Commit 07f29bc5 authored Dec 07, 2009 by Damian Lukowski Committed by David S. Miller Dec 08, 2009

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
6fa12c85 (Revert Backoff [v3]:
Calculate TCP's connection close threshold as a time value.).

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

Reported-by: Frederic Leroy <fredo@starox.org>
Signed-off-by: Damian Lukowski <damian@tvk.rwth-aachen.de>
Acked-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
Signed-off-by: David S. Miller <davem@davemloft.net>

parent cb190546

include/net/tcp.h

+8 −2

Original line number	Diff line number	Diff line
		@@ -1265,14 +1265,20 @@ 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)
		@@ -1281,7 +1287,7 @@ 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)

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