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

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Message-ID: <20180607083229.GJ12198@hirez.programming.kicks-ass.net>
Date:   Thu, 7 Jun 2018 10:32:29 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        Viresh Kumar <viresh.kumar@...aro.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 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.