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

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Message-ID: <5270F21C.3080805@imgtec.com>
Date:	Wed, 30 Oct 2013 11:48:44 +0000
From:	James Hogan <james.hogan@...tec.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Vince Weaver <vince@...ter.net>,
	Victor Kaplansky <VICTORK@...ibm.com>,
	Oleg Nesterov <oleg@...hat.com>,
	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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-metag <linux-metag@...r.kernel.org>
Subject: Re: perf events ring buffer memory barrier on powerpc

Hi Peter,

On 30/10/13 10:42, Peter Zijlstra wrote:
> Subject: perf, tool: Add required memory barriers
> 
> To match patch bf378d341e48 ("perf: Fix perf ring buffer memory
> ordering") change userspace to also adhere to the ordering outlined.
> 
> Most barrier implementations were gleaned from
> arch/*/include/asm/barrier.h and with the exception of metag I'm fairly
> sure they're correct.

Yeh...

Short answer:
For Meta you're probably best off assuming
CONFIG_METAG_SMP_WRITE_REORDERING=n and just using compiler barriers.

Long answer:
The issue with write reordering between Meta's hardware threads beyond
the cache is only with a particular SoC, and SMP is not used in
production on it.
It is possible to make the LINSYSEVENT_WR_COMBINE_FLUSH register
writable to userspace (it's in a non-mappable region already) but even
then the write to that register needs odd placement to be effective
(before the shmem write rather than after - which isn't a place any
existing barriers are guaranteed to be placed). I'm fairly confident we
get away with it in the kernel, and userland normally just uses linked
load/store instructions for atomicity which works fine.

Cheers
James

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