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

[David Laight]

From: Thomas Gleixner
> Sent: 11 December 2020 21:11
> 
> 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.

If it is a counter or timestamp then the high:0 value happened
some time between when you started trying to read the value and
when you finished trying to read it.

As such it is a perfectly reasonable return value.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)