RE: [patch 14/30] drm/i915/pmu: Replace open coded kstat_irqs() copy

[Thomas Gleixner <tglx@xxxxxxxxxxxxx>]

On Fri, Dec 11 2020 at 14:19, David Laight wrote:
> From: Thomas Gleixner
>> You can't catch that. If this really becomes an issue you need a
>> sequence counter around it.
>
> Or just two copies of the high word.
> Provided the accesses are sequenced:
> writer:
>       load high:low
>       add small_value,high:low
>       store high
>       store low
>       store high_copy
> reader:
>       load high_copy
>       load low
>       load high
>       if (high != high_copy)
>               low = 0;

And low = 0 is solving what? You need to loop back and retry until it's
consistent and then it's nothing else than an open coded sequence count.

Thanks,

        tglx