[PATCH 3.16.y-ckt 013/135] drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES

[Luis Henriques]

3.16.7-ckt6 -stable review patch.  If anyone has any objections, please let me know.

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

From: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>

commit 226e5ae9e5f9108beb0bde4ac69f68fe6210fed9 upstream.

If CONFIG_DEBUG_MUTEXES is set, the mutex->owner field is only cleared
if the mutex debugging is enabled which introduces a race in our
mutex_is_locked_by() - i.e. we may inspect the old owner value before it
is acquired by the new task.

This is the root cause of this error:

 diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
 index 5cf6731..3ef3736 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);
  }

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=87955
Signed-off-by: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Signed-off-by: Jani Nikula <jani.nikula@xxxxxxxxx>
Signed-off-by: Luis Henriques <luis.henriques@xxxxxxxxxxxxx>
---
 drivers/gpu/drm/i915/i915_gem.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index ef3b4798da02..b34571c52ea1 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -4990,7 +4990,7 @@ static bool mutex_is_locked_by(struct mutex *mutex, struct task_struct *task)
 	if (!mutex_is_locked(mutex))
 		return false;
 
-#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_MUTEXES)
+#if defined(CONFIG_SMP) && !defined(CONFIG_DEBUG_MUTEXES)
 	return mutex->owner == task;
 #else
 	/* Since UP may be pre-empted, we cannot assume that we own the lock */
-- 
2.1.4

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