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

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Date:	Thu, 12 Sep 2013 11:28:58 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
Cc:	Stephen Warren <swarren@...dotorg.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	"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 12 September 2013 00:12, Srivatsa S. Bhat
<srivatsa.bhat@...ux.vnet.ibm.com> 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]?
>
> Basically here is my hunch: say CPUs 2 and 3 are part of a policy and
> 3 is the policy->cpu. During suspend, CPU 2 will be taken offline first,
> and we hit this code:
>
> 1199         if (cpu != policy->cpu && !frozen) {
> 1200                 sysfs_remove_link(&dev->kobj, "cpufreq");
> 1201         } else if (cpus > 1) {
> 1202
> 1203                 new_cpu = cpufreq_nominate_new_policy_cpu(policy, cpu, frozen);
> 1204                 if (new_cpu >= 0) {
> 1205                         WARN_ON(lock_policy_rwsem_write(cpu));
> 1206                         update_policy_cpu(policy, new_cpu);
> 1207                         unlock_policy_rwsem_write(cpu);
> 1208
> 1209                         if (!frozen) {
> 1210                                 pr_debug("%s: policy Kobject moved to cpu: %d "
> 1211                                          "from: %d\n",__func__, new_cpu, cpu);
> 1212                         }
> 1213                 }
> 1214         }
>
> At this point, the first 'if' condition fails because frozen == true.
> So it enters the else part. But, policy->cpu is actually 3, not 2,
> and hence we invoke nominate_...() unnecessarily. That function returns
> 3 since that's the only CPU remaining in the mask, and so we call
> update_policy_cpu() with new_cpu = 3, and old_cpu was also 3! And that
> is the perfect recipe for disaster, with the current implementation of
> update_policy_cpu().

The problem here is not the wrong implementation of update_policy_cpu()
but

1199         if (cpu != policy->cpu && !frozen) {

Though I have fixed it before looking into your replies :)
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