diff --git a/drivers/tty/tty_io.c b/drivers/tty/tty_io.c
index b008e2b..adbd9ac 100644
--- a/drivers/tty/tty_io.c
+++ b/drivers/tty/tty_io.c
@@ -275,6 +275,7 @@
 	return 0;
 }
 
+/* Caller must hold tty_lock */
 static int check_tty_count(struct tty_struct *tty, const char *routine)
 {
 #ifdef CHECK_TTY_COUNT