linux-kernel - Re: [PATCH v2 01/10] devm-helpers: introduce devm_mutex

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Date:   Wed, 6 Dec 2023 21:16:35 -0500
From:   Waiman Long <longman@...hat.com>
To:     George Stark <gnstark@...utedevices.com>,
        Hans de Goede <hdegoede@...hat.com>, pavel@....cz,
        lee@...nel.org, vadimp@...dia.com, mpe@...erman.id.au,
        npiggin@...il.com, christophe.leroy@...roup.eu,
        mazziesaccount@...il.com, andy.shevchenko@...il.com,
        jic23@...nel.org, peterz@...radead.org, boqun.feng@...il.com,
        will@...nel.org, mingo@...hat.com
Cc:     linux-leds@...r.kernel.org, linux-kernel@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, kernel@...utedevices.com
Subject: Re: [PATCH v2 01/10] devm-helpers: introduce devm_mutex_init

On 12/6/23 19:37, George Stark wrote:
> Hello Waiman
>
> Thanks for the review.
>
> On 12/7/23 00:02, Waiman Long wrote:
>> On 12/6/23 14:55, Hans de Goede wrote:
>>> Hi,
>>>
>>> On 12/6/23 19:58, George Stark wrote:
>>>> Hello Hans
>>>>
>>>> Thanks for the review.
>>>>
>>>> On 12/6/23 18:01, Hans de Goede wrote:
>>>>> Hi George,
>>>>>
> ...
>>>>> mutex_destroy() only actually does anything if CONFIG_DEBUG_MUTEXES
>>>>> is set, otherwise it is an empty inline-stub.
>>>>>
>>>>> Adding a devres resource to the device just to call an empty inline
>>>>> stub which is a no-op seems like a waste of resources. IMHO it
>>>>> would be better to change this to:
>>>>>
>>>>> static inline int devm_mutex_init(struct device *dev, struct mutex 
>>>>> *lock)
>>>>> {
>>>>>      mutex_init(lock);
>>>>> #ifdef CONFIG_DEBUG_MUTEXES
>>>>>      return devm_add_action_or_reset(dev, devm_mutex_release, lock);
>>>>> #else
>>>>>      return 0;
>>>>> #endif
>>>>> }
>>>>>
>>>>> To avoid the unnecessary devres allocation when
>>>>> CONFIG_DEBUG_MUTEXES is not set.
>>>> Honestly saying I don't like unnecessary devres allocation either 
>>>> but the proposed approach has its own price:
>>>>
>>>> 1) we'll have more than one place with branching if mutex_destroy 
>>>> is empty or not using  indirect condition. If suddenly 
>>>> mutex_destroy is extended for non-debug code (in upstream branch or 
>>>> e.g. by someone for local debug) than there'll be a problem.
>>>>
>>>> 2) If mutex_destroy is empty or not depends on CONFIG_PREEMPT_RT 
>>>> option too. When CONFIG_PREEMPT_RT is on mutex_destroy is always 
>>>> empty.
>>>>
>>>> As I see it only the mutex interface (mutex.h) has to say 
>>>> definitely if mutex_destroy must be called. Probably we could add 
>>>> some define to include/linux/mutex.h,like IS_MUTEX_DESTROY_REQUIRED 
>>>> and declare it near mutex_destroy definition itself.
>>> That (a  IS_MUTEX_DESTROY_REQUIRED define) is an interesting idea. 
>>> Lets s>
>>>>> Adding a devres resource to the device just to call an empty inline
>>>>> stub which is a no-op seems like a waste of resources. IMHO it
>>>>> would be better to change this to:
>>>>>
>>>>> static inline int devm_mutex_init(struct device *dev, struct mutex 
>>>>> *lock)
>>>>> {
>>>>>      mutex_init(lock);
>>>>> #ifdef CONFIG_DEBUG_MUTEXES
>>>>>      return devm_add_action_or_reset(dev, devm_mutex_release, lock);
>>>>> #else
>>>>>      return 0;
>>>>> #endif
>>>>> }
>>>>> ee for v3 if the mutex maintainers will accept that and if not 
>>> then I guess we will just need to live with the unnecessary devres 
>>> allocation.
>>
>> The purpose of calling mutex_destroy() is to mark a mutex as being 
>> destroyed so that any subsequent call to mutex_lock/unlock will cause 
>> a warning to be printed when CONFIG_DEBUG_MUTEXES is defined. I would 
>> not say that mutex_destroy() is required. Rather it is a nice to have 
>> for catching programming error.
>
> This is quite understandable but probably mutex_destroy() is not the 
> best name for an optional API. Questions are asked over and over again
> if it can be safely ignored taking into account that it could be 
> extended in the future. Every maintainer makes decision on that question
> in his own way and it leads to inconsistency.
>
> devm_mutex_init could take responsibility for calling/dropping 
> mutex_destroy() on its own.

The DEBUG_MUTEXES code is relatively old and there was no major change 
to it for a number of years. I don't see we will make major change to it 
in the near future. Of course, thing may change if there are new 
requirement that may affect the DEBUG_MUTEXES code.

Cheers,
Longman

>
>> Cheers,
>> Longman
>>
>