static bool tcp_try_undo_partial(struct sock *sk, const int acked)
{
	struct tcp_sock *tp = tcp_sk(sk);

	if (tp->undo_marker && tcp_packet_delayed(tp)) {
		/* Plain luck! Hole if filled with delayed
		 * packet, rather than with a retransmit.
		 */
		tcp_update_reordering(sk, tcp_fackets_out(tp) + acked, 1);

		/* We are getting evidence that the reordering degree is higher
		 * than we realized. If there are no retransmits out then we
		 * can undo. Otherwise we clock out new packets but do not
		 * mark more packets lost or retransmit more.
		 */
		if (tp->retrans_out)
			return true;

		if (!tcp_any_retrans_done(sk))
			tp->retrans_stamp = 0;

		DBGUNDO(sk, "partial recovery");
		tcp_undo_cwnd_reduction(sk, true);
		NET_INC_STATS(sock_net(sk), LINUX_MIB_TCPPARTIALUNDO);
		tcp_try_keep_open(sk);
		return true;
	}
	return false;
}
Unknown