linux-kernel - Re: [PATCH v3 05/10] perf auxtrace: Drop legacy _

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Date:   Sun, 11 Jul 2021 15:41:16 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mike Leach <mike.leach@...aro.org>,
        linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
        coresight@...ts.linaro.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 05/10] perf auxtrace: Drop legacy __sync functions

On Sat, Jul 10, 2021 at 03:34:24PM +0300, Adrian Hunter wrote:
> On 4/07/21 10:16 am, Leo Yan wrote:
> > The main purpose for using __sync built-in functions is to support
> > compat mode for 32-bit perf with 64-bit kernel.  But using these
> > built-in functions might cause couple potential issues.
> > 
> > Firstly, __sync functions originally support Intel Itanium processoer [1]
> > but it cannot promise to support all 32-bit archs.  Now these
> > functions have become the legacy functions.
> > 
> > As Peter also pointed out the logic issue in the function
> > auxtrace_mmap__write_tail(), it does a cmpxchg with 0 values to load
> > old_tail, and then executes a further cmpxchg with old_tail to write
> > the new tail.  If consider the aux_tail might be assigned to '0' in the
> > middle of loops, this can introduce mess for AUX buffer if the kernel
> > fetches the temporary value '0'.
> 
> That is not exactly true. The definition of __sync_*_compare_and_swap is
> "if the current value of *ptr is oldval, then write newval into *pt"
> so replacing zero with zero won't make any difference, but it will return
> the old value in any case.  Probably better to leave out that paragraph.

Okay, I admit the paragraph is not right, will drop it to avoid
confusion.  Thanks for review!

Leo