linux-kernel - Re: [PATCH] cpufreq: convert the cpufreq

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Date:	Wed, 3 Apr 2013 11:37:10 -0500
From:	Nathan Zimmer <nzimmer@....com>
To:	Viresh Kumar <viresh.kumar@...aro.org>
CC:	<rjw@...k.pl>, <cpufreq@...r.kernel.org>,
	<linux-pm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] cpufreq: convert the cpufreq_driver to use the rcu

On 04/03/2013 10:32 AM, Viresh Kumar wrote:
> Please always mention Version number and history. Not everybody
> remembers what changed after last version.
Your right.  I was rushing and forgot.
I need to develop the habit of adding some history to my git commits 
when I amend them.

>
> On 3 April 2013 20:33, Nathan Zimmer <nzimmer@....com> wrote:
>> We eventually would like to remove the rwlock cpufreq_driver_lock or convert
>> it back to a spinlock and protect the read sections with RCU.  The first step in
> Why do we want to convert it back to spinlock?
Documentation/spinlocks.txt:84
I am not sure why but there is the directive I am following.
>> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
>>   bool have_governor_per_policy(void)
>>   {
>> -       return cpufreq_driver->have_governor_per_policy;
>> +       bool have_governor;
> Name it have_governor_per_policy, it looks wrong otherwise.
>
>> +       rcu_read_lock();
>> +       have_governor = rcu_dereference(cpufreq_driver)->have_governor_per_policy;
>> +       rcu_read_unlock();
>> +       return have_governor;
>>   }
Will do.
>>   static ssize_t show_scaling_driver(struct cpufreq_policy *policy, char *buf)
>>   {
>> -       return scnprintf(buf, CPUFREQ_NAME_PLEN, "%s\n", cpufreq_driver->name);
>> +       char *name;
>> +       rcu_read_lock();
>> +       name = rcu_dereference(cpufreq_driver)->name;
>> +       rcu_read_unlock();
>> +       return scnprintf(buf, CPUFREQ_NAME_PLEN, "%s\n", name);
>>   }
> This is the definition of struct cpufreq_driver:
>
> struct cpufreq_driver {
> 	struct module           *owner;
> 	char			name[CPUFREQ_NAME_LEN];
>
>         ...
> };
>
> Purpose of rcu read_lock/unlock are to define the rcu critical section
> after which rcu layer is free to free the memory allocated to earlier
> instance of cpufreq_driver.
>
> So, after the unlock() call you _should_not_ use the memory allocated to
> cpufreq_driver instance. And here, you are using memory allocated to name[]
> after the unlock() call.
Ok I'll fix this spot.

> Which looks to be wrong... I left other parts of driver upto you to fix for this
> "rule of thumb".
In places like show_bios_limit and cpufreq_add_dev_interface we know 
that the memory will still
be there since the cpufreq_driver->owner is held.

> Sorry for not pointing this earlier but rcu is as new to me as it is
> to you. I know
> you must be frustrated with so many versions of this patch, and everytime we
> get a new problem to you... Don't get disheartened with it.. Keep the good work
> going :)
Making a learners mistake isn't really discouraging to me, even when I 
do it twice.

> --
> viresh

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