linux-kernel - Re: [PATCH v2 1/1] doc: Add remote CPU access details and others to this_cpu

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Message-ID: <53C85FE9.6010304@gmail.com>
Date:	Thu, 17 Jul 2014 19:44:41 -0400
From:	Pranith Kumar <bobby.prani@...il.com>
To:	Christoph Lameter <cl@...two.org>
CC:	rdunlap@...radead.org, linux-doc@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] doc: Add remote CPU access details and others
 to this_cpu_ops.txt

On 07/17/2014 11:19 AM, Christoph Lameter wrote:
> Regarding atomic_t in per cpu areas: I am uncomfortable especially
> because both locked and unlocked RMW write operations could be acting on
> values in the same cacheline. I am concerned that the unlocked operation
> could have an unpredictable result.
> 
> 
> f.e. the following per cpu data structure
> 
> struct test {
> 	atomic_t a;
> 	int b;
> } onecacheline;
> 
> 
> Local cpu does
> 
> 	this_cpu_inc(onecacheline.b);
> 
> If this is racing with a remote cpus:
> 
> 	atomic_inc(percpu(&a, cpu))
> 
> then we have on x86 a increment operation with locked semantics racing
> with an unlocked one on the same cacheline.
> 

OK, I will add this as a warning in the documentation. Thanks!

--
Pranith
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