tcp: remove redundant code from tcp_mark_lost_retrans

Arnd Hannemann <hannemann@nets.rwth-aachen.de> noticed and was
puzzled by the fact that !tcp_is_fack(tp) leads to early return
near the beginning and the later on tcp_is_fack(tp) was still
used in an if condition. The later check was a left-over from
RFC3517 SACK stuff (== !tcp_is_fack(tp) behavior nowadays) as
there wasn't clear way how to handle this particular check
cheaply in the spirit of RFC3517 (using only SACK blocks, not
holes + SACK blocks as with FACK). I sort of left it there as
a reminder but since it's confusing other people just remove
it and comment the missing-feature stuff instead.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Cc: Arnd Hannemann <hannemann@nets.rwth-aachen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 12 insertions, 4 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 3f2f09091bcf..125b4517f368 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1178,10 +1178,18 @@ static void tcp_mark_lost_retrans(struct sock *sk)
                 if (!(TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_RETRANS))
                         continue;
 
-                if (after(received_upto, ack_seq) &&
-                    (tcp_is_fack(tp) ||
-                     !before(received_upto,
-                             ack_seq + tp->reordering * tp->mss_cache))) {
+                /* TODO: We would like to get rid of tcp_is_fack(tp) only
+                 * constraint here (see above) but figuring out that at
+                 * least tp->reordering SACK blocks reside between ack_seq
+                 * and received_upto is not easy task to do cheaply with
+                 * the available datastructures.
+                 *
+                 * Whether FACK should check here for tp->reordering segs
+                 * in-between one could argue for either way (it would be
+                 * rather simple to implement as we could count fack_count
+                 * during the walk and do tp->fackets_out - fack_count).
+                 */
+                if (after(received_upto, ack_seq)) {
                         TCP_SKB_CB(skb)->sacked &= ~TCPCB_SACKED_RETRANS;
                         tp->retrans_out -= tcp_skb_pcount(skb);