md: factor out a mddev_alloc_unit helper from mddev_find

Split out a self contained helper to find a free minor for the md
"unit" number.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>

drivers/md/md.c

@@ -745,6 +745,27 @@ static struct mddev *mddev_find_locked(dev_t unit)
 	return NULL;
 }
 
+/* find an unused unit number */
+static dev_t mddev_alloc_unit(void)
+{
+	static int next_minor = 512;
+	int start = next_minor;
+	bool is_free = 0;
+	dev_t dev = 0;
+
+	while (!is_free) {
+		dev = MKDEV(MD_MAJOR, next_minor);
+		next_minor++;
+		if (next_minor > MINORMASK)
+			next_minor = 0;
+		if (next_minor == start)
+			return 0;		/* Oh dear, all in use. */
+		is_free = !mddev_find_locked(dev);
+	}
+
+	return dev;
+}
+
 static struct mddev *mddev_find(dev_t unit)
 {
 	struct mddev *mddev;
@@ -787,27 +808,13 @@ static struct mddev *mddev_find_or_alloc(dev_t unit)
 			return new;
 		}
 	} else if (new) {
-		/* find an unused unit number */
-		static int next_minor = 512;
-		int start = next_minor;
-		int is_free = 0;
-		int dev = 0;
-		while (!is_free) {
-			dev = MKDEV(MD_MAJOR, next_minor);
-			next_minor++;
-			if (next_minor > MINORMASK)
-				next_minor = 0;
-			if (next_minor == start) {
-				/* Oh dear, all in use. */
+		new->unit = mddev_alloc_unit();
+		if (!new->unit) {
 			spin_unlock(&all_mddevs_lock);
 			kfree(new);
 			return NULL;
 		}
-
-			is_free = !mddev_find_locked(dev);
-		}
-		new->unit = dev;
-		new->md_minor = MINOR(dev);
+		new->md_minor = MINOR(new->unit);
 		new->hold_active = UNTIL_STOP;
 		list_add(&new->all_mddevs, &all_mddevs);
 		spin_unlock(&all_mddevs_lock);