From: Dmitry Vyukov <dvyukov@google.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Florian Westphal <fw@strlen.de>,
Anatol Pomozov <anatol.pomozov@gmail.com>,
"Paul E. McKenney" <paulmck@linux.ibm.com>,
LKML <linux-kernel@vger.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()
Date: Thu, 10 Jan 2019 13:53:28 +0100 [thread overview]
Message-ID: <CACT4Y+YZ-bOFM_vHN5GiE5Yh=qqGjO_cgsJD4cnNyupOd6-A-Q@mail.gmail.com> (raw)
In-Reply-To: <20190110124123.GA21224@hirez.programming.kicks-ass.net>
On Thu, Jan 10, 2019 at 1:41 PM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Tue, Jan 08, 2019 at 11:37:46PM +0100, Florian Westphal wrote:
> > Anatol Pomozov <anatol.pomozov@gmail.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?
We want it to be compiled to an actual memory access and then it's up
to hardware to make it visible to other CPUs.
smp_wmb should most likely have this as a side effect too, but
somewhat indirect.
Also race-detector-friendly.
> > 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.
>
next prev parent reply other threads:[~2019-01-10 12:53 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-01-08 19:33 seqcount usage in xt_replace_table() Anatol Pomozov
2019-01-08 22:37 ` Florian Westphal
2019-01-10 12:41 ` Peter Zijlstra
2019-01-10 12:53 ` Dmitry Vyukov [this message]
2019-01-10 20:18 ` Peter Zijlstra
2019-01-10 14:48 ` Florian Westphal
2019-01-10 20:20 ` Peter Zijlstra
2019-01-10 20:25 ` Peter Zijlstra
2019-01-10 22:29 ` Florian Westphal
2019-01-11 8:34 ` Peter Zijlstra
2019-01-11 14:08 ` Paul E. McKenney
2019-01-10 14:52 ` Paul E. McKenney
2019-01-09 0:02 ` Andrea Parri
2019-01-09 0:36 ` Anatol Pomozov
2019-01-09 5:35 ` Dmitry Vyukov
2019-01-09 11:24 ` Andrea Parri
2019-01-09 11:55 ` Dmitry Vyukov
2019-01-09 12:11 ` Andrea Parri
2019-01-09 12:29 ` Dmitry Vyukov
2019-01-09 17:10 ` Paul E. McKenney
2019-01-10 8:49 ` Dmitry Vyukov
2019-01-10 12:30 ` Andrea Parri
2019-01-10 12:38 ` Dmitry Vyukov
2019-01-10 12:46 ` Andrea Parri
2019-01-10 13:25 ` Dmitry Vyukov
2019-01-10 14:50 ` Paul E. McKenney
2019-01-10 12:44 ` Peter Zijlstra
2019-01-10 12:54 ` Dmitry Vyukov
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