linux-kernel - [ 60/66] tcp: frto should not set snd

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Message-Id: <20130217225008.595345988@decadent.org.uk>
Date:	Sun, 17 Feb 2013 22:51:01 +0000
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	akpm@...ux-foundation.org, Eric Dumazet <edumazet@...gle.com>,
	Pasi KÃ€rkkÃ€inen <pasik@....fi>,
	Neal Cardwell <ncardwell@...gle.com>,
	Ilpo JÃ€rvinen <ilpo.jarvinen@...sinki.fi>,
	Yuchung Cheng <ycheng@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [ 60/66] tcp: frto should not set snd_cwnd to 0

3.2-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Eric Dumazet <edumazet@...gle.com>

[ Upstream commit 2e5f421211ff76c17130b4597bc06df4eeead24f ]

Commit 9dc274151a548 (tcp: fix ABC in tcp_slow_start())
uncovered a bug in FRTO code :
tcp_process_frto() is setting snd_cwnd to 0 if the number
of in flight packets is 0.

As Neal pointed out, if no packet is in flight we lost our
chance to disambiguate whether a loss timeout was spurious.

We should assume it was a proper loss.

Reported-by: Pasi Kärkkäinen <pasik@....fi>
Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
Cc: Yuchung Cheng <ycheng@...gle.com>
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 net/ipv4/tcp_input.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index aab8f08..308b5b4 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3635,7 +3635,8 @@ static int 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)) {
+	if (!before(tp->snd_una, tp->frto_highmark) ||
+	    !tcp_packets_in_flight(tp)) {
 		tcp_enter_frto_loss(sk, (tp->frto_counter == 1 ? 2 : 3), flag);
 		return 1;
 	}


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