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From:   Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
To:     maarten.lankhorst@linux.intel.com, mripard@kernel.org,
        tzimmermann@suse.de, airlied@linux.ie, daniel@ffwll.ch,
        sumit.semwal@linaro.org, christian.koenig@amd.com
Cc:     Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>,
        dri-devel@lists.freedesktop.org, intel-gfx@lists.freedesktop.org,
        linux-kernel@vger.kernel.org, linux-media@vger.kernel.org,
        linaro-mm-sig@lists.linaro.org, skhan@linuxfoundation.org,
        gregkh@linuxfoundation.org,
        linux-kernel-mentees@lists.linuxfoundation.org,
        emil.l.velikov@gmail.com
Subject: [PATCH v6 2/4] drm: avoid circular locks in __drm_mode_object_find
Date:   Wed, 30 Jun 2021 22:54:02 +0800
Message-Id: <20210630145404.5958-3-desmondcheongzx@gmail.com>
In-Reply-To: <20210630145404.5958-1-desmondcheongzx@gmail.com>
References: <20210630145404.5958-1-desmondcheongzx@gmail.com>
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In a future patch, _drm_lease_held will dereference drm_file->master
only after making a call to drm_file_get_master which increments the
reference count of drm_file->master while holding a lock on
drm_device.master_mutex.

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 inverting the lock hierarchy for
&dev->master_mutex --> &dev->mode_config.idr_mutex

Signed-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.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..63d35f1f98dd 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