diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index e8768d33efd6..8b9740142152 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -938,10 +938,12 @@ static int get_serial_info(struct tty_struct *tty, struct serial_struct *ss)
 	struct acm *acm = tty->driver_data;
 
 	ss->line = acm->minor;
+	mutex_lock(&acm->port.mutex);
 	ss->close_delay	= jiffies_to_msecs(acm->port.close_delay) / 10;
 	ss->closing_wait = acm->port.closing_wait == ASYNC_CLOSING_WAIT_NONE ?
 				ASYNC_CLOSING_WAIT_NONE :
 				jiffies_to_msecs(acm->port.closing_wait) / 10;
+	mutex_unlock(&acm->port.mutex);
 	return 0;
 }