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

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Date:	Wed, 11 Sep 2013 14:07:12 -0600
From:	Stephen Warren <swarren@...dotorg.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
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 09/11/2013 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.

...
> 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.

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