		printk(KERN_EMERG "list_sort() returned list of"
+						"different length!\n");
+		return 1;
+	}
+	return 0;
+}
+module_init(list_sort_test);
+#endif