netdev - Re: act_mirred: remove spinlock in fast path

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Message-ID: <57655062.1080407@mojatatu.com>
Date:	Sat, 18 Jun 2016 09:45:06 -0400
From:	Jamal Hadi Salim <jhs@...atatu.com>
To:	Eric Dumazet <edumazet@...gle.com>,
	Cong Wang <xiyou.wangcong@...il.com>
Cc:	David Miller <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: act_mirred: remove spinlock in fast path

On 16-06-17 06:03 PM, Eric Dumazet wrote:
> On Fri, Jun 17, 2016 at 2:59 PM, Cong Wang <xiyou.wangcong@...il.com> wrote:
>
>> Generally speaking I worry about we change multiple fields in a struct
>> meanwhile we could still read them any time in the middle, we may
>> get them correct for some easy case, but it is hard to insure the
>> correctness when the struct becomes large.
>>
>> I am thinking to make more tc actions lockless, so this problem
>> comes up immediately for other complex cases than mirred.
>
> I certainly wont object to a patch.
>
> Also note that instead of RCU with a pointer and the usual kfree_rcu() stuff,
> we now can use seqcount_latch infra which might allow to not increase
> memory foot print.
>

Given an update/replace of an action is such a rare occassion, what
is wrong with init doing a spin lock on existing action?
Sure, there is performance impact on fast path at that point - but:
as established update/replace is _a rare occassion_ ;->

cheers,
jamal