Re: [PATCH v4 2/2] cpufreq: Convert the cpufreq_driver_lock to usethe rcu

From: Viresh Kumar
Date: Fri Feb 22 2013 - 22:39:50 EST

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@xxxxxxx> 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.

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