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

[Hans de Goede <hdegoede@redhat.com>]

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,
>>
>> 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.

That (a  IS_MUTEX_DESTROY_REQUIRED define) is an interesting idea. Lets see 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.

Regards,

Hans