dm bufio: avoid a possible ABBA deadlock

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>

1 files changed, 5 insertions, 4 deletions

diff --git a/drivers/md/dm-bufio.c b/drivers/md/dm-bufio.c
index 125aedc3875f..d0745965f8ec 100644
--- a/drivers/md/dm-bufio.c
+++ b/drivers/md/dm-bufio.c
@@ -923,10 +923,11 @@ 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 >>