1 files changed, 18 insertions, 12 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 051f0f815f17..7641b2761a14 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1265,20 +1265,15 @@ tcp_sacktag_write_queue(struct sock *sk, struct sk_buff *ack_skb, u32 prior_snd_
         return flag;
 }
 
-/* F-RTO can only be used if these conditions are satisfied:
- *  - there must be some unsent new data
- *  - the advertised window should allow sending it
- *  - TCP has never retransmitted anything other than head (SACK enhanced
- *    variant from Appendix B of RFC4138 is more robust here)
+/* F-RTO can only be used if TCP has never retransmitted anything other than
+ * head (SACK enhanced variant from Appendix B of RFC4138 is more robust here)
  */
 int tcp_use_frto(struct sock *sk)
 {
         const struct tcp_sock *tp = tcp_sk(sk);
         struct sk_buff *skb;
 
-        if (!sysctl_tcp_frto || !tcp_send_head(sk) ||
-                after(TCP_SKB_CB(tcp_send_head(sk))->end_seq,
-                      tp->snd_una + tp->snd_wnd))
+        if (!sysctl_tcp_frto)
                 return 0;
 
         if (IsSackFrto())
@@ -2642,7 +2637,9 @@ static void tcp_undo_spur_to_response(struct sock *sk, int flag)
  *                  algorithm is not part of the F-RTO detection algorithm
  *                  given in RFC4138 but can be selected separately).
  * Otherwise (basically on duplicate ACK), RTO was (likely) caused by a loss
- * and TCP falls back to conventional RTO recovery.
+ * and TCP falls back to conventional RTO recovery. F-RTO allows overriding
+ * of Nagle, this is done using frto_counter states 2 and 3, when a new data
+ * segment of any size sent during F-RTO, state 2 is upgraded to 3.
  *
  * Rationale: if the RTO was spurious, new ACKs should arrive from the
  * original window even after we transmit two new data segments.
@@ -2671,7 +2668,7 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
                 inet_csk(sk)->icsk_retransmits = 0;
 
         if (!before(tp->snd_una, tp->frto_highmark)) {
-                tcp_enter_frto_loss(sk, tp->frto_counter + 1, flag);
+                tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag);
                 return 1;
         }
 
@@ -2697,7 +2694,7 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
                         return 1;
                 }
 
-                if ((tp->frto_counter == 2) &&
+                if ((tp->frto_counter >= 2) &&
                     (!(flag&FLAG_FORWARD_PROGRESS) ||
                      ((flag&FLAG_DATA_SACKED) && !(flag&FLAG_ONLY_ORIG_SACKED)))) {
                         /* RFC4138 shortcoming (see comment above) */
@@ -2710,10 +2707,19 @@ static int tcp_process_frto(struct sock *sk, u32 prior_snd_una, int flag)
         }
 
         if (tp->frto_counter == 1) {
+                /* Sending of the next skb must be allowed or no FRTO */
+                if (!tcp_send_head(sk) ||
+                    after(TCP_SKB_CB(tcp_send_head(sk))->end_seq,
+                                     tp->snd_una + tp->snd_wnd)) {
+                        tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3),
+                                            flag);
+                        return 1;
+                }
+
                 tp->snd_cwnd = tcp_packets_in_flight(tp) + 2;
                 tp->frto_counter = 2;
                 return 1;
-        } else /* frto_counter == 2 */ {
+        } else {
                 switch (sysctl_tcp_frto_response) {
                 case 2:
                         tcp_undo_spur_to_response(sk, flag);