Re: [PATCH v2 1/6] sched_clock: Expose struct clock_read_data

From: "Ahmed S. Darwish" <a.darwish@linutronix.de>
To: peterz@infradead.org
Cc: Mark Rutland <mark.rutland@arm.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Jiri Olsa <jolsa@redhat.com>,
	Adrian Hunter <adrian.hunter@intel.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	linux-kernel@vger.kernel.org,
	Paul Cercueil <paul@crapouillou.net>,
	Ingo Molnar <mingo@redhat.com>, Leo Yan <leo.yan@linaro.org>,
	Namhyung Kim <namhyung@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	jogness@linutronix.de, Will Deacon <will@kernel.org>,
	"Ben Dooks \(Codethink\)" <ben.dooks@codethink.co.uk>,
	linux-arm-kernel@lists.infradead.org,
	Kan Liang <kan.liang@linux.intel.com>
Subject: Re: [PATCH v2 1/6] sched_clock: Expose struct clock_read_data
Date: Wed, 15 Jul 2020 11:23:45 +0200	[thread overview]
Message-ID: <20200715092345.GA231464@debian-buster-darwi.lab.linutronix.de> (raw)
In-Reply-To: <20200715081443.GB43129@hirez.programming.kicks-ass.net>

On Wed, Jul 15, 2020 at 10:14:43AM +0200, peterz@infradead.org wrote:
> On Wed, Jul 15, 2020 at 10:12:22AM +0200, Peter Zijlstra wrote:
> > 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@infradead.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().
> > >
> > > 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.
> >
> > It's supposed to be a dependent load, so READ_ONCE() is sufficient.
> > Except, of course, the C standard has other ideas, so a compiler is
> > allowed to wreck that, but they mostly don't :-)
>
> Also see:
>
>   https://lkml.kernel.org/r/20200625085745.GD117543@hirez.programming.kicks-ass.net

Oh, spot on.

Can we then please replace the raw_read_seqcount(), in the original
patch which started this discussion, with raw_read_seqcount_latch()?

I see that you already did something similar for timekeeping.c:
7fc26327b756 ("seqlock: Introduce raw_read_seqcount_latch()").

Confession time: I have an internal patch series which introduces
seqcount_latch_t. Having a standardized set of accessors for the
seqcount latch read and write paths will make everything much more
streamlined :-)

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH

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