linux-kernel - Re: [PATCH v6 12/23] PM: EM: Add helpers to read under RCU lock the EM table

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Date: Thu, 4 Jan 2024 20:55:54 +0100
From: "Rafael J. Wysocki" <rafael@...nel.org>
To: Lukasz Luba <lukasz.luba@....com>
Cc: linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org, rafael@...nel.org, 
	dietmar.eggemann@....com, rui.zhang@...el.com, amit.kucheria@...durent.com, 
	amit.kachhap@...il.com, daniel.lezcano@...aro.org, viresh.kumar@...aro.org, 
	len.brown@...el.com, pavel@....cz, mhiramat@...nel.org, qyousef@...alina.io, 
	wvw@...gle.com
Subject: Re: [PATCH v6 12/23] PM: EM: Add helpers to read under RCU lock the
 EM table

On Thu, Jan 4, 2024 at 6:15 PM Lukasz Luba <lukasz.luba@....com> wrote:
>
> To use the runtime modifiable EM table there is a need to use RCU
> read locking properly. Add helper functions for the device drivers and
> frameworks to make sure it's done properly.
>
> Signed-off-by: Lukasz Luba <lukasz.luba@....com>
> ---
>  include/linux/energy_model.h | 19 +++++++++++++++++++
>  1 file changed, 19 insertions(+)
>
> diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
> index f33257ed83fd..cfaf5d8b1aad 100644
> --- a/include/linux/energy_model.h
> +++ b/include/linux/energy_model.h
> @@ -338,6 +338,20 @@ static inline int em_pd_nr_perf_states(struct em_perf_domain *pd)
>         return pd->nr_perf_states;
>  }
>
> +static inline struct em_perf_state *em_get_table(struct em_perf_domain *pd)
> +{
> +       struct em_perf_table __rcu *table;
> +
> +       rcu_read_lock();
> +       table = rcu_dereference(pd->em_table);
> +       return table->state;
> +}
> +
> +static inline void em_put_table(void)
> +{
> +       rcu_read_unlock();
> +}

The lack of symmetry between em_get_table() and em_put_table() is kind
of confusing.

I don't really like these wrappers.

IMO it would be better to use rcu_read_lock()/rcu_read_unlock()
directly everywhere they are needed and there can be a wrapper around
rcu_dereference(pd->em_table), something like

static inline struct em_perf_state *em_perf_state_from_pd(struct
em_perf_domain *pd)
{
        return rcu_dereference(pd->em_table)->state;
}

> +
>  #else
>  struct em_data_callback {};
>  #define EM_ADV_DATA_CB(_active_power_cb, _cost_cb) { }
> @@ -384,6 +398,11 @@ int em_dev_update_perf_domain(struct device *dev,
>  {
>         return -EINVAL;
>  }
> +static inline struct em_perf_state *em_get_table(struct em_perf_domain *pd)
> +{
> +       return NULL;
> +}
> +static inline void em_put_table(void) {}
>  #endif
>
>  #endif
> --