linux-kernel - Re: seqcount usage in xt_replace

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Message-ID: <20190108223746.shuwx3ro7cgwz7hh@breakpoint.cc>
Date:   Tue, 8 Jan 2019 23:37:46 +0100
From:   Florian Westphal <fw@...len.de>
To:     Anatol Pomozov <anatol.pomozov@...il.com>
Cc:     fw@...len.de, Dmitry Vyukov <dvyukov@...gle.com>,
        paulmck@...ux.ibm.com, LKML <linux-kernel@...r.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()

Anatol Pomozov <anatol.pomozov@...il.com> wrote:
> Or maybe xt_replace_table() can be enhanced? When I hear that
> something waits until an event happens on all CPUs I think about
> wait_event() function. Would it be better for xt_replace_table() to
> introduce an atomic counter that is decremented by CPUs, and the main
> CPU waits until the counter gets zero?

That would mean placing an additional atomic op into the
iptables evaluation path (ipt_do_table and friends).

Only alternative I see that might work is synchronize_rcu (the
_do_table functions are called with rcu read lock held).

I guess current scheme is cheaper though.