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    Subject[PATCH 4.9 024/164] dm bufio: avoid a possible ABBA deadlock
    Date
    4.9-stable review patch.  If anyone has any objections, please let me know.

    ------------------

    From: Mikulas Patocka <mpatocka@redhat.com>

    commit 1b0fb5a5b2dc0dddcfa575060441a7176ba7ac37 upstream.

    __get_memory_limit() tests if dm_bufio_cache_size changed and calls
    __cache_size_refresh() if it did. It takes dm_bufio_clients_lock while
    it already holds the client lock. However, lock ordering is violated
    because in cleanup_old_buffers() dm_bufio_clients_lock is taken before
    the client lock.

    This results in a possible deadlock and lockdep engine warning.

    Fix this deadlock by changing mutex_lock() to mutex_trylock(). If the
    lock can't be taken, it will be re-checked next time when a new buffer
    is allocated.

    Also add "unlikely" to the if condition, so that the optimizer assumes
    that the condition is false.

    Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
    Signed-off-by: Mike Snitzer <snitzer@redhat.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    drivers/md/dm-bufio.c | 9 +++++----
    1 file changed, 5 insertions(+), 4 deletions(-)

    --- a/drivers/md/dm-bufio.c
    +++ b/drivers/md/dm-bufio.c
    @@ -923,10 +923,11 @@ static void __get_memory_limit(struct dm
    {
    unsigned long buffers;

    - if (ACCESS_ONCE(dm_bufio_cache_size) != dm_bufio_cache_size_latch) {
    - mutex_lock(&dm_bufio_clients_lock);
    - __cache_size_refresh();
    - mutex_unlock(&dm_bufio_clients_lock);
    + if (unlikely(ACCESS_ONCE(dm_bufio_cache_size) != dm_bufio_cache_size_latch)) {
    + if (mutex_trylock(&dm_bufio_clients_lock)) {
    + __cache_size_refresh();
    + mutex_unlock(&dm_bufio_clients_lock);
    + }
    }

    buffers = dm_bufio_cache_size_per_client >>

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