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

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Date:   Thu, 7 Jun 2018 10:46:04 +0200
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Viresh Kumar <viresh.kumar@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     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 10:42, Viresh Kumar wrote:
> On 07-06-18, 10:32, Peter Zijlstra wrote:
>> On Thu, Jun 07, 2018 at 10:18:27AM +0200, Daniel Lezcano wrote:
>>> So IIUC, neither atomic or WRITE|READ_ONCE are necessary in this code
>>> because of the wake_up_process() barrier is enough, right ?
>>
>> I didn't look hard enough; if there ever is a time where the loads and
>> stores happen concurrently, you need READ/WRITE_ONCE(). If there is no
>> concurrency on the variables, you don't need anything.
>>
>> Neither atomic_read/set() nor REAd/WRITE_ONCE() will help with ordering,
>> which is what the wake_up_process() would provide here, different things
>> entirely.
> 
> Right and you still need the READ/WRITE_ONCE() thing as
> idle_injection_set_duration() may run in parallel with the idle_injection_fn()
> thread.
> 
> And I don't think the purpose of atomic_read/write was ever to take care of the
> ordering issues in this code, it was always about parallel loads/stores.

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 ?


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