tcp: fix for zero packets_in_flight was too broad

There are transients during normal FRTO procedure during which
the packets_in_flight can go to zero between write_queue state
updates and firing the resulting segments out. As FRTO processing
occurs during that window the check must be more precise to
not match "spuriously" :-). More specificly, e.g., when
packets_in_flight is zero but FLAG_DATA_ACKED is true the problematic
branch that set cwnd into zero would not be taken and new segments
might be sent out later.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Tested-by: Eric Dumazet <edumazet@google.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 6 insertions, 2 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 680c4224ed96..ad70a962c20e 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3484,8 +3484,7 @@ static bool tcp_process_frto(struct sock *sk, int flag)
             ((tp->frto_counter >= 2) && (flag & FLAG_RETRANS_DATA_ACKED)))
                 tp->undo_marker = 0;
 
-        if (!before(tp->snd_una, tp->frto_highmark) ||
-            !tcp_packets_in_flight(tp)) {
+        if (!before(tp->snd_una, tp->frto_highmark)) {
                 tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag);
                 return true;
         }
@@ -3505,6 +3504,11 @@ static bool tcp_process_frto(struct sock *sk, int flag)
                 }
         } else {
                 if (!(flag & FLAG_DATA_ACKED) && (tp->frto_counter == 1)) {
+                        if (!tcp_packets_in_flight(tp)) {
+                                tcp_enter_frto_loss(sk, 2, flag);
+                                return true;
+                        }
+
                         /* Prevent sending of new data. */
                         tp->snd_cwnd = min(tp->snd_cwnd,
                                            tcp_packets_in_flight(tp));