drm: Fix possible race conditions while unplugging DRM device

When unplugging a hotpluggable DRM device we first unregister it
with drm_dev_unregister and then set drm_device.unplugged flag which
is used to mark device critical sections with drm_dev_enter()/
drm_dev_exit() preventing access to device resources that are not
available after the device is gone.
But drm_dev_unregister may lead to hotplug uevent(s) fired to
user-space on card and/or connector removal, thus making it possible
for user-space to try accessing a disconnected device.

Fix this by first making sure device is properly marked as
disconnected and only then unregister it.

Fixes: bee330f3d672 ("drm: Use srcu to protect drm_device.unplugged")

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reported-by: Andrii Chepurnyi <andrii_chepurnyi@epam.com>
Cc: "Noralf Trønnes" <noralf@tronnes.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20180522141304.18646-1-andr2000@gmail.com

drivers/gpu/drm/drm_drv.c

@@ -369,13 +369,6 @@ EXPORT_SYMBOL(drm_dev_exit);
  */
 void drm_dev_unplug(struct drm_device *dev)
 {
-	drm_dev_unregister(dev);
-
-	mutex_lock(&drm_global_mutex);
-	if (dev->open_count == 0)
-		drm_dev_put(dev);
-	mutex_unlock(&drm_global_mutex);
-
 	/*
 	 * After synchronizing any critical read section is guaranteed to see
 	 * the new value of ->unplugged, and any critical section which might
@@ -384,6 +377,13 @@ void drm_dev_unplug(struct drm_device *dev)
 	 */
 	dev->unplugged = true;
 	synchronize_srcu(&drm_unplug_srcu);
+
+	drm_dev_unregister(dev);
+
+	mutex_lock(&drm_global_mutex);
+	if (dev->open_count == 0)
+		drm_dev_put(dev);
+	mutex_unlock(&drm_global_mutex);
 }
 EXPORT_SYMBOL(drm_dev_unplug);