drm: safely free connectors from connector_iter

In

commit 613051dac40da1751ab269572766d3348d45a197
Author: Daniel Vetter <[e-mail]>
Date: Wed Dec 14 00:08:06 2016 +0100

drm: locking&new iterators for connector_list

we've went to extreme lengths to make sure connector iterations works
in any context, without introducing any additional locking context.
This worked, except for a small fumble in the implementation:

When we actually race with a concurrent connector unplug event, and
our temporary connector reference turns out to be the final one, then
everything breaks: We call the connector release function from
whatever context we happen to be in, which can be an irq/atomic
context. And connector freeing grabs all kinds of locks and stuff.

Fix this by creating a specially safe put function for connetor_iter,
which (in this rare case) punts the cleanup to a worker.

Reported-by: Ben Widawsky <[e-mail]>
Cc: Ben Widawsky <[e-mail]>
Fixes: 613051dac40d ("drm: locking&new iterators for connector_list")
Cc: Dave Airlie <[e-mail]>
Cc: Chris Wilson <[e-mail]>
Cc: Sean Paul <[e-mail]>
Cc: <[e-mail]> # v4.11+
Reviewed-by: Dave Airlie <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/drm_connector.c

···
 	connector->funcs->destroy(connector);
 }
 
+static void drm_connector_free_work_fn(struct work_struct *work)
+{
+	struct drm_connector *connector =
+		container_of(work, struct drm_connector, free_work);
+	struct drm_device *dev = connector->dev;
+
+	drm_mode_object_unregister(dev, &connector->base);
+	connector->funcs->destroy(connector);
+}
+
 /**
  * drm_connector_init - Init a preallocated connector
  * @dev: DRM device
···
 				    false, drm_connector_free);
 	if (ret)
 		return ret;
+
+	INIT_WORK(&connector->free_work, drm_connector_free_work_fn);
 
 	connector->base.properties = &connector->properties;
 	connector->dev = dev;
···
 }
 EXPORT_SYMBOL(drm_connector_list_iter_begin);
 
+/*
+ * Extra-safe connector put function that works in any context. Should only be
+ * used from the connector_iter functions, where we never really expect to
+ * actually release the connector when dropping our final reference.
+ */
+static void
+drm_connector_put_safe(struct drm_connector *conn)
+{
+	if (refcount_dec_and_test(&conn->base.refcount.refcount))
+		schedule_work(&conn->free_work);
+}
+
 /**
  * drm_connector_list_iter_next - return next connector
  * @iter: connectr_list iterator
···
 	spin_unlock_irqrestore(&config->connector_list_lock, flags);
 
 	if (old_conn)
-		drm_connector_put(old_conn);
+		drm_connector_put_safe(old_conn);
 
 	return iter->conn;
 }
···
 {
 	iter->dev = NULL;
 	if (iter->conn)
-		drm_connector_put(iter->conn);
+		drm_connector_put_safe(iter->conn);
 	lock_release(&connector_list_iter_dep_map, 0, _RET_IP_);
 }
 EXPORT_SYMBOL(drm_connector_list_iter_end);

drivers/gpu/drm/drm_mode_config.c

···
 		drm_connector_put(connector);
 	}
 	drm_connector_list_iter_end(&conn_iter);
+	/* connector_iter drops references in a work item. */
+	flush_scheduled_work();
 	if (WARN_ON(!list_empty(&dev->mode_config.connector_list))) {
 		drm_connector_list_iter_begin(dev, &conn_iter);
 		drm_for_each_connector_iter(connector, &conn_iter)

include/drm/drm_connector.h

···
 	uint8_t num_h_tile, num_v_tile;
 	uint8_t tile_h_loc, tile_v_loc;
 	uint16_t tile_h_size, tile_v_size;
+
+	/**
+	 * @free_work:
+	 *
+	 * Work used only by &drm_connector_iter to be able to clean up a
+	 * connector from any context.
+	 */
+	struct work_struct free_work;
 };
 
 #define obj_to_connector(x) container_of(x, struct drm_connector, base)