io-wq: serialize hash clear with wakeup

commit d3e3c102

 upstream.

We need to ensure that we serialize the stalled and hash bits with the
wait_queue wait handler, or we could be racing with someone modifying
the hashed state after we find it busy, but before we then give up and
wait for it to be cleared. This can cause random delays or stalls when
handling buffered writes for many files, where some of these files cause
hash collisions between the worker threads.

Cc: stable@vger.kernel.org
Reported-by: Daniel Black <daniel@mariadb.org>
Fixes: e941894e

 ("io-wq: make buffered file write hashed work map per-ctx")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>