linux-kernel - Re: seqcount usage in xt_replace

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Message-ID: <20190110222920.jqpzzv74huc2lb7f@breakpoint.cc>
Date:   Thu, 10 Jan 2019 23:29:20 +0100
From:   Florian Westphal <fw@...len.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Florian Westphal <fw@...len.de>,
        Anatol Pomozov <anatol.pomozov@...il.com>,
        Dmitry Vyukov <dvyukov@...gle.com>, paulmck@...ux.ibm.com,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()

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().

> 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.