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

[Oleksandr Natalenko]

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?

Regards,
  Oleksandr.

[1] http://i.piccy.info/
i9/6f5cb187c4ff282d189f78c63f95af43/1450729403/283985/951663/panic.jpg
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