net: Add warning if any lower device is still in adjacency list

Lower list should be empty just like upper.
Signed-off-by: NDavid Ahern <dsa@cumulusnetworks.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/core/dev.c

@@ -5219,6 +5219,20 @@ struct net_device *netdev_master_upper_dev_get(struct net_device *dev)
 }
 EXPORT_SYMBOL(netdev_master_upper_dev_get);
 
+/**
+ * netdev_has_any_lower_dev - Check if device is linked to some device
+ * @dev: device
+ *
+ * Find out if a device is linked to a lower device and return true in case
+ * it is. The caller must hold the RTNL lock.
+ */
+static bool netdev_has_any_lower_dev(struct net_device *dev)
+{
+	ASSERT_RTNL();
+
+	return !list_empty(&dev->adj_list.lower);
+}
+
 void *netdev_adjacent_get_private(struct list_head *adj_list)
 {
 	struct netdev_adjacent *adj;
@@ -6616,6 +6630,7 @@ static void rollback_registered_many(struct list_head *head)
 
 		/* Notifier chain MUST detach us all upper devices. */
 		WARN_ON(netdev_has_any_upper_dev(dev));
+		WARN_ON(netdev_has_any_lower_dev(dev));
 
 		/* Remove entries from kobject tree */
 		netdev_unregister_kobject(dev);