Re: [PATCH v3 2/2] cpufreq: Simplify and fix mutual exclusion with hotplug

[Viresh Kumar <viresh.kumar@linaro.org>]

On 16 July 2014 04:17, Saravana Kannan <skannan@codeaurora.org> wrote:

Again, just too many things in a single patch. That's not acceptable.
Few of these might be bug fixes, which must go in before any other updates.
And so it must have been added as first patch.

Even the other stuff you are trying to fix (by checking policy->cpus) should go
before 1/2, otherwise 1/2 will actually break things inbetween, i.e. show values
even when no CPUs of a cluster are online.

> Since we no longer alloc and destroy/freeze policy and sysfs nodes during
> hotplug and suspend, we don't need to lock sysfs with hotplug. We can
> achieve the same effect by checking if policy->cpus is empty.

Are you talking about the changes in store()?

> Hotplug mutual exclusion was only done for sysfs writes. But reads need the
> same protection too.  So, this patch adds that too.

How? How is checking for policy->cpus enough?

> Also, cpufreq driver (un)register can race with hotplug since CPU online
> state can change between adding/removing the currently online devices and
> registering/unregistering for hotplug notifiers. So, fix that by
> registering for hotplug notifiers first before adding devices and
> unregistering from hotplug notifiers first before removing devices.

Couldn't get it, tell us an example race and what will go wrong due to it.
Also this should have had a separate patch for itself.

> Signed-off-by: Saravana Kannan <skannan@codeaurora.org>
> ---
>  drivers/cpufreq/cpufreq.c | 44 ++++++++++++++++++++------------------------
>  1 file changed, 20 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index a0a2ec2..f72b2b7 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -748,17 +748,18 @@ static ssize_t show(struct kobject *kobj, struct attribute *attr, char *buf)
>  {
>         struct cpufreq_policy *policy = to_policy(kobj);
>         struct freq_attr *fattr = to_attr(attr);
> -       ssize_t ret;
> +       ssize_t ret = -EINVAL;
>
>         if (!down_read_trylock(&cpufreq_rwsem))
> -               return -EINVAL;
> -
> +               return ret;
>         down_read(&policy->rwsem);
>
> -       if (fattr->show)
> -               ret = fattr->show(policy, buf);
> -       else
> -               ret = -EIO;
> +       if (!cpumask_empty(policy->cpus)) {
> +               if (fattr->show)
> +                       ret = fattr->show(policy, buf);
> +               else
> +                       ret = -EIO;
> +       }

Makes sense upto this point.

>         up_read(&policy->rwsem);
>         up_read(&cpufreq_rwsem);
> @@ -773,26 +774,19 @@ static ssize_t store(struct kobject *kobj, struct attribute *attr,
>         struct freq_attr *fattr = to_attr(attr);
>         ssize_t ret = -EINVAL;
>
> -       get_online_cpus();
> -
> -       if (!cpu_online(policy->cpu))
> -               goto unlock;
> -

@Srivatsa: what do you say?

>         if (!down_read_trylock(&cpufreq_rwsem))
> -               goto unlock;
> -
> +               return ret;
>         down_write(&policy->rwsem);
>
> -       if (fattr->store)
> -               ret = fattr->store(policy, buf, count);
> -       else
> -               ret = -EIO;
> +       if (!cpumask_empty(policy->cpus)) {
> +               if (fattr->store)
> +                       ret = fattr->store(policy, buf, count);
> +               else
> +                       ret = -EIO;
> +       }
>
>         up_write(&policy->rwsem);
> -
>         up_read(&cpufreq_rwsem);
> -unlock:
> -       put_online_cpus();
>
>         return ret;
>  }
> @@ -2270,6 +2264,8 @@ int cpufreq_register_driver(struct cpufreq_driver *driver_data)
>                 }
>         }
>
> +       register_hotcpu_notifier(&cpufreq_cpu_notifier);
> +
>         ret = subsys_interface_register(&cpufreq_interface);
>         if (ret)
>                 goto err_boost_unreg;
> @@ -2293,13 +2289,13 @@ int cpufreq_register_driver(struct cpufreq_driver *driver_data)
>                 }
>         }
>
> -       register_hotcpu_notifier(&cpufreq_cpu_notifier);
>         pr_debug("driver %s up and running\n", driver_data->name);
>
>         return 0;
>  err_if_unreg:
>         subsys_interface_unregister(&cpufreq_interface);
>  err_boost_unreg:
> +       unregister_hotcpu_notifier(&cpufreq_cpu_notifier);
>         if (cpufreq_boost_supported())
>                 cpufreq_sysfs_remove_file(&boost.attr);
>  err_null_driver:
> @@ -2327,12 +2323,12 @@ int cpufreq_unregister_driver(struct cpufreq_driver *driver)
>
>         pr_debug("unregistering driver %s\n", driver->name);
>
> +       unregister_hotcpu_notifier(&cpufreq_cpu_notifier);
> +
>         subsys_interface_unregister(&cpufreq_interface);
>         if (cpufreq_boost_supported())
>                 cpufreq_sysfs_remove_file(&boost.attr);
>
> -       unregister_hotcpu_notifier(&cpufreq_cpu_notifier);
> -
>         down_write(&cpufreq_rwsem);
>         write_lock_irqsave(&cpufreq_driver_lock, flags);

Normally the order of register/unregister should be just opposite.
Isn't that true here? Yeah, it was broken earlier as well...