drm/i915: Fix mutex->owner inspection race under DEBUG_MUTEXES

commit 226e5ae9 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=87955Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/gpu/drm/i915/i915_gem.c

@@ -4978,7 +4978,7 @@ 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 */