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

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Date:   Thu, 7 Jun 2018 11:32:01 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Daniel Lezcano <daniel.lezcano@...aro.org>
Cc:     Viresh Kumar <viresh.kumar@...aro.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 Thu, Jun 07, 2018 at 11:09:13AM +0200, Daniel Lezcano wrote:
> 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 ?

I think the above can happen when the compiler uses the variable as a
scratch pad -- very rare I would say.

In general a compiler needs to proof that doing this makes no observable
difference ("as-if" rule). And since it is a regular variable it can
assume data-race-free and do the above (or something like that). Because
if there is a data-race it is UB and it can still do whatever it
pleases.

And here I think the point is that regular variables are considered only
in the context of a single linear execution context. Locks are assumed
to bound observability.

And here the "volatile" and "_atomic" type specifiers again tell the
compiler something 'special' is going on and you should not muck with
things.