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Date: Thu, 4 Jan 2024 16:45:33 +0100
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Subject: Re: [PATCH v5 11/23] PM: EM: Add API for updating the runtime
 modifiable EM
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To: Lukasz Luba <lukasz.luba@arm.com>
Cc: rui.zhang@intel.com, amit.kucheria@verdurent.com,
 linux-kernel@vger.kernel.org, amit.kachhap@gmail.com,
 daniel.lezcano@linaro.org, viresh.kumar@linaro.org, len.brown@intel.com,
 pavel@ucw.cz, mhiramat@kernel.org, qyousef@layalina.io, wvw@google.com,
 linux-pm@vger.kernel.org, rafael@kernel.org
References: <20231129110853.94344-1-lukasz.luba@arm.com>
 <20231129110853.94344-12-lukasz.luba@arm.com>
 <2236f098-b889-4d70-b863-11a3f246889c@arm.com>
 <22c41b1a-333e-42b1-839f-a755d88313af@arm.com>
From: Dietmar Eggemann <dietmar.eggemann@arm.com>
In-Reply-To: <22c41b1a-333e-42b1-839f-a755d88313af@arm.com>
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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?