linux-kernel - [PATCH 3.19 10/27] tcp: fix FRTO undo on cumulative ACK of SACKed range

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Message-Id: <20150426134603.815930429@linuxfoundation.org>
Date:	Sun, 26 Apr 2015 16:15:41 +0200
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Neal Cardwell <ncardwell@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [PATCH 3.19 10/27] tcp: fix FRTO undo on cumulative ACK of SACKed range

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

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

From: Neal Cardwell <ncardwell@...gle.com>

[ Upstream commit 666b805150efd62f05810ff0db08f44a2370c937 ]

On processing cumulative ACKs, the FRTO code was not checking the
SACKed bit, meaning that there could be a spurious FRTO undo on a
cumulative ACK of a previously SACKed skb.

The FRTO code should only consider a cumulative ACK to indicate that
an original/unretransmitted skb is newly ACKed if the skb was not yet
SACKed.

The effect of the spurious FRTO undo would typically be to make the
connection think that all previously-sent packets were in flight when
they really weren't, leading to a stall and an RTO.

Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
Signed-off-by: Yuchung Cheng <ycheng@...gle.com>
Fixes: e33099f96d99c ("tcp: implement RFC5682 F-RTO")
Signed-off-by: David S. Miller <davem@...emloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 net/ipv4/tcp_input.c |    7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -3104,10 +3104,11 @@ static int tcp_clean_rtx_queue(struct so
 			if (!first_ackt.v64)
 				first_ackt = last_ackt;
 
-			if (!(sacked & TCPCB_SACKED_ACKED))
+			if (!(sacked & TCPCB_SACKED_ACKED)) {
 				reord = min(pkts_acked, reord);
-			if (!after(scb->end_seq, tp->high_seq))
-				flag |= FLAG_ORIG_SACK_ACKED;
+				if (!after(scb->end_seq, tp->high_seq))
+					flag |= FLAG_ORIG_SACK_ACKED;
+			}
 		}
 
 		if (sacked & TCPCB_SACKED_ACKED)


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