Re: [PATCH v5 11/23] PM: EM: Add API for updating the runtime modifiable EM

[Lukasz Luba]

On 1/4/24 15:45, Dietmar Eggemann wrote:
> On 20/12/2023 09:06, Lukasz Luba wrote:
> > On 12/12/23 18:50, Dietmar Eggemann wrote:
> > > On 29/11/2023 12:08, Lukasz Luba wrote:
>
> [...]
>
> > > > +int em_dev_update_perf_domain(struct device *dev,
> > > > +                  struct em_perf_table __rcu *new_table)
> > > > +{
> > > > +    struct em_perf_table __rcu *old_table;
> > > > +    struct em_perf_domain *pd;
> > > > +
> > > > +    /*
> > > > +     * The lock serializes update and unregister code paths. When the
> > > > +     * EM has been unregistered in the meantime, we should capture that
> > > > +     * when entering this critical section. It also makes sure that
> > >
> > > What do you want to capture here? You want to block in this moment,
> > > right? Don't understand the 2. sentence here.
> > >
> > > [...]
> >
> > There is general issue with module... they can reload. A driver which
> > registered EM can than later disappear. I had similar issues for the
> > devfreq cooling. It can happen at any time. In this scenario let's
> > consider scenario w/ 2 kernel drivers:
> > 1. Main driver which registered EM, e.g. GPU driver
> > 2. Thermal driver which updates that EM
> > When 1. starts unload process, it has to make sure that it will
> > not free the main EM 'pd', because the 2. might try to use e.g.
> > 'pd->nr_perf_states' while doing update at the moment.
> > Thus, this 'pd' has local mutex, to avoid issues of
> > module unload vs. EM update. The EM unregister will block on
> > that mutex and let the background update finish it's critical
> > section.
>
> All true but wouldn't
>
>      /* Serialize update/unregister or concurrent updates */
>
> be sufficient as a comment here?

Sounds good, I'll change that.