diff --git a/drivers/usb/class/cdc-acm.c b/drivers/usb/class/cdc-acm.c
index e8c564a..3a97fec 100644
--- a/drivers/usb/class/cdc-acm.c
+++ b/drivers/usb/class/cdc-acm.c
@@ -570,8 +570,8 @@ static void acm_tty_close(struct tty_struct *tty, struct file *filp)
 	if (!acm)
 		return;
 
-	mutex_lock(&open_mutex);
 	if (tty_port_close_start(&acm->port, tty, filp) == 0) {
+		mutex_lock(&open_mutex);
 		if (!acm->dev) {
 			tty_port_tty_set(&acm->port, NULL);
 			acm_tty_unregister(acm);
@@ -580,6 +580,7 @@ static void acm_tty_close(struct tty_struct *tty, struct file *filp)
 		mutex_unlock(&open_mutex);
 		return;
 	}
+	mutex_lock(&open_mutex);
 	acm_port_down(acm);
 	tty_port_close_end(&acm->port, tty);
 	tty_port_tty_set(&acm->port, NULL);