linux-kernel - Re: seqcount usage in xt_replace

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Message-ID: <20190110201829.GI2861@worktop.programming.kicks-ass.net>
Date:   Thu, 10 Jan 2019 21:18:29 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dmitry Vyukov <dvyukov@...gle.com>
Cc:     Florian Westphal <fw@...len.de>,
        Anatol Pomozov <anatol.pomozov@...il.com>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()

On Thu, Jan 10, 2019 at 01:53:28PM +0100, Dmitry Vyukov wrote:
> On Thu, Jan 10, 2019 at 1:41 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > 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.
> 
> Do we want WRITE_ONCE here then?

The smp_store_release() already implies WRITE_ONCE().