// SPDX-License-Identifier: GPL-2.0
#include <linux/tcp.h>
#include <net/tcp.h>

static u32 tcp_rack_reo_wnd(const struct sock *sk)
{
        const struct tcp_sock *tp = tcp_sk(sk);

        if (!tp->reord_seen) {
                /* If reordering has not been observed, be aggressive during
                 * the recovery or starting the recovery by DUPACK threshold.
                 */
                if (inet_csk(sk)->icsk_ca_state >= TCP_CA_Recovery)
                        return 0;

                if (tp->sacked_out >= tp->reordering &&
                    !(READ_ONCE(sock_net(sk)->ipv4.sysctl_tcp_recovery) &
                      TCP_RACK_NO_DUPTHRESH))
                        return 0;
        }

        /* To be more reordering resilient, allow min_rtt/4 settling delay.
         * Use min_rtt instead of the smoothed RTT because reordering is
         * often a path property and less related to queuing or delayed ACKs.
         * Upon receiving DSACKs, linearly increase the window up to the
         * smoothed RTT.
         */
        return min((tcp_min_rtt(tp) >> 2) * tp->rack.reo_wnd_steps,
                   tp->srtt_us >> 3);
}

s32 tcp_rack_skb_timeout(struct tcp_sock *tp, struct sk_buff *skb, u32 reo_wnd)
{
        return tp->rack.rtt_us + reo_wnd -
               tcp_stamp_us_delta(tp->tcp_mstamp, tcp_skb_timestamp_us(skb));
}

/* RACK loss detection (IETF RFC8985):
 *
 * Marks a packet lost, if some packet sent later has been (s)acked.
 * The underlying idea is similar to the traditional dupthresh and FACK
 * but they look at different metrics:
 *
 * dupthresh: 3 OOO packets delivered (packet count)
 * FACK: sequence delta to highest sacked sequence (sequence space)
 * RACK: sent time delta to the latest delivered packet (time domain)
 *
 * The advantage of RACK is it applies to both original and retransmitted
 * packet and therefore is robust against tail losses. Another advantage
 * is being more resilient to reordering by simply allowing some
 * "settling delay", instead of tweaking the dupthresh.
 *
 * When tcp_rack_detect_loss() detects some packets are lost and we
 * are not already in the CA_Recovery state, either tcp_rack_reo_timeout()
 * or tcp_time_to_recover()'s "Trick#1: the loss is proven" code path will
 * make us enter the CA_Recovery state.
 */
static void tcp_rack_detect_loss(struct sock *sk, u32 *reo_timeout)
{
        struct tcp_sock *tp = tcp_sk(sk);
        struct sk_buff *skb, *n;
        u32 reo_wnd;

        *reo_timeout = 0;
        reo_wnd = tcp_rack_reo_wnd(sk);
        list_for_each_entry_safe(skb, n, &tp->tsorted_sent_queue,
                                 tcp_tsorted_anchor) {
                struct tcp_skb_cb *scb = TCP_SKB_CB(skb);
                s32 remaining;

                /* Skip ones marked lost but not yet retransmitted */
                if ((scb->sacked & TCPCB_LOST) &&
                    !(scb->sacked & TCPCB_SACKED_RETRANS))
                        continue;

                if (!tcp_skb_sent_after(tp->rack.mstamp,
                                        tcp_skb_timestamp_us(skb),
                                        tp->rack.end_seq, scb->end_seq))
                        break;

                /* A packet is lost if it has not been s/acked beyond
                 * the recent RTT plus the reordering window.
                 */
                remaining = tcp_rack_skb_timeout(tp, skb, reo_wnd);
                if (remaining <= 0) {
                        tcp_mark_skb_lost(sk, skb);
                        list_del_init(&skb->tcp_tsorted_anchor);
                } else {
                        /* Record maximum wait time */
                        *reo_timeout = max_t(u32, *reo_timeout, remaining);
                }
        }
}

bool tcp_rack_mark_lost(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 timeout;

        if (!tp->rack.advanced)
                return false;

        /* Reset the advanced flag to avoid unnecessary queue scanning */
        tp->rack.advanced = 0;
        tcp_rack_detect_loss(sk, &timeout);
        if (timeout) {
                timeout = usecs_to_jiffies(timeout + TCP_TIMEOUT_MIN_US);
                inet_csk_reset_xmit_timer(sk, ICSK_TIME_REO_TIMEOUT,
                                          timeout, inet_csk(sk)->icsk_rto);
        }
        return !!timeout;
}

/* We have waited long enough to accommodate reordering. Mark the expired
 * packets lost and retransmit them.
 */
void tcp_rack_reo_timeout(struct sock *sk)
{
        struct tcp_sock *tp = tcp_sk(sk);
        u32 timeout, prior_inflight;
        u32 lost = tp->lost;

        prior_inflight = tcp_packets_in_flight(tp);
        tcp_rack_detect_loss(sk, &timeout);
        if (prior_inflight != tcp_packets_in_flight(tp)) {
                if (inet_csk(sk)->icsk_ca_state != TCP_CA_Recovery) {
                        tcp_enter_recovery(sk, false);
                        if (!inet_csk(sk)->icsk_ca_ops->cong_control)
                                tcp_cwnd_reduction(sk, 1, tp->lost - lost, 0);
                }
                tcp_xmit_retransmit_queue(sk);
        }
        if (inet_csk(sk)->icsk_pending != ICSK_TIME_RETRANS)
                tcp_rearm_rto(sk);
}

/* RFC6582 NewReno recovery for non-SACK connection. It simply retransmits
 * the next unacked packet upon receiving
 * a) three or more DUPACKs to start the fast recovery
 * b) an ACK acknowledging new data during the fast recovery.
 */
void tcp_newreno_mark_lost(struct sock *sk, bool snd_una_advanced)
{
        const u8 state = inet_csk(sk)->icsk_ca_state;
        struct tcp_sock *tp = tcp_sk(sk);

        if ((state < TCP_CA_Recovery && tp->sacked_out >= tp->reordering) ||
            (state == TCP_CA_Recovery && snd_una_advanced)) {
                struct sk_buff *skb = tcp_rtx_queue_head(sk);
                u32 mss;

                if (TCP_SKB_CB(skb)->sacked & TCPCB_LOST)
                        return;

                mss = tcp_skb_mss(skb);
                if (tcp_skb_pcount(skb) > 1 && skb->len > mss)
                        tcp_fragment(sk, TCP_FRAG_IN_RTX_QUEUE, skb,
                                     mss, mss, GFP_ATOMIC);

                tcp_mark_skb_lost(sk, skb);
        }
}