On 9/12/2013 2:39 AM, Viresh Kumar wrote:
On 11 September 2013 18:48, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> 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()
Which platform?
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 :)
Yes, if you can point to a specific driver having this problem.
Thanks,
Rafael
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