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
|