[Intel-gfx] [PATCH] drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES

[Chris Wilson]

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 at chris-wilson.co.uk>
Cc: stable at vger.kernel.org
---
 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 ad55b06a3cb1..4c14db9587a6 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -5024,7 +5024,7 @@ 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