Re: [PATCH v2 01/10] devm-helpers: introduce devm_mutex_init

[George Stark <gnstark@salutedevices.com>]

Hello Christophe

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 :
>>> [Vous ne recevez pas souvent de courriers de
>>> gnstark@salutedevices.com. Découvrez pourquoi ceci est important à
>>> https://aka.ms/LearnAboutSenderIdentification ]
>>>
>>> Hello Hans
>>>
>>> Thanks for the review.
>>>
>>> On 12/6/23 18:01, Hans de Goede wrote:
>>>> Hi George,
>>>>
>>>> On 12/4/23 19:05, George Stark wrote:
>>>>> Using of devm API leads to certain order of releasing resources.
>>>>> So all dependent resources which are not devm-wrapped should be deleted
>>>>> with respect to devm-release order. Mutex is one of such objects that
>>>>> often is bound to other resources and has no own devm wrapping.
>>>>> Since mutex_destroy() actually does nothing in non-debug builds
>>>>> frequently calling mutex_destroy() is just ignored which is safe for
>>>>> now
>>>>> but wrong formally and can lead to a problem if mutex_destroy() is
>>>>> extended so introduce devm_mutex_init().
>>>>>
>>>>> Signed-off-by: George Stark <gnstark@salutedevices.com>
>>>>> ---
>>>>>    include/linux/devm-helpers.h | 18 ++++++++++++++++++
>>>>>    1 file changed, 18 insertions(+)
>>>>>
>>>>> diff --git a/include/linux/devm-helpers.h
>>>>> b/include/linux/devm-helpers.h
>>>>> index 74891802200d..2f56e476776f 100644
>>>>> --- a/include/linux/devm-helpers.h
>>>>> +++ b/include/linux/devm-helpers.h
>>>>> @@ -76,4 +76,22 @@ static inline int devm_work_autocancel(struct
>>>>> device *dev,
>>>>>       return devm_add_action(dev, devm_work_drop, w);
>>>>>    }
>>>>>
>>>>> +static inline void devm_mutex_release(void *res)
>>>>> +{
>>>>> +    mutex_destroy(res);
>>>>> +}
>>>>> +
>>>>> +/**
>>>>> + * devm_mutex_init - Resource-managed mutex initialization
>>>>> + * @dev:    Device which lifetime work is bound to
>>>>> + * @lock:   Pointer to a mutex
>>>>> + *
>>>>> + * Initialize mutex which is automatically destroyed when driver is
>>>>> detached.
>>>>> + */
>>>>> +static inline int devm_mutex_init(struct device *dev, struct mutex
>>>>> *lock)
>>>>> +{
>>>>> +    mutex_init(lock);
>>>>> +    return devm_add_action_or_reset(dev, devm_mutex_release, lock);
>>>>> +}
>>>>> +
>>>>>    #endif
>>>>
>>>> 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 ?
> 
> Christophe

-- 
Best regards
George