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

[George Stark]

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@xxxxxxxxxxxxxxxxx>
> > ---
> >   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.

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.

> Regards,
>
> Hans

--
Best regards
George