Re: [PATCH] cpufreq: convert the cpufreq_driver to use the rcu

[Nathan Zimmer]

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@xxxxxxx> 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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