Patch "dm bufio: avoid a possible ABBA deadlock" has been added to the 4.4-stable tree

This is a note to let you know that I've just added the patch titled

dm bufio: avoid a possible ABBA deadlock

to the 4.4-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
dm-bufio-avoid-a-possible-abba-deadlock.patch
and it can be found in the queue-4.4 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Mikulas Patocka <[email protected]>
Date: Sun, 30 Apr 2017 17:33:26 -0400
Subject: dm bufio: avoid a possible ABBA deadlock

From: Mikulas Patocka <[email protected]>

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.

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

@@ -914,10 +914,11 @@ 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 >>

Patches currently in stable-queue which might be from [email protected] are

queue-4.4/dm-bufio-avoid-a-possible-abba-deadlock.patch queue-4.4/dm-cache-metadata-fail-operations-if-fail_io-mode-has-been-established.patch queue-4.4/dm-raid-select-the-kconfig-option-config_md_raid0.patch queue-4.4/dm-bufio-make-the-parameter-retain_bytes-unsigned-long.patch queue-4.4/dm-bufio-check-new-buffer-allocation-watermark-every-30-seconds.patch