linux-kernel - Re: "cpufreq: fix serialization issues with freq change notifiers" breaks cpufreq too

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Date:	Thu, 12 Sep 2013 06:09:38 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Guennadi Liakhovetski <g.liakhovetski@....de>,
	Greg KH <greg@...ah.com>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	SH-Linux <linux-sh@...r.kernel.org>,
	Magnus Damm <magnus.damm@...il.com>
Subject: Re: "cpufreq: fix serialization issues with freq change notifiers"
 breaks cpufreq too

On 11 September 2013 18:48, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> On Wednesday, September 11, 2013 02:08:44 PM Viresh Kumar wrote:

>> That looked like a straight forward issue/bug to me and so I haven't
>> gotten deep into it..
>
> Which you should always do when you're going to deal with concurrency issues.
> Even if they appear to be obvious, they often are far from that, like in this
> case.

/me Nods

>> Scenario 2:
>> --------------
>> Governor is changing freq and has called __cpufreq_driver_target().
>> At the same time we are changing scaling_{min|max}_freq from
>> sysfs, which would eventually end up calling governors:
>> CPUFREQ_GOV_LIMITS notification, that will also call:
>> __cpufreq_driver_target()..
>>
>> So, we eventually have two concurrent calls to ->target() and we
>> don't really know how hardware will behave in this case.. Most of
>> the implementations of ->target() routines just go and change
>> freq/voltage without checking if we are already in progress of doing
>> that (i.e. based on expectation that this call is not re entrant)..
>>
>> Now anything can happen at hardware level, which I don't have
>> all insight of :(
>
> That is more theoretical, however.

Maybe we can get more deeper into it then :)
Platform have something like this in their target()

A. If new freq is more than old: Increase voltage
B. Change freq
C. If new freq is less than old: decrease voltage

Now, two concurrent calls to target are X and Y, where X is trying to increase
freq and Y is trying to decrease it..

And this is the sequence that followed due to races..

X.A: voltage increased for larger freq
Y.A: nothing happened here
Y.B: freq decreased
Y.C: voltage decreased
X.B: freq increased
X.C: nothing happened..

We ended up setting a freq which is not supported by the voltage we have
set.. That will probably make clock to CPU unstable and system wouldn't
be workable anymore...

And so I think even this case must also get some space in the changelog :)
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