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SubjectRe: [PATCH v2 linux-next 2/2] cpufreq: Convert the cpufreq_driver_lock to use the rcu
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On Tuesday, February 05, 2013 08:04:50 PM Nathan Zimmer wrote:
> In general rwlocks are discourged so we are moving it to use the rcu instead.
>
> Cc: Viresh Kumar <viresh.kumar@linaro.org>
> Cc: "Rafael J. Wysocki" <rjw@sisk.pl>
> Signed-off-by: Nathan Zimmer <nzimmer@sgi.com>
> ---
> drivers/cpufreq/cpufreq.c | 173 +++++++++++++++++++++++++---------------------
> 1 file changed, 96 insertions(+), 77 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index ef25244..a04ceb9 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -39,13 +39,13 @@
> * level driver of CPUFreq support, and its spinlock. This lock
> * also protects the cpufreq_cpu_data array.
> */
> -static struct cpufreq_driver *cpufreq_driver;
> +static struct cpufreq_driver __rcu *cpufreq_driver;
> static DEFINE_PER_CPU(struct cpufreq_policy *, cpufreq_cpu_data);
> #ifdef CONFIG_HOTPLUG_CPU
> /* This one keeps track of the previously set governor of a removed CPU */
> static DEFINE_PER_CPU(char[CPUFREQ_NAME_LEN], cpufreq_cpu_governor);
> #endif
> -static DEFINE_RWLOCK(cpufreq_driver_lock);
> +static DEFINE_SPINLOCK(cpufreq_driver_lock);
>
> /*
> * cpu_policy_rwsem is a per CPU reader-writer semaphore designed to cure
> @@ -143,21 +143,20 @@ static DEFINE_MUTEX(cpufreq_governor_mutex);
> static struct cpufreq_policy *__cpufreq_cpu_get(unsigned int cpu, bool sysfs)
> {
> struct cpufreq_policy *data;
> - unsigned long flags;
> + struct cpufreq_driver *driver;
>
> if (cpu >= nr_cpu_ids)
> goto err_out;
>
> /* get the cpufreq driver */
> - read_lock_irqsave(&cpufreq_driver_lock, flags);
> -
> - if (!cpufreq_driver)
> + rcu_read_lock();
> + driver = rcu_dereference(cpufreq_driver);
> + if (!driver)
> goto err_out_unlock;
>
> - if (!try_module_get(cpufreq_driver->owner))
> + if (!try_module_get(driver->owner))
> goto err_out_unlock;
>
> -
> /* get the CPU */
> data = per_cpu(cpufreq_cpu_data, cpu);
>
> @@ -167,13 +166,13 @@ static struct cpufreq_policy *__cpufreq_cpu_get(unsigned int cpu, bool sysfs)
> if (!sysfs && !kobject_get(&data->kobj))
> goto err_out_put_module;
>
> - read_unlock_irqrestore(&cpufreq_driver_lock, flags);
> + rcu_read_unlock();
> return data;
>
> err_out_put_module:
> - module_put(cpufreq_driver->owner);
> + module_put(driver->owner);
> err_out_unlock:
> - read_unlock_irqrestore(&cpufreq_driver_lock, flags);
> + rcu_read_unlock();
> err_out:
> return NULL;
> }
> @@ -196,7 +195,7 @@ static void __cpufreq_cpu_put(struct cpufreq_policy *data, bool sysfs)
> {
> if (!sysfs)
> kobject_put(&data->kobj);
> - module_put(cpufreq_driver->owner);
> + module_put(rcu_dereference(cpufreq_driver)->owner);
> }
>
> void cpufreq_cpu_put(struct cpufreq_policy *data)
> @@ -267,13 +266,16 @@ static inline void adjust_jiffies(unsigned long val, struct cpufreq_freqs *ci)
> void cpufreq_notify_transition(struct cpufreq_freqs *freqs, unsigned int state)
> {
> struct cpufreq_policy *policy;
> + struct cpufreq_driver *driver;
>
> BUG_ON(irqs_disabled());
>
> if (cpufreq_disabled())
> return;
>
> - freqs->flags = cpufreq_driver->flags;
> + driver = rcu_dereference(cpufreq_driver);

Pardon me for not asking that question earlier, but what sense does it make
to use rcu_dereference() outside of an rcu_read_lock()/rcu_read_unlock()
scope? Here and in the other places?

Rafael


--
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.


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