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Subject: Re: [PATCH v1 2/2] perf auxtrace: Optimize barriers with load-acquire
 and store-release
To:     Leo Yan <leo.yan@linaro.org>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        linux-perf-users@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20210519140319.1673043-1-leo.yan@linaro.org>
 <20210519140319.1673043-2-leo.yan@linaro.org>
 <20210531151049.GE9324@leoy-ThinkPad-X240s>
From:   Adrian Hunter <adrian.hunter@intel.com>
Organization: Intel Finland Oy, Registered Address: PL 281, 00181 Helsinki,
 Business Identity Code: 0357606 - 4, Domiciled in Helsinki
Message-ID: <cc3810cd-5edc-26d3-9c77-8bb6479152c1@intel.com>
Date:   Mon, 31 May 2021 22:03:33 +0300
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On 31/05/21 6:10 pm, Leo Yan wrote:
> Hi Peter, Adrian,
> 
> On Wed, May 19, 2021 at 10:03:19PM +0800, Leo Yan wrote:
>> Load-acquire and store-release are one-way permeable barriers, which can
>> be used to guarantee the memory ordering between accessing the buffer
>> data and the buffer's head / tail.
>>
>> This patch optimizes the memory ordering with the load-acquire and
>> store-release barriers.
> 
> Is this patch okay for you?
> 
> Besides this patch, I have an extra question.  You could see for
> accessing the AUX buffer's head and tail, it also support to use
> compiler build-in functions for atomicity accessing:
> 
>   __sync_val_compare_and_swap()
>   __sync_bool_compare_and_swap()
> 
> Since now we have READ_ONCE()/WRITE_ONCE(), do you think we still need
> to support __sync_xxx_compare_and_swap() atomicity?

I don't remember, but it seems to me atomicity is needed only
for a 32-bit perf running with a 64-bit kernel.

> 
> I checked the code for updating head and tail for the perf ring buffer
> (see ring_buffer_read_head() and ring_buffer_write_tail() in the file
> tools/include/linux/ring_buffer.h), which doesn't support
> __sync_xxx_compare_and_swap() anymore.  This is why I wander if should
> drop __sync_xxx_compare_and_swap() atomicity for AUX ring buffer as
> well.
> 
> Thanks,
> Leo
>