ovl: do not take a name snapshot in RCU walk

A path lookup on an overlay over FUSE loops in the kernel for ever and
stalls RCU.

take_dentry_name_snapshot() needs a dentry that the caller keeps alive,
because for a dead dentry the refcount of an external name stays at
zero and the function retries without end. In RCU walk
ovl_revalidate_real() holds no reference on the real dentry, and the
eviction of the overlay inode on another CPU can drop the last one.

With a delay in ovl_revalidate_real(), a stat() loop on a 48 byte name
in an overlay over FUSE, raced with drop_caches, spun for ever in
take_dentry_name_snapshot() and stalled RCU.

Fix this by using try_take_dentry_name_snapshot() in RCU walk, and by
returning -ECHILD when it fails.

Fixes: 5be1fa8abd7b ("Pass parent directory inode and expected name to ->d_revalidate()")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
[amir: use try_take_dentry_name_snapshot() instead of open coded snapshot]
Signed-off-by: Amir Goldstein <amir73il@gmail.com>

0 files changed, 0 insertions, 0 deletions