linux-kernel - [PATCH] mutex: Always clear owner field upon mutex

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Message-Id: <1420540175-30204-1-git-send-email-chris@chris-wilson.co.uk>
Date:	Tue,  6 Jan 2015 10:29:35 +0000
From:	Chris Wilson <chris@...is-wilson.co.uk>
To:	linux-kernel@...r.kernel.org
Cc:	Chris Wilson <chris@...is-wilson.co.uk>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>
Subject: [PATCH] mutex: Always clear owner field upon mutex_unlock()

Currently if DEBUG_MUTEXES is enabled, the mutex->owner field is only
cleared iff debug_locks is active. This exposes a race to other users of
the field where the mutex->owner may be still set to a stale value,
potentially upsetting mutex_spin_on_owner() among others.

References: https://bugs.freedesktop.org/show_bug.cgi?id=87955
Signed-off-by: Chris Wilson <chris@...is-wilson.co.uk>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Daniel Vetter <daniel.vetter@...ll.ch>
---
 kernel/locking/mutex-debug.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
index 5cf6731b98e9..3ef3736002d8 100644
--- a/kernel/locking/mutex-debug.c
+++ b/kernel/locking/mutex-debug.c
@@ -80,13 +80,13 @@ void debug_mutex_unlock(struct mutex *lock)
 			DEBUG_LOCKS_WARN_ON(lock->owner != current);
 
 		DEBUG_LOCKS_WARN_ON(!lock->wait_list.prev && !lock->wait_list.next);
-		mutex_clear_owner(lock);
 	}
 
 	/*
 	 * __mutex_slowpath_needs_to_unlock() is explicitly 0 for debug
 	 * mutexes so that we can do it here after we've verified state.
 	 */
+	mutex_clear_owner(lock);
 	atomic_set(&lock->count, 1);
 }
 
-- 
2.1.4

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