[PATCH v2 1/3] drm: use the lookup lock in drm_is_current_master

[Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>]

Inside drm_is_current_master, using the outer drm_device.master_mutex
to protect reads of drm_file.master makes the function prone to creating
lock hierarchy inversions. Instead, we can use the
drm_file.master_lookup_lock that sits at the bottom of the lock
hierarchy.

Reported-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@xxxxxxxxx>
Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
---
 drivers/gpu/drm/drm_auth.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index f00354bec3fb..9c24b8cc8e36 100644
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -63,8 +63,9 @@
 
 static bool drm_is_current_master_locked(struct drm_file *fpriv)
 {
-	lockdep_assert_held_once(&fpriv->minor->dev->master_mutex);
-
+	/* Either drm_device.master_mutex or drm_file.master_lookup_lock
+	 * should be held here.
+	 */
 	return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
 }
 
@@ -82,9 +83,9 @@ bool drm_is_current_master(struct drm_file *fpriv)
 {
 	bool ret;
 
-	mutex_lock(&fpriv->minor->dev->master_mutex);
+	spin_lock(&fpriv->master_lookup_lock);
 	ret = drm_is_current_master_locked(fpriv);
-	mutex_unlock(&fpriv->minor->dev->master_mutex);
+	spin_unlock(&fpriv->master_lookup_lock);
 
 	return ret;
 }
-- 
2.25.1