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

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Date:	Wed, 11 Sep 2013 17:29:07 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
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 11 September 2013 16:44, Srivatsa S. Bhat
<srivatsa.bhat@...ux.vnet.ibm.com> wrote:
> Hmm? The problem is not about merely updating the policy->cpu field; the
> main issue is that the existing code was not letting the cpufreq-stats
> code know that we updated the policy->cpu under the hood. It is important
> for cpufreq-stats to know this because it maintains the reference to its
> stats structure by associating it with the policy->cpu. So if policy->cpu
> changes under the hood, it loses track of its reference. So we need to
> keep that code informed about changes to policy->cpu. Thus, we need to call
> update_policy_cpu() in the CPU online path (during resume). I don't see
> how we can skip that.

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..

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..

Not 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..

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..

But for the boot cluster if we can call ->init() somehow at resume time,
then things would be fairly similar in both cases..

I am running of time now, as need to leave office now...
I hope I made the problem more clear or probably the way I see it :)

--
viresh
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