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

[ethan zhao <ethan.zhao@oracle.com>]

On 2015/2/2 12:54, Viresh Kumar wrote:
> On 2 February 2015 at 10:15, ethan zhao <ethan.zhao@oracle.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
  Closely enough this time, understood, thanks for your explanation.


  Ethan
> and that will only happen once a corresponding cpufreq_cpu_put()
> is issued.