linux-kernel - Re: [REGRESSION] tcp/ipv4: kernel panic because of (possible) division by zero

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Date:	Wed, 6 Jan 2016 10:19:10 -0800
From:	Yuchung Cheng <ycheng@...gle.com>
To:	Oleksandr Natalenko <oleksandr@...alenko.name>
Cc:	netdev <netdev@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Patrick McHardy <kaber@...sh.net>,
	Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
	James Morris <jmorris@...ei.org>,
	Alexey Kuznetsov <kuznet@....inr.ac.ru>,
	"David S. Miller" <davem@...emloft.net>
Subject: Re: [REGRESSION] tcp/ipv4: kernel panic because of (possible)
 division by zero

On Wed, Jan 6, 2016 at 8:50 AM, Oleksandr Natalenko
<oleksandr@...alenko.name> wrote:
>
> Unfortunately, the patch didn't help -- I've got the same stacktrace with slightly different offset (+3) within the function.
>
> Now trying to get full stacktrace via netconsole. Need more time.
>
> Meanwhile, any other ideas on what went wrong?

That's odd b/c the patch already checks and avoids div0. Can post me
the stacktrace and kernel warnings if any ...

One possibility is that tcp_cwnd_reduction() may set a cwnd of 0,
which then gets used to start another recovery phase. This may or may
not be the culprit of this div0 issue because I wasn't able to
reproduce exactly your issue on our servers. But I will post the fix
today and CC you.

>
>
> On December 22, 2015 4:10:32 AM EET, Yuchung Cheng <ycheng@...gle.com> wrote:
> >On Mon, Dec 21, 2015 at 12:25 PM, Oleksandr Natalenko
> ><oleksandr@...alenko.name> wrote:
> >> Commit 3759824da87b30ce7a35b4873b62b0ba38905ef5 (tcp: PRR uses CRB
> >mode by
> >> default and SS mode conditionally) introduced changes to
> >net/ipv4/tcp_input.c
> >> tcp_cwnd_reduction() that, possibly, cause division by zero, and
> >therefore,
> >> kernel panic in interrupt handler [1].
> >>
> >> Reverting 3759824da87b30ce7a35b4873b62b0ba38905ef5 seems to fix the
> >issue.
> >>
> >> I'm able to reproduce the issue on 4.3.0–4.3.3 once per several day
> >> (occasionally).
> >>
> >> What could be done to help in debugging this issue?
> >Do you have ECN enabled (i.e. sysctl net.ipv4.tcp_ecn > 0)?
> >
> >If so I suspect an ACK carrying ECE during CA_Loss causes entering CWR
> >state w/o calling tcp_init_cwnd_reduct() to set tp->prior_cwnd. Can
> >you try this debug / quick-fix patch and send me the error message if
> >any?
> >
> >
> >>
> >> Regards,
> >>   Oleksandr.
> >>
> >> [1] http://i.piccy.info/
> >>
> >i9/6f5cb187c4ff282d189f78c63f95af43/1450729403/283985/951663/panic.jpg
>
> --
> Sent from my Android device with K-9 Mail. Please excuse my brevity.
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