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

[Andy Shevchenko]

On Thu, Dec 7, 2023 at 2:31 PM Christophe Leroy
<christophe.leroy at csgroup.eu> wrote:
> Le 07/12/2023 à 12:59, Andy Shevchenko a écrit :
> > On Thu, Dec 7, 2023 at 1:23 AM George Stark <gnstark at salutedevices.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);

^^^^ (1)

> >>>>>> #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 ?

Yes, see (1) above. If you want to have it in the header, you must
provide an API, which is located in device.h. The idea about
mutex-debug.c is interesting, but the file naming and the devm_*() API
for _initing_ the mutex seems confusing.

-- 
With Best Regards,
Andy Shevchenko