linux-kernel - RE: perf events ring buffer memory barrier on powerpc

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Date:	Thu, 31 Oct 2013 14:55:25 +0200
From:	Victor Kaplansky <VICTORK@...ibm.com>
To:	"David Laight" <David.Laight@...lab.com>
Cc:	"Anton Blanchard" <anton@...ba.org>,
	"Frederic Weisbecker" <fweisbec@...il.com>,
	"LKML" <linux-kernel@...r.kernel.org>,
	"Linux PPC dev" <linuxppc-dev@...abs.org>,
	"Mathieu Desnoyers" <mathieu.desnoyers@...ymtl.ca>,
	"Michael Neuling" <mikey@...ling.org>,
	"Oleg Nesterov" <oleg@...hat.com>, paulmck@...ux.vnet.ibm.com,
	"Peter Zijlstra" <peterz@...radead.org>
Subject: RE: perf events ring buffer memory barrier on powerpc

"David Laight" <David.Laight@...lab.com> wrote on 10/31/2013 02:28:56 PM:

> So even though the wmb() in the writer ensures the writes are correctly
> ordered, the reader can read the old value from the second location from
> its local cache.

In case of circular buffer, the only thing that producer reads is @tail,
and nothing wrong will happen if producer reads old value of @tail.
Moreover,
adherents of smp_mb() insert it *after* the read of @tail, so it cannot
prevent reading of old value anyway.
-- Victor

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