dm bufio: avoid a possible ABBA deadlock

__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.

Cc: stable@vger.kernel.org
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index 280b56c..fd28146 100644 (file)
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -940,10 +940,11 @@ static void __get_memory_limit(struct dm_bufio_client *c,
 {
        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 >>