From: Peter Zijlstra <peterz@infradead.org>
To: Florian Westphal <fw@strlen.de>
Cc: Anatol Pomozov <anatol.pomozov@gmail.com>,
Dmitry Vyukov <dvyukov@google.com>,
paulmck@linux.ibm.com, LKML <linux-kernel@vger.kernel.org>
Subject: Re: seqcount usage in xt_replace_table()
Date: Thu, 10 Jan 2019 21:20:28 +0100 [thread overview]
Message-ID: <20190110202028.GJ2861@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <20190110144812.mkbokbj2iyryj6lv@breakpoint.cc>
On Thu, Jan 10, 2019 at 03:48:12PM +0100, Florian Westphal wrote:
> Peter Zijlstra <peterz@infradead.org> wrote:
> > /*
> > * 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.
>
> IIRC I added this at the request of a reviewer of an earlier iteration
> of that patch.
>
> IIRC the concern was that compiler/hw could re-order
>
> smb_wmb();
> table->private = newinfo;
> /* wait until all cpus are done with old table */
>
> into:
>
> smb_wmb();
> /* wait until all cpus are done with old table */
> ...
> table->private = newinfo;
>
> and that (obviously) makes the wait-loop useless.
The thing is, the 'wait for all cpus' thing is pure loads, not stores,
so smp_wmb() is a complete NOP there.
If you want to ensure those loads happen after that store (which does
indeed seem like a sensible thing), you're going to have to use
smp_mb().
next prev parent reply other threads:[~2019-01-10 20:20 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
2019-01-10 20:18 ` Peter Zijlstra
2019-01-10 14:48 ` Florian Westphal
2019-01-10 20:20 ` Peter Zijlstra [this message]
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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