Re: [PATCH 3/3] cpufreq: Don't use cpu removed during cpufreq_driver_unregister

Srivatsa S. Bhat
2013-01-03T14:25:54Z

Hi Viresh,

On 12/16/2012 11:20 AM, Viresh Kumar wrote:
> This is how the core works:
> cpufreq_driver_unregister()
> - subsys_interface_unregister()
> - for_each_cpu() call cpufreq_remove_dev(), i.e. 0,1,2,3,4 when we
> unregister.
>
> cpufreq_remove_dev():
> - Remove policy node
> - Call cpufreq_add_dev() for next cpu, sharing mask with removed cpu.
> i.e. When cpu 0 is removed, we call it for cpu 1. And when called for cpu 2,
> we call it for cpu 3.
> - cpufreq_add_dev() would call cpufreq_driver->init()
> - init would return mask as AND of 2, 3 and 4 for cluster A7.
> - cpufreq core would do online_cpu && policy->cpus
> Here is the BUG(). Because cpu hasn't died but we have just unregistered
> the cpufreq driver, online cpu would still have cpu 2 in it. And so thing
> go bad again.
>
> Solution: Keep cpumask of cpus that are registered with cpufreq core and clear
> cpus when we get a call from subsys_interface_unregister() via
> cpufreq_remove_dev().
>

I took a quick look at the problem you described above, and the cpufreq code..
If we cannot avoid calling cpufreq_add_dev() from cpufreq_remove_dev(), then I can't
think of anything better than what your patch does.

BTW, off-topic, while going through that path, I think I found a memory leak
in __cpufreq_remove_dev():

if (unlikely(cpumask_weight(data->cpus) > 1)) {
for_each_cpu(j, data->cpus) {
if (j == cpu)
continue;
per_cpu(cpufreq_cpu_data, j) = NULL;
}
}

We are assigning NULL without freeing that memory.

Regards,
Srivatsa S. Bhat

> Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
> ---
> drivers/cpufreq/cpufreq.c | 8 +++++++-
> 1 file changed, 7 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index a0a33bd..271d3be 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -47,6 +47,9 @@ static DEFINE_PER_CPU(char[CPUFREQ_NAME_LEN], cpufreq_cpu_governor);
> #endif
> static DEFINE_SPINLOCK(cpufreq_driver_lock);
>
> +/* Used when we unregister cpufreq driver */
> +struct cpumask cpufreq_online_mask;
> +
> /*
> * cpu_policy_rwsem is a per CPU reader-writer semaphore designed to cure
> * all cpufreq/hotplug/workqueue/etc related lock issues.
> @@ -981,6 +984,7 @@ static int cpufreq_add_dev(struct device *dev, struct subsys_interface *sif)
> * managing offline cpus here.
> */
> cpumask_and(policy->cpus, policy->cpus, cpu_online_mask);
> + cpumask_and(policy->cpus, policy->cpus, &cpufreq_online_mask);
>
> policy->user_policy.min = policy->min;
> policy->user_policy.max = policy->max;
> @@ -1064,7 +1068,6 @@ static int __cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif
> }
> per_cpu(cpufreq_cpu_data, cpu) = NULL;
>
> -
> #ifdef CONFIG_SMP
> /* if this isn't the CPU which is the parent of the kobj, we
> * only need to unlink, put and exit
> @@ -1185,6 +1188,7 @@ static int cpufreq_remove_dev(struct device *dev, struct subsys_interface *sif)
> if (unlikely(lock_policy_rwsem_write(cpu)))
> BUG();
>
> + cpumask_clear_cpu(cpu, &cpufreq_online_mask);
> retval = __cpufreq_remove_dev(dev, sif);
> return retval;
> }
> @@ -1903,6 +1907,8 @@ int cpufreq_register_driver(struct cpufreq_driver *driver_data)
> cpufreq_driver = driver_data;
> spin_unlock_irqrestore(&cpufreq_driver_lock, flags);
>
> + cpumask_setall(&cpufreq_online_mask);
> +
> ret = subsys_interface_register(&cpufreq_interface);
> if (ret)
> goto err_null_driver;
>