1 files changed, 12 insertions, 18 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index cd8ea972dc65..8d480542aa07 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1909,6 +1909,7 @@ void tcp_enter_loss(struct sock *sk)
         struct tcp_sock *tp = tcp_sk(sk);
         struct net *net = sock_net(sk);
         struct sk_buff *skb;
+        bool new_recovery = icsk->icsk_ca_state < TCP_CA_Recovery;
         bool is_reneg;                  /* is receiver reneging on SACKs? */
         bool mark_lost;
 
@@ -1967,15 +1968,12 @@ void tcp_enter_loss(struct sock *sk)
         tp->high_seq = tp->snd_nxt;
         tcp_ecn_queue_cwr(tp);
 
-        /* F-RTO RFC5682 sec 3.1 step 1 mandates to disable F-RTO
-         * if a previous recovery is underway, otherwise it may incorrectly
-         * call a timeout spurious if some previously retransmitted packets
-         * are s/acked (sec 3.2). We do not apply that retriction since
-         * retransmitted skbs are permanently tagged with TCPCB_EVER_RETRANS
-         * so FLAG_ORIG_SACK_ACKED is always correct. But we do disable F-RTO
-         * on PTMU discovery to avoid sending new data.
+        /* F-RTO RFC5682 sec 3.1 step 1: retransmit SND.UNA if no previous
+         * loss recovery is underway except recurring timeout(s) on
+         * the same SND.UNA (sec 3.2). Disable F-RTO on path MTU probing
          */
         tp->frto = net->ipv4.sysctl_tcp_frto &&
+                   (new_recovery || icsk->icsk_retransmits) &&
                    !inet_csk(sk)->icsk_mtup.probe_size;
 }
 
@@ -2628,18 +2626,14 @@ static void tcp_process_loss(struct sock *sk, int flag, bool is_dupack,
             tcp_try_undo_loss(sk, false))
                 return;
 
-        /* The ACK (s)acks some never-retransmitted data meaning not all
-         * the data packets before the timeout were lost. Therefore we
-         * undo the congestion window and state. This is essentially
-         * the operation in F-RTO (RFC5682 section 3.1 step 3.b). Since
-         * a retransmitted skb is permantly marked, we can apply such an
-         * operation even if F-RTO was not used.
-         */
-        if ((flag & FLAG_ORIG_SACK_ACKED) &&
-            tcp_try_undo_loss(sk, tp->undo_marker))
-                return;
-
         if (tp->frto) { /* F-RTO RFC5682 sec 3.1 (sack enhanced version). */
+                /* Step 3.b. A timeout is spurious if not all data are
+                 * lost, i.e., never-retransmitted data are (s)acked.
+                 */
+                if ((flag & FLAG_ORIG_SACK_ACKED) &&
+                    tcp_try_undo_loss(sk, true))
+                        return;
+
                 if (after(tp->snd_nxt, tp->high_seq)) {
                         if (flag & FLAG_DATA_SACKED || is_dupack)
                                 tp->frto = 0; /* Step 3.a. loss was real */