linux-kernel - [55/74] tcp: undo spurious timeout after SACK reneging

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Message-ID: <lsq.1365374742.154097199@decadent.org.uk>
Date:	Sun, 07 Apr 2013 23:45:42 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:	akpm@...ux-foundation.org, "Neal Cardwell" <ncardwell@...gle.com>,
	"Yuchung Cheng" <ycheng@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [55/74] tcp: undo spurious timeout after SACK reneging

3.2.43-rc1 review patch.  If anyone has any objections, please let me know.

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

From: Yuchung Cheng <ycheng@...gle.com>

[ Upstream commit 7ebe183c6d444ef5587d803b64a1f4734b18c564 ]

On SACK reneging the sender immediately retransmits and forces a
timeout but disables Eifel (undo). If the (buggy) receiver does not
drop any packet this can trigger a false slow-start retransmit storm
driven by the ACKs of the original packets. This can be detected with
undo and TCP timestamps.

Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
Acked-by: Neal Cardwell <ncardwell@...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 | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 1b1f7af..3124e17 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -2265,11 +2265,8 @@ void tcp_enter_loss(struct sock *sk, int how)
 	if (tcp_is_reno(tp))
 		tcp_reset_reno_sack(tp);
 
-	if (!how) {
-		/* Push undo marker, if it was plain RTO and nothing
-		 * was retransmitted. */
-		tp->undo_marker = tp->snd_una;
-	} else {
+	tp->undo_marker = tp->snd_una;
+	if (how) {
 		tp->sacked_out = 0;
 		tp->fackets_out = 0;
 	}

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