btrfs: move root node locking helpers to locking.c

The helpers are related to locking so move them there, update comments.
Reviewed-by: NAnand Jain <anand.jain@oracle.com>
Reviewed-by: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -143,44 +143,6 @@ struct extent_buffer *btrfs_root_node(struct btrfs_root *root)
 	return eb;
 }
 
-/* loop around taking references on and locking the root node of the
- * tree until you end up with a lock on the root.  A locked buffer
- * is returned, with a reference held.
- */
-struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root)
-{
-	struct extent_buffer *eb;
-
-	while (1) {
-		eb = btrfs_root_node(root);
-		btrfs_tree_lock(eb);
-		if (eb == root->node)
-			break;
-		btrfs_tree_unlock(eb);
-		free_extent_buffer(eb);
-	}
-	return eb;
-}
-
-/* loop around taking references on and locking the root node of the
- * tree until you end up with a lock on the root.  A locked buffer
- * is returned, with a reference held.
- */
-struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
-{
-	struct extent_buffer *eb;
-
-	while (1) {
-		eb = btrfs_root_node(root);
-		btrfs_tree_read_lock(eb);
-		if (eb == root->node)
-			break;
-		btrfs_tree_read_unlock(eb);
-		free_extent_buffer(eb);
-	}
-	return eb;
-}
-
 /* cowonly root (everything not a reference counted cow subvolume), just get
  * put onto a simple dirty list.  transaction.c walks this to make sure they
  * get properly updated on disk.

fs/btrfs/locking.c

@@ -523,3 +523,45 @@ void btrfs_unlock_up_safe(struct btrfs_path *path, int level)
 		path->locks[i] = 0;
 	}
 }
+
+/*
+ * Loop around taking references on and locking the root node of the tree until
+ * we end up with a lock on the root node.
+ *
+ * Return: root extent buffer with write lock held
+ */
+struct extent_buffer *btrfs_lock_root_node(struct btrfs_root *root)
+{
+	struct extent_buffer *eb;
+
+	while (1) {
+		eb = btrfs_root_node(root);
+		btrfs_tree_lock(eb);
+		if (eb == root->node)
+			break;
+		btrfs_tree_unlock(eb);
+		free_extent_buffer(eb);
+	}
+	return eb;
+}
+
+/*
+ * Loop around taking references on and locking the root node of the tree until
+ * we end up with a lock on the root node.
+ *
+ * Return: root extent buffer with read lock held
+ */
+struct extent_buffer *btrfs_read_lock_root_node(struct btrfs_root *root)
+{
+	struct extent_buffer *eb;
+
+	while (1) {
+		eb = btrfs_root_node(root);
+		btrfs_tree_read_lock(eb);
+		if (eb == root->node)
+			break;
+		btrfs_tree_read_unlock(eb);
+		free_extent_buffer(eb);
+	}
+	return eb;
+}