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

[peterz at infradead.org]

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 at 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