Re: perf events ring buffer memory barrier on powerpc

From: James Hogan
Date: Wed Oct 30 2013 - 07:50:24 EST

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.


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.


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