saner umount_tree()/release_mounts(), part 1

global list of release_mounts() fodder, protected by namespace_sem;
eventually, all umount_tree() callers will use it as kill list.
Helper picking the contents of that list, releasing namespace_sem
and doing release_mounts() on what it got.
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namespace.c

@@ -1119,6 +1119,8 @@ int may_umount(struct vfsmount *mnt)
 
 EXPORT_SYMBOL(may_umount);
 
+static LIST_HEAD(unmounted);	/* protected by namespace_sem */
+
 void release_mounts(struct list_head *head)
 {
 	struct mount *mnt;
@@ -1143,6 +1145,14 @@ void release_mounts(struct list_head *head)
 	}
 }
 
+static void namespace_unlock(void)
+{
+	LIST_HEAD(head);
+	list_splice_init(&unmounted, &head);
+	up_write(&namespace_sem);
+	release_mounts(&head);
+}
+
 /*
  * vfsmount lock must be held for write
  * namespace_sem must be held for write
@@ -1252,17 +1262,16 @@ static int do_umount(struct mount *mnt, int flags)
 	event++;
 
 	if (!(flags & MNT_DETACH))
-		shrink_submounts(mnt, &umount_list);
+		shrink_submounts(mnt, &unmounted);
 
 	retval = -EBUSY;
 	if (flags & MNT_DETACH || !propagate_mount_busy(mnt, 2)) {
 		if (!list_empty(&mnt->mnt_list))
-			umount_tree(mnt, 1, &umount_list);
+			umount_tree(mnt, 1, &unmounted);
 		retval = 0;
 	}
 	br_write_unlock(&vfsmount_lock);
-	up_write(&namespace_sem);
-	release_mounts(&umount_list);
+	namespace_unlock();
 	return retval;
 }