Re: [PATCH v6 1/2] cpufreq: split the cpufreq_driver_lock and usethe rcu

[Nathan Zimmer]

On Tue, Apr 02, 2013 at 08:29:12PM +0530, Viresh Kumar wrote:
> On 2 April 2013 20:25, Nathan Zimmer <nzimmer@xxxxxxx> wrote:
> > The lock is unneeded if we expect register and unregister driver to not be
> > called from muliple threads at once.  I didn't make that assumption.
> 
> Hmm.. But doesn't rcu part take care of that too?? Two writers
> updating stuff simultaneously?

My concern is in the cpufreq_register_driver.  Since we are only to set the
pointer when it is null we have have to hold the lock over both operations.

int cpufreq_register_driver(struct cpufreq_driver *driver_data)
{
...
        spin_lock_irqsave(&cpufreq_driver_lock, flags);
        if (rcu_access_pointer(cpufreq_driver)) {
                spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
                return -EBUSY;
        }
        rcu_assign_pointer(cpufreq_driver, driver_data);
        spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
        synchronize_rcu();
...
}


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/