netdev - Re: act_mirred: remove spinlock in fast path

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Date:	Sat, 18 Jun 2016 08:16:06 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Jamal Hadi Salim <jhs@...atatu.com>
Cc:	Eric Dumazet <edumazet@...gle.com>,
	Cong Wang <xiyou.wangcong@...il.com>,
	David Miller <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: act_mirred: remove spinlock in fast path

On Sat, 2016-06-18 at 09:45 -0400, Jamal Hadi Salim wrote:
> 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_ ;->

The potential 'problem' is not the write side, but the read side.

If you read say 3 values <A, B, C>  you might want to read them in a
consistent way, instead of <new_A, old_B, old_C>