netdev - Re: tcp crash in net-2.6 tree

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Message-Id: <20070330150603.edeb2023.akpm@linux-foundation.org>
Date:	Fri, 30 Mar 2007 15:06:03 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	David Miller <davem@...emloft.net>
Cc:	ilpo.jarvinen@...sinki.fi, netdev@...r.kernel.org
Subject: Re: tcp crash in net-2.6 tree

On Fri, 30 Mar 2007 14:43:47 -0700 (PDT)
David Miller <davem@...emloft.net> wrote:

> From: "Ilpo_J__rvinen" <ilpo.jarvinen@...sinki.fi>
> Date: Fri, 30 Mar 2007 17:33:28 +0300 (EEST)
> 
> > If there is nothing at high_seq (application hasn't given any data to/past 
> > that point), the search fails to find any skb and returns NULL... But I 
> > have no idea how this can happen? As TCP does after(skb->seq, 
> > tp->high_seq) (even in the quoted code block) guaranteeing that something 
> > is there after the high_seq for TCP to step temporarily on... So at least 
> > one skb should have it's end_seq after tp->high_seq (actually there 
> > should be at least two valid skbs after tp->high_seq since the used 
> > sequence number space does not have holes), which should be enough to get 
> > an existing skb from write_queue_find?!
> > 
> > I also checked all call paths to tcp_update_scoreboard_fack to make sure 
> > that snd_una hasn't gone past high_seq and found nothing suspicious (and 
> > that wouldn't return NULL anyway I think)...
> 
> Let's not speculate, let's find out for sure if snd_una is
> surpassing high_seq while we're in this state.
> 
> Andrew please give this debugging patch a spin,

OK, will take a look at that this evening, hopefully.

> and also what
> is your workload?  I'd like to play with it too.

I use an x86_64 box as a distcc server: shove .i fiels at it, get .o files
sent back.  I was using it thusly and noticed that it had died.

Also, an x86_64 box I have here at google was hanging yesterday and that
appears to have stopped since I removed a couple of x86_64 patches and
git-net.  I'm in the process of working out what fixed it...


> I've tried to code this patch so that if the bug triggers your
> machine shouldn't crash and burn completely, just spit out the
> log message.

ok..  I don't know how repeatable the distcc crash is.  We'll see.

distccd seems to be rather good at triggering networking problems - I think
that's the third one I've seen in the past few years.


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