netdev - Re: [PATCHv2 2/2] tcp: Stalling connections: Fix timeout calculation routine

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Date:	Mon, 7 Dec 2009 14:08:03 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Damian Lukowski <damian@....rwth-aachen.de>
cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Netdev <netdev@...r.kernel.org>,
	Frederic Leroy <fredo@...rox.org>,
	David Miller <davem@...emloft.net>,
	Herbert Xu <herbert@...dor.apana.org.au>,
	Greg KH <gregkh@...e.de>
Subject: Re: [PATCHv2 2/2] tcp: Stalling connections: Fix timeout calculation
 routine

On Mon, 7 Dec 2009, Damian Lukowski wrote:

> 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.
> 
> Thanks to Ilpo Järvinen for code suggestions and Frederic Leroy for
> testing.
> 
> Signed-off-by: Damian Lukowski <damian@....rwth-aachen.de>
> ---
>  net/ipv4/tcp_timer.c |    9 ++++++++-
>  1 files changed, 8 insertions(+), 1 deletions(-)
> 
> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
> index 5c5f739..a9d2891 100644
> --- a/net/ipv4/tcp_timer.c
> +++ b/net/ipv4/tcp_timer.c
> @@ -140,10 +140,17 @@ static bool retransmits_timed_out(const 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(
> +					(struct sock *)sk))->when;

Grr, a cast.... 

> +	else
> +		start_ts = tcp_sk(sk)->retrans_stamp;
> +
>  	linear_backoff_thresh = ilog2(TCP_RTO_MAX/TCP_RTO_MIN);
>  
>  	if (boundary <= linear_backoff_thresh)
> @@ -152,7 +159,7 @@ static 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;
>  }
>  
>  /* A write timeout has occurred. Process the after effects. */
> 

Also, in here it's more useful to provide the fix as the first patch (1/2) 
since it's going to stable and those people don't want the move patch 
there.

-- 
 i.