lib: Introduce generic list_sort function

There are two copies of list_sort() in the tree already, one in the DRM
code, another in ubifs.  Now XFS needs this as well.  Create a generic
list_sort() function from the ubifs version and convert existing users
to it so we don't end up with yet another copy in the tree.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Acked-by: Dave Airlie <airlied@redhat.com>
Acked-by: Artem Bityutskiy <dedekind@infradead.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 102 insertions, 0 deletions

diff --git a/lib/list_sort.c b/lib/list_sort.c
new file mode 100644
index 00000000000..19d11e0bb95
--- /dev/null
+++ b/lib/list_sort.c
@@ -0,0 +1,102 @@
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/list_sort.h>
+#include <linux/slab.h>
+#include <linux/list.h>
+
+/**
+ * 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.
+ *
+ * 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))
+{
+	struct list_head *p, *q, *e, *list, *tail, *oldhead;
+	int insize, nmerges, psize, qsize, i;
+
+	if (list_empty(head))
+		return;
+
+	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;
+			}
+			p = q;
+		}
+
+		tail->next = list;
+		list->prev = tail;
+
+		if (nmerges <= 1)
+			break;
+
+		insize *= 2;
+	}
+
+	head->next = list;
+	head->prev = list->prev;
+	list->prev->next = head;
+	list->prev = head;
+}
+
+EXPORT_SYMBOL(list_sort);