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Subject: Re: [PATCH V5] powercap/drivers/idle_injection: Add an idle injection
 framework
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Viresh Kumar <viresh.kumar@linaro.org>, rjw@rjwysocki.net,
        linux-kernel@vger.kernel.org,
        Eduardo Valentin <edubezval@gmail.com>,
        Javi Merino <javi.merino@kernel.org>,
        Leo Yan <leo.yan@linaro.org>,
        Kevin Wangtao <kevin.wangtao@linaro.org>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Rui Zhang <rui.zhang@intel.com>,
        Daniel Thompson <daniel.thompson@linaro.org>,
        "open list:POWER MANAGEMENT CORE" <linux-pm@vger.kernel.org>
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From:   Daniel Lezcano <daniel.lezcano@linaro.org>
Message-ID: <d0d3f817-6cd1-fd4c-91e3-304090270efe@linaro.org>
Date:   Thu, 7 Jun 2018 14:31:54 +0200
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On 07/06/2018 11:39, Peter Zijlstra wrote:
> On Thu, Jun 07, 2018 at 11:32:01AM +0200, Peter Zijlstra wrote:
>> 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.
> 
> Also, I think, more likely:
> 
> 	if (cond)
> 		X = 5;
> 	else
> 		X = 4;
> 
> is allowed to be transformed into:
> 
> 	X = 4;
> 	if (cond)
> 		X = 5;
> 
> as long as cond doesn't involve a sequence point of sorts (think
> function call).
> 
> For the single execution context case, this transformation is valid, but
> it is not in the threaded case. But then we go back to the assumption
> that regular variables are data-race-free.

Thank you very much for the explanations.

  -- Daniel


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