linux-kernel - Re: [PATCH v1 2/2] perf auxtrace: Optimize barriers with load-acquire and store-release

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Message-ID: <YLUG7f3D4deX0/BF@hirez.programming.kicks-ass.net>
Date:   Mon, 31 May 2021 17:55:25 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] perf auxtrace: Optimize barriers with
 load-acquire and store-release

On Mon, May 31, 2021 at 11:10:49PM +0800, 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?

Not without actual numbers; that's some terrible ifdef soup.

> 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 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.

I'm not sure wth that code is even trying to do, that's some seriously
dodgy code.