linux-kernel - Re: [PATCH V5] powercap/drivers/idle_injection: Add an idle injection framework

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Message-ID: <d2b8c964-ea0f-624c-a178-721537893541@linaro.org>
Date:   Thu, 7 Jun 2018 11:09:13 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, rjw@...ysocki.net,
        linux-kernel@...r.kernel.org,
        Eduardo Valentin <edubezval@...il.com>,
        Javi Merino <javi.merino@...nel.org>,
        Leo Yan <leo.yan@...aro.org>,
        Kevin Wangtao <kevin.wangtao@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Rui Zhang <rui.zhang@...el.com>,
        Daniel Thompson <daniel.thompson@...aro.org>,
        "open list:POWER MANAGEMENT CORE" <linux-pm@...r.kernel.org>
Subject: Re: [PATCH V5] powercap/drivers/idle_injection: Add an idle injection
 framework

On 07/06/2018 10:49, Viresh Kumar wrote:
> On 07-06-18, 10:46, Daniel Lezcano wrote:
>> Yes, correct.
>>
>> But if we don't care about who wins to store to value, is there a risk
>> of scramble variable if we just assign a value ?
> 
> Normally no, as the compiler wouldn't screw it up badly. But there is no rule
> which stops the compiler from doing this:
> 
> idle_duration_ms = 5;
> idle_duration_ms = -5;
> idle_duration_ms = 0;
> idle_duration_ms = <real-value-we-want-to-write>;
> 
> So we *must* use READ/WRITE_ONCE() to make sure garbage values aren't seen by
> readers.

Ok understood. Why would a compiler do this kind of things ?

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