linux-kernel - [ 072/167] [PATCH 12/26] tcp: fix tcp_rcv_rtt

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Message-Id: <20120509055039.814414757@decadent.org.uk>
Date:	Wed, 09 May 2012 06:51:41 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	alan@...rguk.ukuu.org.uk, Neal Cardwell <ncardwell@...gle.com>,
	Eric Dumazet <edumazet@...gle.com>,
	"David S. Miller" <davem@...emloft.net>
Subject: [ 072/167] [PATCH 12/26] tcp: fix tcp_rcv_rtt_update() use of an unscaled RTT
 sample

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

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

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

[ Upstream commit 18a223e0b9ec8979320ba364b47c9772391d6d05 ]

Fix a code path in tcp_rcv_rtt_update() that was comparing scaled and
unscaled RTT samples.

The intent in the code was to only use the 'm' measurement if it was a
new minimum.  However, since 'm' had not yet been shifted left 3 bits
but 'new_sample' had, this comparison would nearly always succeed,
leading us to erroneously set our receive-side RTT estimate to the 'm'
sample when that sample could be nearly 8x too high to use.

The overall effect is to often cause the receive-side RTT estimate to
be significantly too large (up to 40% too large for brief periods in
my tests).

Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
Acked-by: Eric Dumazet <edumazet@...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, 5 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index e4d1e4a..4c46fa7 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -473,8 +473,11 @@ static void tcp_rcv_rtt_update(struct tcp_sock *tp, u32 sample, int win_dep)
 		if (!win_dep) {
 			m -= (new_sample >> 3);
 			new_sample += m;
-		} else if (m < new_sample)
-			new_sample = m << 3;
+		} else {
+			m <<= 3;
+			if (m < new_sample)
+				new_sample = m;
+		}
 	} else {
 		/* No previous measure. */
 		new_sample = m << 3;
-- 
1.7.10



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