[PATCH 3.16 101/134] dm bufio: avoid a possible ABBA deadlock

Ben Hutchings
2017-08-18T13:15:50Z

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: Ben Hutchings <ben@decadent.org.uk>
---
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
@@ -873,10 +873,11 @@ 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 >>