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

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Date:   Wed, 6 Dec 2023 21:58:18 +0300
From:   George Stark <gnstark@...utedevices.com>
To:     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>,
        Waiman Long <longman@...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 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.

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