netdev - [PATCHv3 1/2] tcp: Stalling connections: Fix timeout calculation routine

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Message-id: <4B1D27F7.9000502@tvk.rwth-aachen.de>
Date:	Mon, 07 Dec 2009 17:06:15 +0100
From:	Damian Lukowski <damian@....rwth-aachen.de>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Netdev <netdev@...r.kernel.org>,
	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>,
	Frederic Leroy <fredo@...rox.org>,
	David Miller <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Greg KH <gregkh@...e.de>
Subject: [PATCHv3 1/2] tcp: Stalling connections: Fix timeout calculation
 routine

This patch fixes a problem in the TCP connection timeout calculation.
Currently, timeout decisions are made on the basis of the current
tcp_time_stamp and retrans_stamp, which is usually set at the first
retransmission.
However, if the retransmission fails in tcp_retransmit_skb(),
retrans_stamp is not updated and remains zero. This leads to wrong
decisions in retransmits_timed_out() if tcp_time_stamp is larger than
the specified timeout, which is very likely.
In this case, the TCP connection dies after the first attempted
(and unsuccessful) retransmission.

With this patch, tcp_skb_cb->when is used instead, when retrans_stamp
is not available.

This bug has been introduced together with retransmits_timed_out()
in 2.6.32, as the number of retransmissions has been used for timeout
decisions before. The corresponding commit was
6fa12c85031485dff38ce550c24f10da23b0adaa.

Thanks to Ilpo Järvinen for code suggestions and Frederic Leroy for
testing.

Signed-off-by: Damian Lukowski <damian@....rwth-aachen.de>
---
 include/net/tcp.h |   10 ++++++++--
 1 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/include/net/tcp.h b/include/net/tcp.h
index 03a49c7..842ac4d 100644
--- a/include/net/tcp.h
+++ b/include/net/tcp.h
@@ -1263,14 +1263,20 @@ static inline struct sk_buff *tcp_write_queue_prev(struct sock *sk, struct sk_bu
  * TCP connection after "boundary" unsucessful, exponentially backed-off
  * retransmissions with an initial RTO of TCP_RTO_MIN.
  */
-static inline bool retransmits_timed_out(const struct sock *sk,
+static inline bool retransmits_timed_out(struct sock *sk,
 					 unsigned int boundary)
 {
 	unsigned int timeout, linear_backoff_thresh;
+	unsigned int start_ts;

 	if (!inet_csk(sk)->icsk_retransmits)
 		return false;

+	if (unlikely(!tcp_sk(sk)->retrans_stamp))
+		start_ts = TCP_SKB_CB(tcp_write_queue_head(sk))->when;
+	else
+		start_ts = tcp_sk(sk)->retrans_stamp;
+
 	linear_backoff_thresh = ilog2(TCP_RTO_MAX/TCP_RTO_MIN);

 	if (boundary <= linear_backoff_thresh)
@@ -1279,7 +1285,7 @@ static inline bool retransmits_timed_out(const struct sock *sk,
 		timeout = ((2 << linear_backoff_thresh) - 1) * TCP_RTO_MIN +
 			  (boundary - linear_backoff_thresh) * TCP_RTO_MAX;

-	return (tcp_time_stamp - tcp_sk(sk)->retrans_stamp) >= timeout;
+	return (tcp_time_stamp - start_ts) >= timeout;
 }

 static inline struct sk_buff *tcp_send_head(struct sock *sk)
-- 
1.6.4.4

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