attach_recursive_mnt(): do not lock the covering tree when sliding something under it

If we are propagating across the userns boundary, we need to lock the
mounts added there.  However, in case when something has already
been mounted there and we end up sliding a new tree under that,
the stuff that had been there before should not get locked.

IOW, lock_mnt_tree() should be called before we reparent the
preexisting tree on top of what we are adding.

Fixes: 3bd045cc9c4b ("separate copying and locking mount tree on cross-userns copies")
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namespace.c b/fs/namespace.c
index 14601ec4c2c5bbc8773d5d67adc5e5fdc7dd1200..eed83254492f3097b7835c89a9941adf3b74b1c2 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2768,14 +2768,14 @@ static int attach_recursive_mnt(struct mount *source_mnt,
        hlist_for_each_entry_safe(child, n, &tree_list, mnt_hash) {
                struct mount *q;
                hlist_del_init(&child->mnt_hash);
-               q = __lookup_mnt(&child->mnt_parent->mnt,
-                                child->mnt_mountpoint);
-               if (q)
-                       mnt_change_mountpoint(child, smp, q);
                /* Notice when we are propagating across user namespaces */
                if (child->mnt_parent->mnt_ns->user_ns != user_ns)
                        lock_mnt_tree(child);
                child->mnt.mnt_flags &= ~MNT_LOCKED;
+               q = __lookup_mnt(&child->mnt_parent->mnt,
+                                child->mnt_mountpoint);
+               if (q)
+                       mnt_change_mountpoint(child, smp, q);
                commit_tree(child);
        }
        put_mountpoint(smp);