netdev - Re: act_mirred: remove spinlock in fast path

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Date:	Fri, 17 Jun 2016 14:35:58 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Eric Dumazet <edumazet@...gle.com>
Cc:	Jamal Hadi Salim <jhs@...atatu.com>,
	David Miller <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: act_mirred: remove spinlock in fast path

On Fri, Jun 17, 2016 at 2:24 PM, Eric Dumazet <edumazet@...gle.com> wrote:
> Well, I added a READ_ONCE() to read tcf_action once.
>
> Adding rcu here would mean adding a pointer and extra cache line, to
> deref the values.
>
> IMHO the race here has no effect . You either read the old or new value.

Sure, the point is we may read a new ->tcf_action and an old ->tcfm_eaction,
this is what I am worrying.

If that is not a good example, what about new ->tcf_action and ->tcfm_eaction,
with an old ->tcfm_ifindex?

>
> If the packet is processed before or after the 'change' it would have
> the same 'race'
>

Why? As long as the change is like a transaction, we are safe.

> All these fields are integers, they never are 'partially written'.
>
> The only case m->tcfm_eaction could be read twice is in the error
> path. Who cares ?

This is not what I worry about. I guess you miss read eaction with action.