linux-kernel - [PATCH v7 2/5] drm: separate locks in __drm_mode_object

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Message-Id: <20210701165358.19053-3-desmondcheongzx@gmail.com>
Date:   Fri,  2 Jul 2021 00:53:55 +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,
        sumit.semwal@...aro.org, christian.koenig@....com
Cc:     Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>,
        dri-devel@...ts.freedesktop.org, intel-gfx@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org, linux-media@...r.kernel.org,
        linaro-mm-sig@...ts.linaro.org, skhan@...uxfoundation.org,
        gregkh@...uxfoundation.org,
        linux-kernel-mentees@...ts.linuxfoundation.org,
        emil.l.velikov@...il.com, Daniel Vetter <daniel.vetter@...ll.ch>
Subject: [PATCH v7 2/5] drm: separate locks in __drm_mode_object_find

In a future patch, _drm_lease_held will dereference drm_file->master
only after making a call to drm_file_get_master. This will increment
the reference count of drm_file->master while holding onto a new
drm_file.master_lock.

In preparation for this, the call to _drm_lease_held should be moved
out from the section locked by &dev->mode_config.idr_mutex. This
avoids creating new lock hierarchies between
&dev->mode_config.idr_mutex and &drm_file->master_lock.

Reported-by: Daniel Vetter <daniel.vetter@...ll.ch>
Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@...il.com>
---
 drivers/gpu/drm/drm_mode_object.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/gpu/drm/drm_mode_object.c b/drivers/gpu/drm/drm_mode_object.c
index b26588b52795..83e35ff3b13a 100644
--- a/drivers/gpu/drm/drm_mode_object.c
+++ b/drivers/gpu/drm/drm_mode_object.c
@@ -146,16 +146,18 @@ struct drm_mode_object *__drm_mode_object_find(struct drm_device *dev,
 	if (obj && obj->id != id)
 		obj = NULL;
 
-	if (obj && drm_mode_object_lease_required(obj->type) &&
-	    !_drm_lease_held(file_priv, obj->id))
-		obj = NULL;
-
 	if (obj && obj->free_cb) {
 		if (!kref_get_unless_zero(&obj->refcount))
 			obj = NULL;
 	}
 	mutex_unlock(&dev->mode_config.idr_mutex);
 
+	if (obj && drm_mode_object_lease_required(obj->type) &&
+	    !_drm_lease_held(file_priv, obj->id)) {
+		drm_mode_object_put(obj);
+		obj = NULL;
+	}
+
 	return obj;
 }
 
-- 
2.25.1