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Date:   Thu, 22 Jul 2021 17:29:27 +0800
From:   Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
To:     linux-graphics-maintainer@...are.com, zackr@...are.com,
        airlied@...ux.ie, daniel@...ll.ch,
        maarten.lankhorst@...ux.intel.com, mripard@...nel.org,
        tzimmermann@...e.de
Cc:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        dri-devel@...ts.freedesktop.org, linux-kernel@...r.kernel.org,
        intel-gfx@...ts.freedesktop.org, skhan@...uxfoundation.org,
        gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        Daniel Vetter <daniel.vetter@...ll.ch>
Subject: [PATCH 1/3] 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@...ll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
---
 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

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