diff --git a/drivers/usb/serial/mos7840.c b/drivers/usb/serial/mos7840.c
index aeeb9cb20999..0b29c5383dc8 100644
--- a/drivers/usb/serial/mos7840.c
+++ b/drivers/usb/serial/mos7840.c
@@ -1722,6 +1722,7 @@ static int mos7840_tiocmset(struct usb_serial_port *port, struct file *file,
 	if (mos7840_port == NULL)
 		return -ENODEV;
 
+	/* FIXME: What locks the port registers ? */
 	mcr = mos7840_port->shadowMCR;
 	if (clear & TIOCM_RTS)
 		mcr &= ~MCR_RTS;
diff --git a/drivers/usb/serial/option.c b/drivers/usb/serial/option.c
index d101025a4c63..0e7eeb2820e2 100644
--- a/drivers/usb/serial/option.c
+++ b/drivers/usb/serial/option.c
@@ -458,6 +458,7 @@ static int option_tiocmset(struct usb_serial_port *port, struct file *file,
 
 	portdata = usb_get_serial_port_data(port);
 
+	/* FIXME: what locks portdata fields ? */
 	if (set & TIOCM_RTS)
 		portdata->rts_state = 1;
 	if (set & TIOCM_DTR)