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Date:   Tue, 2 Oct 2018 16:29:24 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Quentin Perret <quentin.perret@arm.com>
Cc:     rjw@rjwysocki.net, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, gregkh@linuxfoundation.org,
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        juri.lelli@redhat.com, edubezval@gmail.com,
        srinivas.pandruvada@linux.intel.com, currojerez@riseup.net,
        javi.merino@kernel.org
Subject: Re: [PATCH v7 03/14] PM: Introduce an Energy Model management
 framework
Message-ID: <20181002142924.GI26858@hirez.programming.kicks-ass.net>
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 <20181002125115.245r3ocusvyiexno@queper01-lin>
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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?

If you want to allow updates, you'll have to do the whole RCU thing, at
which point you'll need rcu_assign_pointer(), which again is exactly
smp_store_release() :-)