drm: use the lookup lock in drm_is_current_master

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@ffwll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210724111824.59266-2-desmondcheongzx@gmail.com

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 3a64a6a79adef3bd23cac50568d9c991a2ce16b5..5b1d92f5afea690d59b61282a114b722c871f4b2 100644 (file)
--- 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;
 }