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

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Date:	Thu, 12 Sep 2013 11:56:34 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	"Rafael J. Wysocki" <rjw@...k.pl>,
	Stephen Warren <swarren@...dotorg.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 11:22 AM, Viresh Kumar wrote:
> Let me fix my mail first.. I was running out of time yesterday and so couldn't
> frame things correctly :)
> 
> On 11 September 2013 17:29, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> Okay.. There are two different ways in which cpufreq_add_dev() work
>> currently..
>>
>> Boot cluster (i.e. policy with boot CPU)
>> ---------------
>>
>> Here cpufreq_remove_dev() is never called for boot cpu but all others.
>> And similarly cpufreq_add_dev() is never called for boot cpu but all others.
>>
>> Now policy->cpu contains meaningful cpu at beginning of resume and
>> we don't need to modify that at all.. For all the remaining CPUs we
>> better call cpufreq_add_policy_cpu() rather..
> 
> And this should be done without your patch. Or actually we will simply
> return from this place. Atleast for systems with single cluster, like Tegra.
> 
> policy->related_cpus is still valid after resume and we haven't removed
> policy from the cpufreq_policy_list (though there is a bug which I have
> fixed separately and sent it to you..).. So no change required for a single
> cluster system..
> 
>> Non-boot Cluster
>> ---------------------
>>
>> All CPUs here are removed and at the end policy->cpu contains the last
>> cpu removed.. So, for a cluster with cpu 2 and 3.... it will contain 3..
>>
>> Now at resume we will add cpu2 first and so need to update policy->cpu
>> to 2..
> 
>> But for all other CPUs in this cluster we return early from
>> cpufreq_add_dev() and call cpufreq_add_policy_cpu() as policy->cpus
>> was fixed by call to ->init() for the first cpu of this cluster..
> 
> This was wrong, we need a valid policy->related_cpus field which is always
> valid and so we return early here too, but not for the first cpu of cluster.
> 
>> And so we never reach the line: policy->cpu = cpu;
>>
>> For the first cpu of non-boot cluster we need to call update_policy_cpu()
>> and not for others..
> 
> that's correct, thought I have one more idea.. :)
> 
>> But for the boot cluster if we can call ->init() somehow at resume time,
>> then things would be fairly similar in both cases..
> 
> Not required.. its all working already.. and so Stephen shouldn't need your
> patch for Tegra, but rather my patches that fix other cpufreq bugs..
> 
> Now coming back to the ideas I have...
> Same code will work if hotplug sequence is fixed a bit. Why aren't we doing
> exact opposite of suspend in resume?
> 
> We are removing CPUs (leaving the boot cpu) in ascending order and then
> adding them back in same order.. Why?
> 
> Why not remove CPUs in descending order and add in ascending order? Or
> remove in ascending order and add in descending order?
> 

I had the same thought when solving this bug.. We have had similar issues with
CPU hotplug notifiers too: why are they invoked in the same order during both
CPU down and up, instead of reversing the order? I even had a patchset to perform
reverse-invocation of notifiers.. http://lwn.net/Articles/508072/
... but people didn't find that very compelling to have.


> That way policy->cpu will be updated with the right cpu and your patch wouldn't
> be required..
> 
> I am not saying that this can't be hacked/fixed in cpufreq but suspend/resume
> may also be fixed and that looks logically more correct to me..
> 

It does to me too, but I think the reason nobody really bothered is because perhaps
not many other subsystems care about the order in which CPUs are torn down or
brought up; they just need the total number to match.. cpufreq is one exception
as we saw with this bug.


Regards,
Srivatsa S. Bhat

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