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>

2 files changed, 103 insertions, 1 deletions

diff --git a/lib/Makefile b/lib/Makefile
index 911b25aed1e7..3b0b4a696db9 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -21,7 +21,7 @@ lib-y	+= kobject.o kref.o klist.o
 
 obj-y += bcd.o div64.o sort.o parser.o halfmd4.o debug_locks.o random32.o \
          bust_spinlocks.o hexdump.o kasprintf.o bitmap.o scatterlist.o \
-         string_helpers.o gcd.o
+         string_helpers.o gcd.o list_sort.o
 
 ifeq ($(CONFIG_DEBUG_KOBJECT),y)
 CFLAGS_kobject.o += -DDEBUG
diff --git a/lib/list_sort.c b/lib/list_sort.c
new file mode 100644
index 000000000000..19d11e0bb958
--- /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);