linux-kernel - Re: [PATCH v2 01/10] devm-helpers: introduce devm_mutex

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Date:   Thu, 7 Dec 2023 03:37:59 +0300
From:   George Stark <gnstark@...utedevices.com>
To:     Waiman Long <longman@...hat.com>,
        Hans de Goede <hdegoede@...hat.com>, <pavel@....cz>,
        <lee@...nel.org>, <vadimp@...dia.com>, <mpe@...erman.id.au>,
        <npiggin@...il.com>, <christophe.leroy@...roup.eu>,
        <mazziesaccount@...il.com>, <andy.shevchenko@...il.com>,
        <jic23@...nel.org>, <peterz@...radead.org>, <boqun.feng@...il.com>,
        <will@...nel.org>, <mingo@...hat.com>
CC:     <linux-leds@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linuxppc-dev@...ts.ozlabs.org>, <kernel@...utedevices.com>
Subject: Re: [PATCH v2 01/10] devm-helpers: introduce devm_mutex_init

Hello Waiman

Thanks for the review.

On 12/7/23 00:02, Waiman Long wrote:
> On 12/6/23 14:55, Hans de Goede wrote:
>> 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,
>>>>
...
>>>> 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 s>
>>>> 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
>>>> }
>>>>ee 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.
> 
> The purpose of calling mutex_destroy() is to mark a mutex as being 
> destroyed so that any subsequent call to mutex_lock/unlock will cause a 
> warning to be printed when CONFIG_DEBUG_MUTEXES is defined. I would not 
> say that mutex_destroy() is required. Rather it is a nice to have for 
> catching programming error.

This is quite understandable but probably mutex_destroy() is not the 
best name for an optional API. Questions are asked over and over again
if it can be safely ignored taking into account that it could be 
extended in the future. Every maintainer makes decision on that question
in his own way and it leads to inconsistency.

devm_mutex_init could take responsibility for calling/dropping 
mutex_destroy() on its own.


> Cheers,
> Longman
> 

-- 
Best regards
George