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From: Thomas Gleixner <tglx@linutronix.de>
Subject: RE: [patch 14/30] drm/i915/pmu: Replace open coded kstat_irqs() copy
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Date: Fri, 11 Dec 2020 22:10:36 +0100
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To: David Laight <David.Laight@ACULAB.COM>, Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>, LKML <linux-kernel@vger.kernel.org>
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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