linux-kernel - Re: [PATCH v2 1/6] sched_clock: Expose struct clock_read

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Date:   Wed, 15 Jul 2020 14:54:07 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     "Ahmed S. Darwish" <a.darwish@...utronix.de>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Cercueil <paul@...pouillou.net>,
        "Ben Dooks (Codethink)" <ben.dooks@...ethink.co.uk>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>, jogness@...utronix.de,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/6] sched_clock: Expose struct clock_read_data

On Wed, Jul 15, 2020 at 07:56:50AM +0200, Ahmed S. Darwish wrote:
> On Wed, Jul 15, 2020 at 10:05:07AM +0800, Leo Yan wrote:
> > From: Peter Zijlstra <peterz@...radead.org>
> >
> ...
> >
> > Provide struct clock_read_data and two (seqcount) helpers so that
> > architectures (arm64 in specific) can expose the numbers to userspace.
> >
> ...
> >
> > +struct clock_read_data *sched_clock_read_begin(unsigned int *seq)
> > +{
> > +	*seq = raw_read_seqcount(&cd.seq);
> > +	return cd.read_data + (*seq & 1);
> > +}
> > +
> ...
> 
> Hmm, this seqcount_t is actually a latch seqcount. I know the original
> code also used raw_read_seqcount(), but while at it, let's use the
> proper read API for seqcount_t latchers: raw_read_seqcount_latch().

Good point.  To be honest, I think myself cannot give a good judgement
for memory barrier related thing :)

I read a bit the document for the latch technique [1], comparing to
raw_read_seqcount_latch(), the function raw_read_seqcount() contains
smp_rmb(), IIUC, the *read* memory barrier is used to support for
kcsan.

The usage for smp_rmb() and kcsan flow is like below:

  sched_clock_read_begin()
    `-> raw_read_seqcount()
          `-> smp_rmb()
                `-> kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX)

  sched_clock_read_retry()
    `-> read_seqcount_retry()
          `-> smp_rmb()
                `-> kcsan_atomic_next(0)

So the question is: should we support kcsan or not in this flow?

> raw_read_seqcount_latch() has no read memory barrier though, and a
> suspicious claim that READ_ONCE() pairs with an smp_wmb() (??). But if
> its implementation is wrong, let's fix it there instead.

I don't think we need pair with smp_wmb(), since it's mainly related
with data reading, so a smp_rmb() would be sufficient [2].

Thanks,
Leo

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/seqlock.h?h=v5.8-rc5#n321
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/linux/seqlock.h?h=v5.8-rc5#n373