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

[Andy Shevchenko]

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);
> >>>> #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.

-- 
With Best Regards,
Andy Shevchenko