linux-kernel - Re: [PATCH v6 1/2] cpufreq: split the cpufreq_driver

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Message-ID: <20130402154021.GA521@gulag1.americas.sgi.com>
Date:	Tue, 2 Apr 2013 10:40:22 -0500
From:	Nathan Zimmer <nzimmer@....com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Nathan Zimmer <nzimmer@....com>, rjw@...k.pl,
	cpufreq@...r.kernel.org, linux-pm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 1/2] cpufreq: split the cpufreq_driver_lock and use
	the rcu

On Tue, Apr 02, 2013 at 08:29:12PM +0530, Viresh Kumar wrote:
> On 2 April 2013 20:25, Nathan Zimmer <nzimmer@....com> 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();
...
}


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