linux-kernel - Re: seqcount usage in xt_replace

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Date:   Thu, 10 Jan 2019 13:41:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Florian Westphal <fw@...len.de>
Cc:     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()

On Tue, Jan 08, 2019 at 11:37:46PM +0100, Florian Westphal wrote:
> 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).
> 

For:

	/*
	 * Ensure contents of newinfo are visible before assigning to
	 * private.
	 */
	smp_wmb();
	table->private = newinfo;

we have:

	smp_store_release(&table->private, newinfo);

But what store does that second smp_wmb() order against? The comment:

	/* make sure all cpus see new ->private value */
	smp_wmb();

makes no sense what so ever, no smp_*() barrier can provide such
guarantees.

> 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.

Is performance a concern in this path? There is no comment justifying
this 'creative' stuff.