linux-kernel - Re: [PATCH v4 2/2] cpufreq: Convert the cpufreq_driver

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Message-ID: <CAKohpom9q93cVD61qeht2WXgA=no64dwXowtMHS0wCy0k6qxXw@mail.gmail.com>
Date:	Sat, 23 Feb 2013 09:09:31 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Nathan Zimmer <nzimmer@....com>
Cc:	rjw@...k.pl, cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 2/2] cpufreq: Convert the cpufreq_driver_lock to use
 the rcu

Hi Nathan,

Sorry for pointing out this so late but i still feel we are missing something
really important.

On 22 February 2013 21:54, Nathan Zimmer <nzimmer@....com> wrote:

> -       read_lock_irqsave(&cpufreq_driver_lock, flags);
> +       rcu_read_lock();
> +       freqs->flags = rcu_dereference(cpufreq_driver)->flags;
>         policy = per_cpu(cpufreq_cpu_data, freqs->cpu);
> -       read_unlock_irqrestore(&cpufreq_driver_lock, flags);
> +       rcu_read_unlock();

> -       write_lock_irqsave(&cpufreq_driver_lock, flags);
> +       spin_lock_irqsave(&cpufreq_driver_lock, flags);
>         for_each_cpu(j, policy->cpus) {
>                 per_cpu(cpufreq_cpu_data, j) = policy;
>                 per_cpu(cpufreq_policy_cpu, j) = policy->cpu;
>         }
> -       write_unlock_irqrestore(&cpufreq_driver_lock, flags);
> +       spin_unlock_irqrestore(&cpufreq_driver_lock, flags);

Look at how we are protecting cpufreq_cpu_data here. rcu_read_[un]lock()
only marks the start/end of critical section. How are we sure here that
cpufreq_cpu_data is not read simultaneously when we are updating it?

rcu lock/unlock only works for cpufreq_driver pointer only and not for
this data. We still need the same locking for for cpufreq_cpu_data.

What do you say?
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