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

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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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