mutex lock inside spin lock?

static int
ppp_async_push(struct asyncppp *ap)
{
	int avail, sent, done = 0;
	struct tty_struct *tty = ap->tty;
	int tty_stuffed = 0;
<!--break-->
	/*
	 * We can get called recursively here if the tty write
	 * function calls our wakeup function.  This can happen
	 * for example on a pty with both the master and slave
	 * set to PPP line discipline.
	 * We use the XMIT_BUSY bit to detect this and get out,
	 * leaving the XMIT_WAKEUP bit set to tell the other
	 * instance that it may now be able to write more now.
	 */
	if (test_and_set_bit(XMIT_BUSY, &ap->xmit_flags))
		return 0;
	spin_lock_bh(&ap->xmit_lock);
	for (;;) {
		if (test_and_clear_bit(XMIT_WAKEUP, &ap->xmit_flags))
			tty_stuffed = 0;
		if (!tty_stuffed && ap->optr < ap->olim) {
			avail = ap->olim - ap->optr;
			set_bit(TTY_DO_WRITE_WAKEUP, &tty->flags);
			sent = tty->ops->write(tty, ap->optr, avail);
			if (sent < 0)
				goto flush;	/* error, e.g. loss of CD */
			ap->optr += sent;
			if (sent < avail)
				tty_stuffed = 1;
			continue;
		}