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

[Yuchung Cheng]

On Mon, Dec 21, 2015 at 12:25 PM, Oleksandr Natalenko
<oleksandr@xxxxxxxxxxxxxx> 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 4.3.0â4.3.3 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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