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Date: Mon, 2 Feb 2015 10:24:05 +0530
From: Viresh Kumar <viresh.kumar@...aro.org>
To: ethan zhao <ethan.zhao@...cle.com>
Cc: Rafael Wysocki <rjw@...ysocki.net>,
santosh shilimkar <santosh.shilimkar@...cle.com>,
Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH Resend] cpufreq: Set cpufreq_cpu_data to NULL before
putting kobject
On 2 February 2015 at 10:15, ethan zhao <ethan.zhao@...cle.com> wrote:
> On 2015/2/2 12:26, Viresh Kumar wrote:
> But there is no checking against refcount in or before
>
> cpufreq_policy_free(), that is one issue I mentioned.
As I said earlier, the completion will only fire once the refcount
is zero. And so there is no need of any check here.
>> That routines doesn't have any tricks and simply frees the policy.
>> Because, before calling cpufreq_policy_put_kobj(), we have set
>> the per-cpu variable to NULL, nobody else will get the policy
>
> It is possible cpufreq_cpu_get() within the PPC thread was called just
> before __cpufreq_remove_dev_finish() is to be called in another thread,
> so you set the per_cpu(cpufreq_cpu_data, cpu) to NULL will not prevent
> the actions between cpufreq_cpu_get and cpufreq_cpu_put().
>
> And then the freeing happens in __cpufreq_remove_dev_finish().
It will.. You aren't looking closely enough. If cpufreq_cpu_get() is called just
before remove-dev, then cpufreq_cpu_get() will take:
read_lock_irqsave(&cpufreq_driver_lock, flags);
And it will do:
read_unlock_irqrestore(&cpufreq_driver_lock, flags);
only after increasing the refcount with kobject_get().
While on the other side __cpufreq_remove_dev_finish() will do this:
write_lock_irqsave(&cpufreq_driver_lock, flags);
policy = per_cpu(cpufreq_cpu_data, cpu);
per_cpu(cpufreq_cpu_data, cpu) = NULL;
write_unlock_irqrestore(&cpufreq_driver_lock, flags);
So, it will wait for the read_lock in cpufreq_cpu_get() to finish before
setting per-cpu variable to NULL. And so, after kobject_put() in
cpufreq_policy_put_kobj(), we will wait for the completion to fire
and that will only happen once a corresponding cpufreq_cpu_put()
is issued.
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