drm: prevent double-(un)registration for connectors

If we're unlucky then the registration from a hotplugged connector
might race with the final registration step on driver load. And since
MST topology discover is asynchronous that's even somewhat likely.

v2: Also update the kerneldoc for @registered!

v3: Review from Chris:
- Improve kerneldoc for late_register/early_unregister callbacks.
- Use mutex_destroy.
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161218133545.2106-1-daniel.vetter@ffwll.ch
(cherry picked from commit e73ab00e)

drivers/gpu/drm/drm_connector.c

@@ -225,6 +225,7 @@ int drm_connector_init(struct drm_device *dev,
 
 	INIT_LIST_HEAD(&connector->probed_modes);
 	INIT_LIST_HEAD(&connector->modes);
+	mutex_init(&connector->mutex);
 	connector->edid_blob_ptr = NULL;
 	connector->status = connector_status_unknown;
 
@@ -359,6 +360,8 @@ void drm_connector_cleanup(struct drm_connector *connector)
 		connector->funcs->atomic_destroy_state(connector,
 						       connector->state);
 
+	mutex_destroy(&connector->mutex);
+
 	memset(connector, 0, sizeof(*connector));
 }
 EXPORT_SYMBOL(drm_connector_cleanup);
@@ -374,14 +377,15 @@ EXPORT_SYMBOL(drm_connector_cleanup);
  */
 int drm_connector_register(struct drm_connector *connector)
 {
-	int ret;
+	int ret = 0;
 
+	mutex_lock(&connector->mutex);
 	if (connector->registered)
-		return 0;
+		goto unlock;
 
 	ret = drm_sysfs_connector_add(connector);
 	if (ret)
-		return ret;
+		goto unlock;
 
 	ret = drm_debugfs_connector_add(connector);
 	if (ret) {
@@ -397,12 +401,14 @@ int drm_connector_register(struct drm_connector *connector)
 	drm_mode_object_register(connector->dev, &connector->base);
 
 	connector->registered = true;
-	return 0;
+	goto unlock;
 
 err_debugfs:
 	drm_debugfs_connector_remove(connector);
 err_sysfs:
 	drm_sysfs_connector_remove(connector);
+unlock:
+	mutex_unlock(&connector->mutex);
 	return ret;
 }
 EXPORT_SYMBOL(drm_connector_register);
@@ -415,8 +421,11 @@ EXPORT_SYMBOL(drm_connector_register);
  */
 void drm_connector_unregister(struct drm_connector *connector)
 {
-	if (!connector->registered)
+	mutex_lock(&connector->mutex);
+	if (!connector->registered) {
+		mutex_unlock(&connector->mutex);
 		return;
+	}
 
 	if (connector->funcs->early_unregister)
 		connector->funcs->early_unregister(connector);
@@ -425,6 +434,7 @@ void drm_connector_unregister(struct drm_connector *connector)
 	drm_debugfs_connector_remove(connector);
 
 	connector->registered = false;
+	mutex_unlock(&connector->mutex);
 }
 EXPORT_SYMBOL(drm_connector_unregister);
 

include/drm/drm_connector.h

@@ -381,6 +381,8 @@ struct drm_connector_funcs {
 	 * core drm connector interfaces. Everything added from this callback
 	 * should be unregistered in the early_unregister callback.
 	 *
+	 * This is called while holding drm_connector->mutex.
+	 *
 	 * Returns:
 	 *
 	 * 0 on success, or a negative error code on failure.
@@ -395,6 +397,8 @@ struct drm_connector_funcs {
 	 * late_register(). It is called from drm_connector_unregister(),
 	 * early in the driver unload sequence to disable userspace access
 	 * before data structures are torndown.
+	 *
+	 * This is called while holding drm_connector->mutex.
 	 */
 	void (*early_unregister)(struct drm_connector *connector);
 
@@ -559,7 +563,6 @@ struct drm_cmdline_mode {
  * @interlace_allowed: can this connector handle interlaced modes?
  * @doublescan_allowed: can this connector handle doublescan?
  * @stereo_allowed: can this connector handle stereo modes?
- * @registered: is this connector exposed (registered) with userspace?
  * @modes: modes available on this connector (from fill_modes() + user)
  * @status: one of the drm_connector_status enums (connected, not, or unknown)
  * @probed_modes: list of modes derived directly from the display
@@ -607,6 +610,13 @@ struct drm_connector {
 
 	char *name;
 
+	/**
+	 * @mutex: Lock for general connector state, but currently only protects
+	 * @registered. Most of the connector state is still protected by the
+	 * mutex in &drm_mode_config.
+	 */
+	struct mutex mutex;
+
 	/**
 	 * @index: Compacted connector index, which matches the position inside
 	 * the mode_config.list for drivers not supporting hot-add/removing. Can
@@ -620,6 +630,10 @@ struct drm_connector {
 	bool interlace_allowed;
 	bool doublescan_allowed;
 	bool stereo_allowed;
+	/**
+	 * @registered: Is this connector exposed (registered) with userspace?
+	 * Protected by @mutex.
+	 */
 	bool registered;
 	struct list_head modes; /* list of modes on this connector */