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

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Message-ID: <20131030153931.GM16117@laptop.programming.kicks-ass.net>
Date:	Wed, 30 Oct 2013 16:39:31 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Victor Kaplansky <VICTORK@...ibm.com>
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

On Wed, Oct 30, 2013 at 04:52:05PM +0200, Victor Kaplansky wrote:
> 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.

Ah indeed, my argument was flawed in that @head is the important part.
But we still do a comparison of @tail against @head before we do further
reads.

Although I suppose speculative reads are allowed -- they don't have the
destructive behaviour speculative writes have -- and thus we could in
fact get reorder issues.

But since it is still a dependent load in that we do that @tail vs @head
comparison before doing other loads, wouldn't a read_barrier_depends()
be sufficient? Or do we still need a complete rmb?

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

Agreed, the ACCESS_ONCE() around tail is superfluous since we're the one
updating tail, so there's no problem with the value changing
unexpectedly.
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