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SubjectRe: [PATCH] cpufreq, store_scaling_governor requires policy->rwsem to be held for duration of changing governors [v2]


On 08/04/2014 06:36 AM, Viresh Kumar wrote:
> Sorry for the delay guys, was away :(
>
>>
>> Possible unsafe locking scenario:
>>
>> CPU0 CPU1
>> ---- ----
>> lock(&policy->rwsem);
>> lock(s_active#9);
>> lock(&policy->rwsem);
>> lock(s_active#9);
>>
>
> Thanks for coming to my rescue Stephen :), I was quite sure I got this
> with ondemand
> as well..

Yeah, I'm confused by it a bit because I wasn't able to reproduce it. But I've
got some pretty clear instructions on how to do it.

>
> I will be looking very closely at the code now to see what's going wrong.
> And btw, does anybody here has the exact understanding of why this
> lockdep does happen? I mean what was the real problem for which we

The issue is the collision between the setup & teardown of the policy's governor
sysfs files.

On creation the kernel does:

down_write(&policy->rwsem)
mutex_lock(kernfs_mutex) <- note this is similar to the "old" sysfs_mutex.

The opposite happens on a governor switch, specifically the existing governor's
exit, and then we get a lockdep warning.

I tried to reproduce with the instructions but was unable to ... ut that was on
Friday ;) and I'm going to try again this morning. I've also ping'd some of the
engineers here in the office who are working on ARM to get access to a system to
do further analysis and testing.

I'll ping back later in the day with some results.

Sorry I don't have a better answer or solution yet, Viresh.

P.


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