netdev - Re: [PATCH net] net, sched: fix soft lockup in tc

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Date:   Thu, 22 Dec 2016 11:05:10 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     David Miller <davem@...emloft.net>,
        Shahar Klein <shahark@...lanox.com>,
        Or Gerlitz <gerlitz.or@...il.com>,
        Roi Dayan <roid@...lanox.com>, Jiri Pirko <jiri@...lanox.com>,
        John Fastabend <john.fastabend@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH net] net, sched: fix soft lockup in tc_classify

On Wed, Dec 21, 2016 at 1:07 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
>
> Ok, you mean for net. In that case I prefer the smaller sized fix to be
> honest. It also covers everything from the point where we fetch the chain
> via cops->tcf_chain() to the end of the function, which is where most of
> the complexity resides, and only the two mentioned commits do the relock,

I really wish the problem is only about relocking, but look at the code,
the deeper reason why we have this bug is the complexity of the logic
inside tc_ctl_tfilter(): 1) the replay logic is hard, we have to make it
idempotent; 2) the request logic itself is hard, because of tc filter design
and implementation.

This is why I worry more than just relocking.

> so as a fix I think it's fine as-is. As mentioned, if there's need to
> refactor tc_ctl_tfilter() net-next would be better, imho.

Refactor is a too strong word, just moving the replay out is not a refactor.
The end result will be still smaller than your commit d936377414fadbafb4,
which is already backported to stable.