"cpufreq: fix serialization issues with freq change notifiers" breaks cpufreq too

[Viresh Kumar]

On 11 September 2013 18:48, Rafael J. Wysocki <rjw at rjwysocki.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 :)