--- x/drivers/tty/n_gsm.c
+++ y/drivers/tty/n_gsm.c
@@ -3126,7 +3126,9 @@ static int gsm_activate_mux(struct gsm_m
 	struct gsm_dlci *dlci;
 	int ret;
 
+	mutex_lock(&gsm->mutex);
 	dlci = gsm_dlci_alloc(gsm, 0);
+	mutex_unlock(&gsm->mutex);
 	if (dlci == NULL)
 		return -ENOMEM;