On Thu, Dec 7, 2023 at 1:23 AM George Stark [off-list ref] wrote:

On 12/7/23 01:37, Christophe Leroy wrote:

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Le 06/12/2023 à 23:14, Christophe Leroy a écrit :

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Le 06/12/2023 à 19:58, George Stark a écrit :

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On 12/6/23 18:01, Hans de Goede wrote:

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On 12/4/23 19:05, George Stark wrote:

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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 (local)

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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;

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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