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

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Message-ID: <CAK6E8=d1FquYTtMXi6v+koruRhA2MUe693-DX5jbU8urRatUoQ@mail.gmail.com>
Date:	Mon, 21 Dec 2015 18:10:32 -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 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

Download attachment "0001-tcp-debug-tcp_cwnd_reduction-div0.patch" of type "application/octet-stream" (1350 bytes)