linux-kernel - Re: cpufreq_stats NULL deref on second system suspend

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Message-ID: <5230CD17.6050200@linux.vnet.ibm.com>
Date:	Thu, 12 Sep 2013 01:35:43 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Stephen Warren <swarren@...dotorg.org>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	cpufreq <cpufreq@...r.kernel.org>
Subject: Re: cpufreq_stats NULL deref on second system suspend

On 09/12/2013 01:37 AM, Stephen Warren wrote:
> On 09/11/2013 01:46 PM, Srivatsa S. Bhat wrote:
>> On 09/12/2013 12:33 AM, Stephen Warren wrote:
>>> On 09/11/2013 12:42 PM, Srivatsa S. Bhat wrote:
>>> ...
>>>> OK, I took a second look at the code, and I suspect that applying the
>>>> second patch might help. So can you try by applying both the patches
>>>> please[1][2]?
>>>>
>>> ...
>>>> [1]. http://marc.info/?l=linux-kernel&m=137889516210816&w=2
>>>> [2]. http://marc.info/?l=linux-kernel&m=137889800511940&w=2
>>>
>>> Yes, with both of those patches applies, the problem is solved:-)
>>>
>>> I was going to test the second patch originally, but it sounded like it
>>> was more of a cleanup rather than a fix for my issue, so I didn't bother
>>> when I found the problem wasn't solved by patch 1. Sorry!
>>>
>>
>> Well, honestly, even I had intended the second patch as a cleanup and
>> hadn't asked you to test it ;-) Only when you reported that the first patch
>> failed to solve your problem, I realized that the second patch was
>> important too! :-) Thanks for testing!
>>
>>> For the record, I'm testing on a 2-CPU system, so I'm not sure whether
>>> your explanation applies; it talks about CPUs 2 and 3 whereas I only
>>> have CPUs 0 and 1, but perhaps your explanation applies equally to any
>>> pair of CPUs?
>>>
>>
>> Yes, it applies to any pair of CPUs, as long as the CPU first taken down
>> is not the policy->cpu. In your case, it applies like this:
>> IIUC, CPU0 is the boot cpu, and hence it wont be taken offline using hotplug.
>> So only CPU 1 is taken offline during suspend. And if it is not the policy->cpu,
>> then it hits the very same bug that I described with the analogy of CPUs 2
>> and 3.
>>
>>> For the record, here's the information you requested in the other email:
>>>
>>> # cat /sys/devices/system/cpu/cpu*/cpufreq/related_cpus
>>> 0 1
>>> 0 1
>>
>> Thanks! It would have been more useful to somehow know which was the
>> policy->cpu. But looking at the problem, certainly CPU0 was the policy->cpu
>> in your case.
> 
> Yes, I believe CPU0 since,
> 
>> # ls -l /sys/devices/system/cpu/cpu1/cpufreq
>> lrwxrwxrwx 1 root root 0 Jan  1 00:01 /sys/devices/system/cpu/cpu1/cpufreq -> ../cpu0/cpufreq
> 
> and cpu0/cpufreq/ has all the files in it.
> 
> ...

Ah, nice!

>> So can you see if patch 1 + this above fix solves your problem as well?
>> Then we can retain the original patch 2 as a cleanup, after these 2 patches.
>> This organization also makes the code look better and understandable.
> 
> Yes, both patch 1+3 and 1+3+2 work fine.
> 

Cool! Thanks a lot for all your testing efforts Stephen! :-)

Regards,
Srivatsa S. Bhat

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