netdev - Re: act_mirred: remove spinlock in fast path

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Message-ID: <CANn89iJiPnNj1nBHn5v0DQ7fB5n8h1=_h-Ev0UhcYmJE4PehDw@mail.gmail.com>
Date:	Fri, 17 Jun 2016 14:40:30 -0700
From:	Eric Dumazet <edumazet@...gle.com>
To:	Cong Wang <xiyou.wangcong@...il.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 2:35 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
> 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.

No I did not. I am referring to the fact that we currently might read
m->tcfm_eaction multiple times.

Please explain what would be wrong reading a wrong pair of values ?

One packet might come to a wrong device in the unlikely case an admin
change all the fields during an update ?

Is it going to crash or reveal highly sensitive security data ?

If yes, then please send a patch. I considered all this when writing
my patch and maybe I was wrong.