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

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Message-ID: <OFF6C78E48.E477DB1E-ON42257C14.0050F9B3-42257C14.0051AC6D@il.ibm.com>
Date:	Wed, 30 Oct 2013 16:52:05 +0200
From:	Victor Kaplansky <VICTORK@...ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Anton Blanchard <anton@...ba.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.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 Ellerman <michael@...erman.id.au>,
	Michael Neuling <mikey@...ling.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: perf events ring buffer memory barrier on powerpc

Peter Zijlstra <peterz@...radead.org> wrote on 10/30/2013 01:25:26 PM:

> Also, I'm not entirely sure on C, that too seems like a dependency, we
> simply cannot read the buffer @tail before we've read the tail itself,
> now can we? Similarly we cannot compare tail to head without having the
> head read completed.

No, this one we cannot omit, because our problem on consumer side is not
with @tail, which is written exclusively by consumer, but with @head.

BTW, it is why you also don't need ACCESS_ONCE() around @tail, but only
around
@head read.

-- Victor

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