fix the softlockups in attach_recursive_mnt()

In case when we mounting something on top of a large stack of overmounts,
all of them being peers of each other, we get quadratic time by the
depth of overmount stack.  Easily fixed by doing commit_tree() before
reparenting the overmount; simplifies commit_tree() as well - it doesn't
need to skip the already mounted stuff that had been reparented on top
of the new mounts.

Since we are holding mount_lock through both reparenting and call of
commit_tree(), the order does not matter from the mount hash point
of view.

Reported-by: "Lai, Yi" <yi1.lai@linux.intel.com>
Tested-by: "Lai, Yi" <yi1.lai@linux.intel.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Fixes: 663206854f02 "copy_tree(): don't link the mounts via mnt_list"
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

diff --git a/fs/namespace.c b/fs/namespace.c
index ddfd4457d338372a6ee88f2fc54075eab25be3a8..1c97f93d18655e9614ee713790eca2f0d9c9b6fc 100644 (file)
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1197,10 +1197,7 @@ static void commit_tree(struct mount *mnt)
 
        if (!mnt_ns_attached(mnt)) {
                for (struct mount *m = mnt; m; m = next_mnt(m, mnt))
-                       if (unlikely(mnt_ns_attached(m)))
-                               m = skip_mnt_tree(m);
-                       else
-                               mnt_add_to_ns(n, m);
+                       mnt_add_to_ns(n, m);
                n->nr_mounts += n->pending_mounts;
                n->pending_mounts = 0;
        }
@@ -2704,6 +2701,7 @@ static int attach_recursive_mnt(struct mount *source_mnt,
                        lock_mnt_tree(child);
                q = __lookup_mnt(&child->mnt_parent->mnt,
                                 child->mnt_mountpoint);
+               commit_tree(child);
                if (q) {
                        struct mountpoint *mp = root.mp;
                        struct mount *r = child;
@@ -2713,7 +2711,6 @@ static int attach_recursive_mnt(struct mount *source_mnt,
                                mp = shorter;
                        mnt_change_mountpoint(r, mp, q);
                }
-               commit_tree(child);
        }
        unpin_mountpoint(&root);
        unlock_mount_hash();