Commit 1b0fb5a5 authored Apr 30, 2017 by Mikulas Patocka Committed by Mike Snitzer May 01, 2017

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>

parent 86331f39

drivers/md/dm-bufio.c

+5 −4

Original line number	Diff line number	Diff line
		@@ -940,11 +940,12 @@ 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);
		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 >>
		(c->sectors_per_block_bits + SECTOR_SHIFT);

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