linux-kernel - Re: [PATCH] cpufreq: Make cpufreq_quick

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Date:	Fri, 11 Mar 2016 03:23:22 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Richard Cochran <rcochran@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	rt@...utronix.de, "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Linux PM list <linux-pm@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: Make cpufreq_quick_get() safe to call.

On Thu, Mar 10, 2016 at 11:20 PM, Rafael J. Wysocki <rjw@...ysocki.net> wrote:
> 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@...il.com>
>> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
>> Cc: Viresh Kumar <viresh.kumar@...aro.org>
>> Signed-off-by: Richard Cochran <rcochran@...utronix.de>
>
> Can you please CC PM-related patches to linux-pm@...r.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);

Sorry, ret_freq is needed outside of this block anyway, so that would be

                   ret_freq = cpufreq_driver->get(cpu);
>
>                 read_unlock_irqrestore(&cpufreq_driver_lock, flags);
>                 return ret_freq;
>         }
>
>         read_unlock_irqrestore(&cpufreq_driver_lock, flags);