Re: [PATCH v7 03/14] PM: Introduce an Energy Model management framework

[Quentin Perret <quentin.perret@arm.com>]

On Tuesday 02 Oct 2018 at 16:29:24 (+0200), Peter Zijlstra wrote:
> On Tue, Oct 02, 2018 at 03:05:23PM +0100, Quentin Perret wrote:
> > On Tuesday 02 Oct 2018 at 15:48:57 (+0200), Peter Zijlstra wrote:
> > > +/**
> > > + * em_cpu_get() - Return the performance domain for a CPU
> > > + * @cpu : CPU to find the performance domain for
> > > + *
> > > + * Return: the performance domain to which 'cpu' belongs, or NULL if it doesn't
> > > + * exist.
> > > + */
> > > +struct em_perf_domain *em_cpu_get(int cpu)
> > > +{
> > > +       return READ_ONCE(per_cpu(em_data, cpu));
> > > +}
> > > +EXPORT_SYMBOL_GPL(em_cpu_get);
> > > 
> > > But your read side doesn't take, not is required to take em_pd_mutex.
> > > 
> > > At that point, the mutex_unlock() doesn't guarantee anything.
> > > 
> > > A CPU observing the em_data store, doesn't need to observe the store
> > > that filled the data structure it points to.
> > 
> > Right but even if I add the smp_store_release(), I can still have a
> > CPU observing em_data while another is in the process of updating it.
> > So, if smp_store_release() doesn't guarantee that readers will see a
> > complete update, do I actually get something interesting from it ?
> > (That's not a rhetorical question, I'm actually wondering :-)
> 
> I thought the update would fail if em_data was already set.
> 
> That is, you can only set this thing up _once_ and then you'll have to
> forever live with it.
> 
> Or did I read that wrong?

No no, that's correct. em_data is populated once and kept as-is
forever.

What I was trying to say is, when em_data is being populated for the
first time, nothing prevents a reader from using em_cpu_get()
concurrently. And in this case, it doesn't matter if you use
smp_store_release() or not, the reader might see the table half-updated.

So, basically, smp_store_release() doesn't guarantee that readers won't
see a half-baked em_data. That's the point I'm trying to make at least :-)