tcp: allow undo from reordered DSACKs

Previously, SACK-enabled connections hung around in TCP_CA_Disorder
state while snd_una==high_seq, just waiting to accumulate DSACKs and
hopefully undo a cwnd reduction. This could and did lead to the
following unfortunate scenario: if some incoming ACKs advance snd_una
beyond high_seq then we were setting undo_marker to 0 and moving to
TCP_CA_Open, so if (due to reordering in the ACK return path) we
shortly thereafter received a DSACK then we were no longer able to
undo the cwnd reduction.

The change: Simplify the congestion avoidance state machine by
removing the behavior where SACK-enabled connections hung around in
the TCP_CA_Disorder state just waiting for DSACKs. Instead, when
snd_una advances to high_seq or beyond we typically move to
TCP_CA_Open immediately and allow an undo in either TCP_CA_Open or
TCP_CA_Disorder if we later receive enough DSACKs.

Other patches in this series will provide other changes that are
necessary to fully fix this problem.

Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 2 insertions, 13 deletions

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 751d39060fb8..a4efdd7cf5a1 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2858,7 +2858,7 @@ static void tcp_try_keep_open(struct sock *sk)
         struct tcp_sock *tp = tcp_sk(sk);
         int state = TCP_CA_Open;
 
-        if (tcp_left_out(tp) || tcp_any_retrans_done(sk) || tp->undo_marker)
+        if (tcp_left_out(tp) || tcp_any_retrans_done(sk))
                 state = TCP_CA_Disorder;
 
         if (inet_csk(sk)->icsk_ca_state != state) {
@@ -3066,17 +3066,6 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
                         }
                         break;
 
-                case TCP_CA_Disorder:
-                        tcp_try_undo_dsack(sk);
-                        if (!tp->undo_marker ||
-                            /* For SACK case do not Open to allow to undo
-                             * catching for all duplicate ACKs. */
-                            tcp_is_reno(tp) || tp->snd_una != tp->high_seq) {
-                                tp->undo_marker = 0;
-                                tcp_set_ca_state(sk, TCP_CA_Open);
-                        }
-                        break;
-
                 case TCP_CA_Recovery:
                         if (tcp_is_reno(tp))
                                 tcp_reset_reno_sack(tp);
@@ -3117,7 +3106,7 @@ static void tcp_fastretrans_alert(struct sock *sk, int pkts_acked,
                                 tcp_add_reno_sack(sk);
                 }
 
-                if (icsk->icsk_ca_state == TCP_CA_Disorder)
+                if (icsk->icsk_ca_state <= TCP_CA_Disorder)
                         tcp_try_undo_dsack(sk);
 
                 if (!tcp_time_to_recover(sk)) {