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From: Oleksandr Natalenko <oleksandr@natalenko.name>
To: Yuchung Cheng <ycheng@google.com>
Cc: netdev <netdev@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Patrick McHardy <kaber@trash.net>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        James Morris <jmorris@namei.org>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        "David S. Miller" <davem@davemloft.net>
Subject: Re: [REGRESSION] tcp/ipv4: kernel panic because of (possible) division by zero
Date: Wed, 06 Jan 2016 20:49:43 +0200
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Sure, but after catching the stacktrace.

On середа, 6 січня 2016 р. 10:43:45 EET Yuchung Cheng wrote:
> Could you turn off ecn (sysctl net.ipv4.tcp_ecn=0) to see if this still
> happen?


> >> On December 22, 2015 4:10:32 AM EET, Yuchung Cheng <ycheng@google.com> 
wrote:
> >> >On Mon, Dec 21, 2015 at 12:25 PM, Oleksandr Natalenko
> >> >
> >> ><oleksandr@natalenko.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.