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

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Date:	Tue, 22 Dec 2015 22:13:36 +0200
From:	Oleksandr Natalenko <oleksandr@...alenko.name>
To:	Yuchung Cheng <ycheng@...gle.com>
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

That is correct, I have net.ipv4.tcp_ecn set to 1.

I've recompiled the kernel with proposed patch, now still waiting for issue to 
be triggered.

Could I manually simulate the erroneous TCP ECN behavior to speed up the 
debugging?

On понеділок, 21 грудня 2015 р. 18:10:32 EET Yuchung Cheng 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


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