Re: [PATCH Resend] cpufreq: Set cpufreq_cpu_data to NULL before putting kobject

[Viresh Kumar]

On 2 February 2015 at 10:15, ethan zhao <ethan.zhao@xxxxxxxxxx> 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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