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

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Message-ID: <be693688-2e82-4e1a-9ead-cf1513ee637b@csgroup.eu>
Date:   Wed, 6 Dec 2023 22:37:11 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     George Stark <gnstark@...utedevices.com>,
        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>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.com>,
        "jic23@...nel.org" <jic23@...nel.org>,
        "peterz@...radead.org" <peterz@...radead.org>,
        Waiman Long <longman@...hat.com>
CC:     "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 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@...utedevices.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@...utedevices.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() ?

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

Wouldn't that work ?

Christophe