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Date: Wed, 30 Oct 2013 11:48:44 +0000
From: James Hogan <james.hogan@imgtec.com>
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To: Peter Zijlstra <peterz@infradead.org>
Subject: Re: perf events ring buffer memory barrier on powerpc
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Cc: Michael Neuling <mikey@neuling.org>,
 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>,
 Vince Weaver <vince@deater.net>, Oleg Nesterov <oleg@redhat.com>,
 Linux PPC dev <linuxppc-dev@ozlabs.org>, Anton Blanchard <anton@samba.org>,
 Frederic Weisbecker <fweisbec@gmail.com>,
 Victor Kaplansky <VICTORK@il.ibm.com>,
 "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
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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