netdev - Re: [bug] stuck localhost TCP connections, v2.6.26-rc3+

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Message-ID: <Pine.LNX.4.64.0805301436260.16829@wrl-59.cs.helsinki.fi>
Date:	Sat, 31 May 2008 00:12:23 +0300 (EEST)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Ingo Molnar <mingo@...e.hu>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Netdev <netdev@...r.kernel.org>,
	"David S. Miller" <davem@...emloft.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [bug] stuck localhost TCP connections, v2.6.26-rc3+

On Fri, 30 May 2008, Ingo Molnar wrote:

> * Ingo Molnar <mingo@...e.hu> wrote:
> 
> > btw., i now also have a hung socket over real network:
> 
> last night i turned off distcc support, and got about 200 successful 
> bootups and zero TCP hangs (as expected - there's not much TCP traffic 
> if the distcc cluster is not utilized).
> 
> but that's 200 overnight tests instead of the expected 600, so this is a 
> major and rather crippling bug to me.
> 
> There's no good way to detect these hung sockets by me from userspace 
> and get rid of them. Has anyone before thought of the obvious: to write 
> a kernel-space "TCP socket watchdog" kernel feature that detects them 
> and tries to free them so that people can become aware of it?
>
> Hung sockets is a re-occuring bug in the TCP stack after all. (and it's 
> a natural property of it: state machine designs are always vulnerable to 
> lost event problems.)

...It is quite easy to kill a TCP flow with enough information (seqnos 
which one could collect e.g., from a tcpdump), just a RST is necessary,
so user-space could do that too.

-- 
 i.
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