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Date: Tue, 12 May 2015 15:00:43 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Douglas Hatch <doug.hatch@hp.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Waiman Long <Waiman.Long@hp.com>,
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	<linux-tip-commits@vger.kernel.org>
Subject: Re: [tip:locking/core] locking/pvqspinlock: Replace xchg() by the
 more descriptive set_mb()
Message-ID: <20150512130043.GF16478@twins.programming.kicks-ass.net>
References: <tip-52c9d2badd1ae4d11c29de57d4e964e48afd3cb4@git.kernel.org>
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On Tue, May 12, 2015 at 10:45:29AM +0200, Peter Zijlstra wrote:
> On Mon, May 11, 2015 at 10:50:42AM -0700, Linus Torvalds wrote:
> > On Mon, May 11, 2015 at 7:54 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> > >
> > > Hmm, so I looked at the set_mb() definitions and I figure we want to do
> > > something like the below, right?
> > 
> > I don't think you need to do this for the non-smp cases. 
> 
> Well, its the store tearing thing again, we use WRITE_ONCE() in
> smp_store_release() for the same reason. We want it to be a single
> store.
> 
> > The whole
> > thing is about smp memory ordering, so on UP you don't even need the
> > WRITE_ONCE(), much less a barrier.

Ah, you meant the memory barrier; indeed, a compiler barrier is
sufficient. I got somewhat confused between Waiman's email and barrier
and barrier() (again!).