diff --git a/src/runtime/runtime2.go b/src/runtime/runtime2.go
index cfea1cd45f..8d749f3d7c 100644
--- a/src/runtime/runtime2.go
+++ b/src/runtime/runtime2.go
@@ -22,6 +22,13 @@ const (
 	// If you add to this list, add to the list
 	// of "okay during garbage collection" status
 	// in mgcmark.go too.
+	//
+	// TODO(austin): The _Gscan bit could be much lighter-weight.
+	// For example, we could choose not to run _Gscanrunnable
+	// goroutines found in the run queue, rather than CAS-looping
+	// until they become _Grunnable. And transitions like
+	// _Gscanwaiting -> _Gscanrunnable are actually okay because
+	// they don't affect stack ownership.
 
 	// _Gidle means this goroutine was just allocated and has not
 	// yet been initialized.