[PATCH v2] locks: ignore same lock in blocked_lock_hash

[Jeff Layton <jlayton@kernel.org>]

Andreas reported that he was seeing the tdbtorture test fail in
some cases with -EDEADLCK when it wasn't before. Some debugging
showed that deadlock detection was sometimes discovering the
caller's lock request itself in a dependency chain.

If posix_locks_deadlock() fails to find a deadlock, the caller_fl
will be passed to __locks_insert_block(), and this wakes up all
locks that are blocked on caller_fl, clearing the fl_blocker link.

So if posix_locks_deadlock() finds caller_fl while searching for
a deadlock, it can be sure that link in the cycle is about to be
broken and it need not treat it as the cause of a deadlock.

URL: https://bugzilla.kernel.org/show_bug.cgi?id=202975
Fixes: 5946c4319ebb ("fs/locks: allow a lock request to block other requests.")
Cc: stable@vger.kernel.org
Reported-by: Andreas Schneider <asn@redhat.com>
Signed-off-by: Neil Brown <neilb@suse.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
---
 fs/locks.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/fs/locks.c b/fs/locks.c
index eaa1cfaf73b0..a939a274dc71 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -1023,6 +1023,19 @@ static int posix_locks_deadlock(struct file_lock *caller_fl,
 	while ((block_fl = what_owner_is_waiting_for(block_fl))) {
 		if (i++ > MAX_DEADLK_ITERATIONS)
 			return 0;
+
+		/*
+		 * It's possible that we're retrying this lock request after
+		 * another task is has blocked on it. A lock request can't
+		 * block itself, and any locks that are blocked on it will
+		 * also be awoken soon (and have their fl_blocker pointer
+		 * cleared). Any dependency chain that contains the request
+		 * itself is therefore about to be broken, so we can safely
+		 * ignore it.
+		 */
+		if (block_fl == caller_fl)
+			return 0;
+
 		if (posix_same_owner(caller_fl, block_fl))
 			return 1;
 	}
-- 
2.20.1