I am not very sure whether this is a real bug.
In function moxa_new_dcdstate() (in file drivers/tty/moxa.c), function tty_hangup() may be called only when “C_CLOCAL(tty)” is true (at line 1363). But in many other places, function tty_hangup() is called only when “!C_CLOCAL(tty)” is true (e.g. codes of function rfcomm_dev_modem_status in file net/bluetooth/rfcomm/tty.c).
The related codes are as following.
moxa_new_dcdstate @@ drivers/tty/moxa.c:1362
1362                tty = tty_port_tty_get(&p->port);
1363                if (tty && C_CLOCAL(tty) && !dcd)
1364                        tty_hangup(tty);
1365                tty_kref_put(tty);

rfcomm_dev_modem_status @@ net/bluetooth/rfcomm/tty.c:625
625        if ((dev->modem_status & TIOCM_CD) && !(v24_sig & RFCOMM_V24_DV)) {
 626                if (dev->tty && !C_CLOCAL(dev->tty))
 627                        tty_hangup(dev->tty);
 628        }

Thanks

RUC_Soft_Sec