1 files changed, 17 insertions, 25 deletions

diff --git a/net/ipv4/tcp_metrics.c b/net/ipv4/tcp_metrics.c
index f6a005c485a9..4a22f3e715df 100644
--- a/net/ipv4/tcp_metrics.c
+++ b/net/ipv4/tcp_metrics.c
@@ -443,7 +443,7 @@ void tcp_init_metrics(struct sock *sk)
         struct dst_entry *dst = __sk_dst_get(sk);
         struct tcp_sock *tp = tcp_sk(sk);
         struct tcp_metrics_block *tm;
-        u32 val;
+        u32 val, crtt = 0; /* cached RTT scaled by 8 */
 
         if (dst == NULL)
                 goto reset;
@@ -478,15 +478,19 @@ void tcp_init_metrics(struct sock *sk)
                 tp->reordering = val;
         }
 
-        val = tcp_metric_get(tm, TCP_METRIC_RTT);
-        if (val == 0 || tp->srtt == 0) {
-                rcu_read_unlock();
-                goto reset;
-        }
-        /* Initial rtt is determined from SYN,SYN-ACK.
-         * The segment is small and rtt may appear much
-         * less than real one. Use per-dst memory
-         * to make it more realistic.
+        crtt = tcp_metric_get_jiffies(tm, TCP_METRIC_RTT);
+        rcu_read_unlock();
+reset:
+        /* The initial RTT measurement from the SYN/SYN-ACK is not ideal
+         * to seed the RTO for later data packets because SYN packets are
+         * small. Use the per-dst cached values to seed the RTO but keep
+         * the RTT estimator variables intact (e.g., srtt, mdev, rttvar).
+         * Later the RTO will be updated immediately upon obtaining the first
+         * data RTT sample (tcp_rtt_estimator()). Hence the cached RTT only
+         * influences the first RTO but not later RTT estimation.
+         *
+         * But if RTT is not available from the SYN (due to retransmits or
+         * syn cookies) or the cache, force a conservative 3secs timeout.
          *
          * A bit of theory. RTT is time passed after "normal" sized packet
          * is sent until it is ACKed. In normal circumstances sending small
@@ -497,21 +501,9 @@ void tcp_init_metrics(struct sock *sk)
          * to low value, and then abruptly stops to do it and starts to delay
          * ACKs, wait for troubles.
          */
-        val = msecs_to_jiffies(val);
-        if (val > tp->srtt) {
-                tp->srtt = val;
-                tp->rtt_seq = tp->snd_nxt;
-        }
-        val = tcp_metric_get_jiffies(tm, TCP_METRIC_RTTVAR);
-        if (val > tp->mdev) {
-                tp->mdev = val;
-                tp->mdev_max = tp->rttvar = max(tp->mdev, tcp_rto_min(sk));
-        }
-        rcu_read_unlock();
-
-        tcp_set_rto(sk);
-reset:
-        if (tp->srtt == 0) {
+        if (crtt > tp->srtt) {
+                inet_csk(sk)->icsk_rto = crtt + max(crtt >> 2, tcp_rto_min(sk));
+        } else if (tp->srtt == 0) {
                 /* RFC6298: 5.7 We've failed to get a valid RTT sample from
                  * 3WHS. This is most likely due to retransmission,
                  * including spurious one. Reset the RTO back to 3secs