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

[Peter Zijlstra]

On Tue, Oct 02, 2018 at 01:51:17PM +0100, Quentin Perret wrote:
> On Tuesday 02 Oct 2018 at 14:30:31 (+0200), Peter Zijlstra wrote:
> > On Wed, Sep 12, 2018 at 10:12:58AM +0100, Quentin Perret wrote:
> > > +/**
> > > + * em_register_perf_domain() - Register the Energy Model of a performance domain
> > > + * @span	: Mask of CPUs in the performance domain
> > > + * @nr_states	: Number of capacity states to register
> > > + * @cb		: Callback functions providing the data of the Energy Model
> > > + *
> > > + * Create Energy Model tables for a performance domain using the callbacks
> > > + * defined in cb.
> > > + *
> > > + * If multiple clients register the same performance domain, all but the first
> > > + * registration will be ignored.
> > > + *
> > > + * Return 0 on success
> > > + */
> > > +int em_register_perf_domain(cpumask_t *span, unsigned int nr_states,
> > > +						struct em_data_callback *cb)
> > > +{
> > > +	unsigned long cap, prev_cap = 0;
> > > +	struct em_perf_domain *pd;
> > > +	int cpu, ret = 0;
> > > +
> > > +	if (!span || !nr_states || !cb)
> > > +		return -EINVAL;
> > > +
> > > +	/*
> > > +	 * Use a mutex to serialize the registration of performance domains and
> > > +	 * let the driver-defined callback functions sleep.
> > > +	 */
> > > +	mutex_lock(&em_pd_mutex);
> > > +
> > > +	for_each_cpu(cpu, span) {
> > > +		/* Make sure we don't register again an existing domain. */
> > > +		if (READ_ONCE(per_cpu(em_data, cpu))) {
> > > +			ret = -EEXIST;
> > > +			goto unlock;
> > > +		}
> > > +
> > > +		/*
> > > +		 * All CPUs of a domain must have the same micro-architecture
> > > +		 * since they all share the same table.
> > > +		 */
> > > +		cap = arch_scale_cpu_capacity(NULL, cpu);
> > > +		if (prev_cap && prev_cap != cap) {
> > > +			pr_err("CPUs of %*pbl must have the same capacity\n",
> > > +							cpumask_pr_args(span));
> > > +			ret = -EINVAL;
> > > +			goto unlock;
> > > +		}
> > > +		prev_cap = cap;
> > > +	}
> > > +
> > > +	/* Create the performance domain and add it to the Energy Model. */
> > > +	pd = em_create_pd(span, nr_states, cb);
> > > +	if (!pd) {
> > > +		ret = -EINVAL;
> > > +		goto unlock;
> > > +	}
> > > +
> > > +	for_each_cpu(cpu, span)
> > > +		WRITE_ONCE(per_cpu(em_data, cpu), pd);
> > 
> > It's not immediately obvious to me why this doesn't need to be
> > smp_store_release(). The moment you publish that pointer, it can be
> > read, right?
> > 
> > Even if you never again change the pointer value, you want to ensure the
> > content of pd is stable before pd itself is observable, right?
> 
> So, I figured the mutex already gives me some of that. I mean, AFAIU it
> should guarantee that concurrent callers to em_register_perf_domain are
> serialized correctly.

+/**
+ * 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.