linux-kernel - Re: seqcount usage in xt_replace

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Date:   Fri, 11 Jan 2019 06:08:10 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Florian Westphal <fw@...len.de>,
        Anatol Pomozov <anatol.pomozov@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()

On Fri, Jan 11, 2019 at 09:34:11AM +0100, Peter Zijlstra wrote:
> On Thu, Jan 10, 2019 at 11:29:20PM +0100, Florian Westphal wrote:
> > Peter Zijlstra <peterz@...radead.org> wrote:
> > > Would using synchronize_rcu() not also mean you can get rid of that
> > > xt_write_recseq*() stuff entirely?
> > 
> > No, because those are used to synchronize with cpus that read
> > the ruleset counters, see
> > 
> > net/ipv4/netfilter/ip_tables.c:get_counters().
> 
> Ah, bummer :/
> 
> > > Anyway, synchronize_rcu() can also take a little while, but I don't
> > > think anywere near 30 seconds.
> > 
> > Ok, I think in that case it would be best to just replace the
> > recseq value sampling with smp_mb + synchronize_rcu plus a comment
> > that explains why its done.
> 
> synchronize_rcu() implies smp_mb() on all CPUs.

Yes, it does, but in the case of idle CPUs, the smp_mb() calls are only
required to follow any pre-existing RCU read-side critical section on
the one hand an precede any RCU read-side critical section completing
after the synchronize_rcu() on the other.

To do more would mean waking up idle CPUs, which does not make the
battery-powered guys happy.  ;-)

							Thanx, Paul