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

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Message-ID: <ad392cd4-b2d0-4671-bb9f-4bb62e2b12bf@arm.com>
Date: Thu, 4 Jan 2024 16:45:33 +0100
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Lukasz Luba <lukasz.luba@....com>
Cc: rui.zhang@...el.com, amit.kucheria@...durent.com,
 linux-kernel@...r.kernel.org, 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,
 linux-pm@...r.kernel.org, rafael@...nel.org
Subject: Re: [PATCH v5 11/23] PM: EM: Add API for updating the runtime
 modifiable EM

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?