From: "Rafael J. Wysocki" <rjw@rjwysocki.net>
To: Richard Cochran <rcochran@linutronix.de>
Cc: linux-kernel@vger.kernel.org, rt@linutronix.de,
        "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Linux PM list <linux-pm@vger.kernel.org>
Subject: Re: [PATCH] cpufreq: Make cpufreq_quick_get() safe to call.
Date: Thu, 10 Mar 2016 23:20:47 +0100
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On Thursday, March 10, 2016 04:10:36 PM Richard Cochran wrote:
> The function, cpufreq_quick_get, accesses the global 'cpufreq_driver' and
> its fields without taking the associated lock, cpufreq_driver_lock.
> 
> Without the locking, nothing guarantees that 'cpufreq_driver' remains
> consistent during the call.  This patch fixes the issue by taking the lock
> before accessing the data structure.
> 
> Cc: Dirk Brandewie <dirk.brandewie@gmail.com>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> Signed-off-by: Richard Cochran <rcochran@linutronix.de>

Can you please CC PM-related patches to linux-pm@vger.kernel.org?  They
are much easier to handle for me then.

> ---
>  drivers/cpufreq/cpufreq.c | 10 +++++++++-
>  1 file changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index e979ec7..ce02b2b 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1457,9 +1457,17 @@ unsigned int cpufreq_quick_get(unsigned int cpu)
>  {
>  	struct cpufreq_policy *policy;
>  	unsigned int ret_freq = 0;
> +	unsigned long flags;
> +
> +	read_lock_irqsave(&cpufreq_driver_lock, flags);
>  
>  	if (cpufreq_driver && cpufreq_driver->setpolicy && cpufreq_driver->get)
> -		return cpufreq_driver->get(cpu);
> +		ret_freq = cpufreq_driver->get(cpu);
> +
> +	read_unlock_irqrestore(&cpufreq_driver_lock, flags);
> +
> +	if (ret_freq)
> +		return ret_freq;
>  
>  	policy = cpufreq_cpu_get(cpu);
>  	if (policy) {
> 

I would prefer something like this:

	read_lock_irqsave(&cpufreq_driver_lock, flags);

  	if (cpufreq_driver && cpufreq_driver->setpolicy && cpufreq_driver->get) {
		unsigned int ret_freq = cpufreq_driver->get(cpu);

		read_unlock_irqrestore(&cpufreq_driver_lock, flags);
		return ret_freq;
	}

	read_unlock_irqrestore(&cpufreq_driver_lock, flags);

Thanks,
Rafael