TTY_LDISC_CHANGING
} while (!test_bit(TTY_LDISC_CHANGING, &tty->flags));
set_bit(TTY_LDISC_CHANGING, &tty->flags);
clear_bit(TTY_LDISC_CHANGING, &tty->flags);
test_bit(TTY_LDISC_CHANGING, &tty->flags);