linux-kernel - [PATCH v2 2/2] drm: add lockdep assert to drm_is_current_master

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Date:   Sat, 31 Jul 2021 16:24:58 +0800
From:   Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
To:     maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de, airlied@...ux.ie, daniel@...ll.ch,
        peterz@...radead.org, mingo@...hat.com, will@...nel.org,
        longman@...hat.com, boqun.feng@...il.com
Cc:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        skhan@...uxfoundation.org, gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: [PATCH v2 2/2] 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@...il.com>
Acked-by: Boqun Feng <boqun.feng@...il.com>
Acked-by: Waiman Long <longman@...hat.com>
---
 drivers/gpu/drm/drm_auth.c | 6 +++---
 include/drm/drm_file.h     | 4 ++++
 2 files changed, 7 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_auth.c b/drivers/gpu/drm/drm_auth.c
index 9c24b8cc8e36..6f4d7ff23c80 100644
--- 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 726cfe0ff5f5..a3acb7ac3550 100644
--- 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
-- 
2.25.1