Subject: Re: [PATCH RFC 3/5] sched/cpufreq: Fix incorrect RCU API usage

From: Joel Fernandes <[email-masked]>
Date: Thu, 21 Feb 2019 10:21:39 -0500

On Thu, Feb 21, 2019 at 10:18:05AM +0100, Peter Zijlstra wrote:
> On Thu, Feb 21, 2019 at 12:49:40AM -0500, Joel Fernandes (Google) wrote:
> > @@ -34,8 +34,12 @@ void cpufreq_add_update_util_hook(int cpu, struct update_util_data *data,
> >  	if (WARN_ON(!data || !func))
> >  		return;
> >  
> > -	if (WARN_ON(per_cpu(cpufreq_update_util_data, cpu)))
> > +	rcu_read_lock();
> > +	if (WARN_ON(rcu_dereference(per_cpu(cpufreq_update_util_data, cpu)))) {
> > +		rcu_read_unlock();
> >  		return;
> > +	}
> > +	rcu_read_unlock();
> >  
> >  	data->func = func;
> >  	rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
> 
> This doesn't make any kind of sense to me.
> 

As per the rcu_assign_pointer() line, I inferred that
cpufreq_update_util_data is expected to be RCU protected. Reading the pointer
value of RCU pointers generally needs to be done from RCU read section, and
using rcu_dereference() (or using rcu_access()).

In this patch, I changed cpufreq_update_util_data to be __rcu annotated to
avoid the sparse error thrown by rcu_assign_pointer().

Instead of doing that, If your intention here is RELEASE barrier, should I
just replace in this function:
	rcu_assign_pointer(per_cpu(cpufreq_update_util_data, cpu), data);
with:
	smp_store_release(per_cpu(cpufreq_update_util_data, cpu), data))
?

It would be nice IMO to be explicit about the intention of release/publish
semantics by using smp_store_release().

thanks,

 - Joel