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

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Message-ID: <469f44fb-2371-4b3b-bc1c-d09ec35a5ec8@redhat.com>
Date:   Wed, 6 Dec 2023 16:02:02 -0500
From:   Waiman Long <longman@...hat.com>
To:     Hans de Goede <hdegoede@...hat.com>,
        George Stark <gnstark@...utedevices.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
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

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,
>>>
>>> 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@...utedevices.com>
>>>> ---
>>>>    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.
> That (a  IS_MUTEX_DESTROY_REQUIRED define) is an interesting idea. Lets see 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.

Cheers,
Longman