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Date:   Fri, 11 Jan 2019 09:34:11 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Florian Westphal <fw@strlen.de>
Cc:     Anatol Pomozov <anatol.pomozov@gmail.com>,
        Dmitry Vyukov <dvyukov@google.com>, paulmck@linux.ibm.com,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()
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On Thu, Jan 10, 2019 at 11:29:20PM +0100, Florian Westphal wrote:
> Peter Zijlstra <peterz@infradead.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.