diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index f5a82e107bcb..ca17b771ad60 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -279,6 +279,7 @@ struct lockdep_map rcu_sched_lock_map = {
 };
 EXPORT_SYMBOL_GPL(rcu_sched_lock_map);
 
+// Tell lockdep when RCU callbacks are being invoked.
 static struct lock_class_key rcu_callback_key;
 struct lockdep_map rcu_callback_map =
 	STATIC_LOCKDEP_MAP_INIT("rcu_callback", &rcu_callback_key);