tcp: disallow cwnd undo when switching congestion control

commit 44abafc4cc094214a99f860f778c48ecb23422fc upstream.

When the sender switches its congestion control during loss
recovery, if the recovery is spurious then it may incorrectly
revert cwnd and ssthresh to the older values set by a previous
congestion control. Consider a congestion control (like BBR)
that does not use ssthresh and keeps it infinite: the connection
may incorrectly revert cwnd to an infinite value when switching
from BBR to another congestion control.

This patch fixes it by disallowing such cwnd undo operation
upon switching congestion control.  Note that undo_marker
is not reset s.t. the packets that were incorrectly marked
lost would be corrected. We only avoid undoing the cwnd in
tcp_undo_cwnd_reduction().

Signed-off-by: Yuchung Cheng <ycheng@google.com>
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: Neal Cardwell <ncardwell@google.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>

diff --git a/net/ipv4/tcp_cong.c b/net/ipv4/tcp_cong.c
index 019c2389a341f6254185c8b9169d9b09c8c16c78..2ca6c080a4bc46ae59b99b203d6a89b2c76b7623 100644 (file)
--- a/net/ipv4/tcp_cong.c
+++ b/net/ipv4/tcp_cong.c
@@ -95,6 +95,7 @@ void tcp_init_congestion_control(struct sock *sk)
                rcu_read_unlock();
        }
 
+       tcp_sk(sk)->prior_ssthresh = 0;
        if (icsk->icsk_ca_ops->init)
                icsk->icsk_ca_ops->init(sk);
 }