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

[Ahmed S. Darwish]

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.

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH