diff --git a/drivers/isdn/i4l/isdn_tty.c b/drivers/isdn/i4l/isdn_tty.c
index 1b2239c1d569..dc1cded716c1 100644
--- a/drivers/isdn/i4l/isdn_tty.c
+++ b/drivers/isdn/i4l/isdn_tty.c
@@ -1437,15 +1437,19 @@ isdn_tty_set_termios(struct tty_struct *tty, struct ktermios *old_termios)
 {
 	modem_info *info = (modem_info *) tty->driver_data;
 
+	mutex_lock(&modem_info_mutex);
 	if (!old_termios)
 		isdn_tty_change_speed(info);
 	else {
 		if (tty->termios.c_cflag == old_termios->c_cflag &&
 		    tty->termios.c_ispeed == old_termios->c_ispeed &&
-		    tty->termios.c_ospeed == old_termios->c_ospeed)
+		    tty->termios.c_ospeed == old_termios->c_ospeed) {
+			mutex_unlock(&modem_info_mutex);
 			return;
+		}
 		isdn_tty_change_speed(info);
 	}
+	mutex_unlock(&modem_info_mutex);
 }
 
 /*