[TCP]: Bidir flow must not disregard SACK blocks for lost marking

It's possible that new SACK blocks that should trigger new LOST
markings arrive with new data (which previously made is_dupack
false). In addition, I think this fixes a case where we get
a cumulative ACK with enough SACK blocks to trigger the fast
recovery (is_dupack would be false there too).

I'm not completely pleased with this solution because readability
of the code is somewhat questionable as 'is_dupack' in SACK case
is no longer about dupacks only but would mean something like
'lost_marker_work_todo' too... But because of Eifel stuff done
in CA_Recovery, the FLAG_DATA_SACKED check cannot be placed to
the if statement which seems attractive solution. Nevertheless,
I didn't like adding another variable just for that either... :-)

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 4 insertions, 1 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 41163ddc312..378ca8a086a 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2112,7 +2112,10 @@ tcp_fastretrans_alert(struct sock *sk, u32 prior_snd_una,
 {
         struct inet_connection_sock *icsk = inet_csk(sk);
         struct tcp_sock *tp = tcp_sk(sk);
-        int is_dupack = (tp->snd_una == prior_snd_una && !(flag&FLAG_NOT_DUP));
+        int is_dupack = (tp->snd_una == prior_snd_una &&
+                         (!(flag&FLAG_NOT_DUP) ||
+                          ((flag&FLAG_DATA_SACKED) &&
+                           (tp->fackets_out > tp->reordering))));
 
         /* Some technical things:
          * 1. Reno does not count dupacks (sacked_out) automatically. */