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

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Date:	Mon, 26 May 2008 18:23:28 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>
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+


* Ilpo Järvinen <ilpo.jarvinen@...sinki.fi> wrote:

> On Mon, 26 May 2008, Ingo Molnar wrote:
> 
> > there's a hung distcc task on the system, waiting for socket action 
> > forever:
> > 
> >   [root@...ope ~]# strace -fp 19578
> >   Process 19578 attached - interrupt to quit
> >   select(5, NULL, [4], [4], {82, 90000} <unfinished ...>
> 
> Hmm, readfds is NULL isn't it?!? Are you sure you straced the right 
> process?

yes, i'm stracing the task that is hung unexpectedly.

> > disturbing that task via strace did not change the state of the 
> > socket - and that's not unexpected as it's a select(). [TCP state 
> > might be affected if strace impacted a recvmsg or a sendmsg wait 
> > directly.]
> 
> I fail to understand this paragraph due to excessive negation... :-)

i mean, sometimes a TCP connection can get 'unstuck' if you strace a 
task - that is because the TCP related syscall the task sits in gets 
interrupted. But in this case it's select() which doesnt explicitly take 
the socket, doesnt do any tcp_push_pending_frames() processing, etc. - 
it just its on the socket waitqueue AFAICS. And that's expected.

	Ingo
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