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

[Ahmed S. Darwish]

On Wed, Jul 15, 2020 at 02:54:07PM +0800, Leo Yan 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@xxxxxxxxxxxxx>
> > >
> > ...
> > >
> > > 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 smp_rmb() has no relation whatsoever to KCSAN. It pairs with the
write memory barriers in the seqcount_t write path.

AFAIK, PeterZ is the author of this patch, so let's wait for his input
here.

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH