tcp: fix SYNACK RTT estimation in Fast Open

tp->lsndtime may not always be the SYNACK timestamp if a passive
Fast Open socket sends data before handshake completes. And if the
remote acknowledges both the data and the SYNACK, the RTT sample
is already taken in tcp_ack(), so no need to call
tcp_update_ack_rtt() in tcp_synack_rtt_meas() aagain.

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 13 insertions, 5 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a16b01b537ba..305cd0526a78 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2871,14 +2871,19 @@ static inline bool tcp_ack_update_rtt(struct sock *sk, const int flag,
 }
 
 /* Compute time elapsed between (last) SYNACK and the ACK completing 3WHS. */
-static void tcp_synack_rtt_meas(struct sock *sk, struct request_sock *req)
+static void tcp_synack_rtt_meas(struct sock *sk, const u32 synack_stamp)
 {
         struct tcp_sock *tp = tcp_sk(sk);
         s32 seq_rtt = -1;
 
-        if (tp->lsndtime && !tp->total_retrans)
-                seq_rtt = tcp_time_stamp - tp->lsndtime;
-        tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, seq_rtt, -1);
+        if (synack_stamp && !tp->total_retrans)
+                seq_rtt = tcp_time_stamp - synack_stamp;
+
+        /* If the ACK acks both the SYNACK and the (Fast Open'd) data packets
+         * sent in SYN_RECV, SYNACK RTT is the smooth RTT computed in tcp_ack()
+         */
+        if (!tp->srtt)
+                tcp_ack_update_rtt(sk, FLAG_SYN_ACKED, seq_rtt, -1);
 }
 
 static void tcp_cong_avoid(struct sock *sk, u32 ack, u32 in_flight)
@@ -5587,6 +5592,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
         struct request_sock *req;
         int queued = 0;
         bool acceptable;
+        u32 synack_stamp;
 
         tp->rx_opt.saw_tstamp = 0;
 
@@ -5669,9 +5675,11 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
                  * so release it.
                  */
                 if (req) {
+                        synack_stamp = tcp_rsk(req)->snt_synack;
                         tp->total_retrans = req->num_retrans;
                         reqsk_fastopen_remove(sk, req, false);
                 } else {
+                        synack_stamp = tp->lsndtime;
                         /* Make sure socket is routed, for correct metrics. */
                         icsk->icsk_af_ops->rebuild_header(sk);
                         tcp_init_congestion_control(sk);
@@ -5694,7 +5702,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
                 tp->snd_una = TCP_SKB_CB(skb)->ack_seq;
                 tp->snd_wnd = ntohs(th->window) << tp->rx_opt.snd_wscale;
                 tcp_init_wl(tp, TCP_SKB_CB(skb)->seq);
-                tcp_synack_rtt_meas(sk, req);
+                tcp_synack_rtt_meas(sk, synack_stamp);
 
                 if (tp->rx_opt.tstamp_ok)
                         tp->advmss -= TCPOLEN_TSTAMP_ALIGNED;