linux-kernel - Re: [PATCH v3 03/11] devm-helpers: introduce devm_mutex

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Message-ID: <c8950992-9b3e-4740-8ad6-f22d5a043fb1@redhat.com>
Date:   Thu, 14 Dec 2023 12:51:53 +0100
From:   Hans de Goede <hdegoede@...hat.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        George Stark <gnstark@...utedevices.com>,
        "andy.shevchenko@...il.com" <andy.shevchenko@...il.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>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "will@...nel.org" <will@...nel.org>,
        "longman@...hat.com" <longman@...hat.com>,
        "boqun.feng@...il.com" <boqun.feng@...il.com>,
        "nikitos.tr@...il.com" <nikitos.tr@...il.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 v3 03/11] devm-helpers: introduce devm_mutex_init

Hi,

On 12/14/23 11:06, Christophe Leroy wrote:
> 
> 
> Le 13/12/2023 à 23:30, 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 ]
>>
>> Using of devm API leads to a 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 wrapper.
>> 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().
> 
> So you abandonned the idea of using mutex.h ?
> 
> I can't see the point to spread mutex functions into devm-helpers.h
> 
> Adding a mutex_destroy macro for this purpose looks odd. And if someone 
> defines a new version of mutex_destroy() and forget the macro, it will 
> go undetected.
> 
> Usually macros of that type serve the purpose of defining a fallback 
> when the macro is not defined. In that case, when someone adds a new 
> version without defining the macro, it gets detected because if 
> conflicts with the fallback.
> But in your case it works the other way round, so I will just go undetected.
> 
> For me the best solution remains to use mutex.h and have 
> devm_mutex_init() defined or declared at the same place as mutex_destroy().

FWIW defining devm_mutex_init() in mutex.h is fine
with me and makes sense to me. I also agree that putting
it there would be better if that is acceptable for
the mutex maintainers.

devm-helpers.h is there for helpers which don't fit
in another place.

Regards,

Hans




> 
> 
>>
>> Signed-off-by: George Stark <gnstark@...utedevices.com>
>> ---
>>   include/linux/devm-helpers.h | 27 +++++++++++++++++++++++++++
>>   1 file changed, 27 insertions(+)
>>
>> diff --git a/include/linux/devm-helpers.h b/include/linux/devm-helpers.h
>> index 74891802200d..4043c3481d2e 100644
>> --- a/include/linux/devm-helpers.h
>> +++ b/include/linux/devm-helpers.h
>> @@ -24,6 +24,7 @@
>>    */
>>
>>   #include <linux/device.h>
>> +#include <linux/mutex.h>
>>   #include <linux/workqueue.h>
>>
>>   static inline void devm_delayed_work_drop(void *res)
>> @@ -76,4 +77,30 @@ static inline int devm_work_autocancel(struct device *dev,
>>          return devm_add_action(dev, devm_work_drop, w);
>>   }
>>
>> +#ifdef mutex_destroy
>> +static inline void devm_mutex_release(void *res)
>> +{
>> +       mutex_destroy(res);
>> +}
>> +#endif
>> +
>> +/**
>> + * devm_mutex_init - Resource-managed mutex initialization
>> + * @dev:       Device which lifetime mutex is bound to
>> + * @lock:      Pointer to a mutex
>> + *
>> + * Initialize mutex which is automatically destroyed when the driver is detached.
>> + *
>> + * Returns: 0 on success or a negative error code on failure.
>> + */
>> +static inline int devm_mutex_init(struct device *dev, struct mutex *lock)
>> +{
>> +       mutex_init(lock);
>> +#ifdef mutex_destroy
>> +       return devm_add_action_or_reset(dev, devm_mutex_release, lock);
>> +#else
>> +       return 0;
>> +#endif
>> +}
>> +
>>   #endif
>> --
>> 2.25.1
>>