linux-kernel - Re: Question about barriers for ARM on tools/perf/

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Message-ID: <554CC9FE.7030509@gmail.com>
Date:	Fri, 08 May 2015 08:36:46 -0600
From:	David Ahern <dsahern@...il.com>
To:	Will Deacon <will.deacon@....com>,
	Peter Zijlstra <peterz@...radead.org>
CC:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...hat.com>,
	Namhyung Kim <namhyung@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Question about barriers for ARM on tools/perf/

On 5/8/15 8:27 AM, Will Deacon wrote:
> On Fri, May 08, 2015 at 03:25:13PM +0100, Peter Zijlstra wrote:
>> On Fri, May 08, 2015 at 03:21:08PM +0100, Will Deacon wrote:
>>> Wouldn't it be better to go the other way, and use compiler builtins for
>>> the memory barriers instead of relying on the kernel? It looks like the
>>> perf_mmap__{read,write}_head functions are basically just acquire/release
>>> operations and could therefore be implemented using something like
>>> __atomic_load_n(&pc->data_head, __ATOMIC_ACQUIRE) and
>>> __atomic_store_n(&pc->data_tail, tail, __ATOMIC_RELEASE).
>>
>> He wants to do smp refcounting, which needs atomic_inc() /
>> atomic_inc_non_zero() / atomic_dec_return() etc..
>
> Right, of course, but GCC has those too:
>
>    https://gcc.gnu.org/onlinedocs/gcc/_005f_005fatomic-Builtins.html

And we need a solution that works from RHEL5 forward. Not sure what gcc 
version that is; RHEL6 uses 4.4.7. We have done a prototype with the 
__sync functions and it worked nicely.

David

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