linux-kernel - [PATCH 4.4 012/103] dm bufio: avoid a possible ABBA deadlock

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Message-Id: <20170523200858.382600912@linuxfoundation.org>
Date:   Tue, 23 May 2017 22:08:38 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Mikulas Patocka <mpatocka@...hat.com>,
        Mike Snitzer <snitzer@...hat.com>
Subject: [PATCH 4.4 012/103] dm bufio: avoid a possible ABBA deadlock

4.4-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Mikulas Patocka <mpatocka@...hat.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@...hat.com>
Signed-off-by: Mike Snitzer <snitzer@...hat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

---
 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
@@ -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 >>