drm: add lockdep assert to drm_is_current_master_locked

In drm_is_current_master_locked, accessing drm_file.master should be
protected by either drm_file.master_lookup_lock or
drm_device.master_mutex. This was previously awkward to assert with
lockdep.

Following patch ("locking/lockdep: Provide lockdep_assert{,_once}()
helpers"), this assertion is now convenient. So we add in the
assertion and explain this lock design in the kerneldoc.

Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Waiman Long <longman@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210802105957.77692-3-desmondcheongzx@gmail.com

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 53e58bb..60a6b21 100644 (file)
--- a/drivers/gpu/drm/drm_auth.c
+++ b/drivers/gpu/drm/drm_auth.c
@@ -63,9 +63,9 @@
 
 static bool drm_is_current_master_locked(struct drm_file *fpriv)
 {
-       /* Either drm_device.master_mutex or drm_file.master_lookup_lock
-        * should be held here.
-        */
+       lockdep_assert_once(lockdep_is_held(&fpriv->master_lookup_lock) ||
+                           lockdep_is_held(&fpriv->minor->dev->master_mutex));
+
        return fpriv->is_master && drm_lease_owner(fpriv->master) == fpriv->minor->dev->master;
 }
 

diff --git a/include/drm/drm_file.h b/include/drm/drm_file.h
index 726cfe0..a3acb7a 100644 (file)
--- a/include/drm/drm_file.h
+++ b/include/drm/drm_file.h
@@ -233,6 +233,10 @@ struct drm_file {
         * this only matches &drm_device.master if the master is the currently
         * active one.
         *
+        * To update @master, both &drm_device.master_mutex and
+        * @master_lookup_lock need to be held, therefore holding either of
+        * them is safe and enough for the read side.
+        *
         * When dereferencing this pointer, either hold struct
         * &drm_device.master_mutex for the duration of the pointer's use, or
         * use drm_file_get_master() if struct &drm_device.master_mutex is not