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

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Message-ID: <3275dec3-fd19-4aa1-8eba-441fd64cc185@csgroup.eu>
Date:   Thu, 7 Dec 2023 12:31:27 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Andy Shevchenko <andy.shevchenko@...il.com>,
        George Stark <gnstark@...utedevices.com>
CC:     Hans de Goede <hdegoede@...hat.com>, "pavel@....cz" <pavel@....cz>,
        "lee@...nel.org" <lee@...nel.org>,
        "vadimp@...dia.com" <vadimp@...dia.com>,
        "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "mazziesaccount@...il.com" <mazziesaccount@...il.com>,
        "jic23@...nel.org" <jic23@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "will@...nel.org" <will@...nel.org>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "linux-leds@...r.kernel.org" <linux-leds@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        "kernel@...utedevices.com" <kernel@...utedevices.com>
Subject: Re: [PATCH v2 01/10] devm-helpers: introduce devm_mutex_init



Le 07/12/2023 à 12:59, Andy Shevchenko a écrit :
> On Thu, Dec 7, 2023 at 1:23 AM George Stark <gnstark@...utedevices.com> wrote:
>> On 12/7/23 01:37, Christophe Leroy wrote:
>>> Le 06/12/2023 à 23:14, Christophe Leroy a écrit :
>>>> Le 06/12/2023 à 19:58, George Stark a écrit :
>>>>> On 12/6/23 18:01, Hans de Goede wrote:
>>>>>> On 12/4/23 19:05, George Stark wrote:
> 
> ...
> 
>>>>>> 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.
>>>>>
>>>>> I tried to put devm_mutex_init itself in mutex.h and it could've helped
>>>>> too but it's not the place for devm API.
>>>>>
>>>>
>>>> What do you mean by "it's not the place for devm API" ?
>>>>
>>>> If you do a 'grep devm_ include/linux/' you'll find devm_ functions in
>>>> almost 100 .h files. Why wouldn't mutex.h be the place for
>>>> devm_mutex_init() ?
>> mutex.h's maintainers believe so.
>>
>> https://lore.kernel.org/lkml/070c174c-057a-46de-ae8e-836e9e20eceb@salutedevices.com/T/#mb42e1d7760816b0cedd3130e08f29690496b5ac2
>>>
>>> Looking at it closer, I have the feeling that you want to do similar to
>>> devm_gpio_request() in linux/gpio.h :
>>>
>>> In linux/mutex.h, add a prototype for devm_mutex_init() when
>>> CONFIG_DEBUG_MUTEXES is defined and an empty static inline otherwise.
>>> Then define devm_mutex_init() in kernel/locking/mutex-debug.c
>>
>> Yes, this would be almost perfect decision. BTW just as in linux/gpio.h
>> we wouldn't have to include whole "linux/device.h" into mutex.h, only
>> add forward declaration of struct device;
>>
>>> Wouldn't that work ?
> 
> No. It will require inclusion of device.h (which is a twisted hell
> from the header perspective) into mutex.h. Completely unappreciated
> move.
> 

I see no reason for including device.h, I think a forward declaration of 
struct device would be enough, as done in linux/gpio.h

Am I missing something ?