Re: [PATCH v4 02/10] locking: introduce devm_mutex_init

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 14/12/2023 à 19:48, Waiman Long a écrit :
> 
> On 12/14/23 12:36, George Stark wrote:
>> Using of devm API leads to a 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() will be
>> extended so introduce devm_mutex_init()
>>
>> Signed-off-by: George Stark <gnstark@salutedevices.com>
>> ---
>>   include/linux/mutex.h        | 23 +++++++++++++++++++++++
>>   kernel/locking/mutex-debug.c | 22 ++++++++++++++++++++++
>>   2 files changed, 45 insertions(+)
>>
>> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
>> index a33aa9eb9fc3..ebd03ff1ef66 100644
>> --- a/include/linux/mutex.h
>> +++ b/include/linux/mutex.h
>> @@ -21,6 +21,8 @@
>>   #include <linux/debug_locks.h>
>>   #include <linux/cleanup.h>
>> +struct device;
>> +
>>   #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>   # define __DEP_MAP_MUTEX_INITIALIZER(lockname)            \
>>           , .dep_map = {                    \
>> @@ -127,6 +129,20 @@ extern void __mutex_init(struct mutex *lock, 
>> const char *name,
>>    */
>>   extern bool mutex_is_locked(struct mutex *lock);
>> +#ifdef CONFIG_DEBUG_MUTEXES
>> +
>> +int devm_mutex_init(struct device *dev, struct mutex *lock);
> Please add "extern" to the function declaration to be consistent with 
> other functional declarations in mutex.h.

'extern' is pointless and deprecated on function prototypes. Already 
having some is not a good reason to add new ones, errors from the past 
should be avoided nowadays. With time they should all disappear so don't 
add new ones.

>> +
>> +#else
>> +
>> +static inline int devm_mutex_init(struct device *dev, struct mutex 
>> *lock)
>> +{
>> +    mutex_init(lock);
>> +    return 0;
>> +}
> 
> I would prefer you to add a devm_mutex_init macro after the function 
> declaration and put this inline function at the end of header if the 
> devm_mutex_init macro isn't defined. In this way, you don't need to 
> repeat this inline function twice as it has no dependency on PREEMPT_RT.

It is already done that way for other functions in that file. Should be 
kept consistant. I agree with you it is not ideal, maybe we should 
rework that file completely but I don't like the idea of a 
devm_mutex_init macro for that.

Christophe

> 
> By doing this, you can also move the function declaration right after 
> mutex_destroy() without the need to add another #ifdef 
> CONFIG_DEBUG_MUTEXES block.
> 
>> +
>> +#endif
>> +
>>   #else /* !CONFIG_PREEMPT_RT */
>>   /*
>>    * Preempt-RT variant based on rtmutexes.
>> @@ -169,6 +185,13 @@ do {                            \
>>                               \
>>       __mutex_init((mutex), #mutex, &__key);        \
>>   } while (0)
>> +
>> +static inline int devm_mutex_init(struct device *dev, struct mutex 
>> *lock)
>> +{
>> +    mutex_init(lock);
>> +    return 0;
>> +}
>> +
>>   #endif /* CONFIG_PREEMPT_RT */
>>   /*
>> diff --git a/kernel/locking/mutex-debug.c b/kernel/locking/mutex-debug.c
>> index bc8abb8549d2..c9efab1a8026 100644
>> --- a/kernel/locking/mutex-debug.c
>> +++ b/kernel/locking/mutex-debug.c
>> @@ -19,6 +19,7 @@
>>   #include <linux/kallsyms.h>
>>   #include <linux/interrupt.h>
>>   #include <linux/debug_locks.h>
>> +#include <linux/device.h>
>>   #include "mutex.h"
>> @@ -104,3 +105,24 @@ void mutex_destroy(struct mutex *lock)
>>   }
>>   EXPORT_SYMBOL_GPL(mutex_destroy);
>> +
>> +static void devm_mutex_release(void *res)
>> +{
>> +    mutex_destroy(res);
>> +}
>> +
>> +/**
>> + * devm_mutex_init - Resource-managed mutex initialization
>> + * @dev:    Device which lifetime mutex is bound to
>> + * @lock:    Pointer to a mutex
>> + *
>> + * Initialize mutex which is automatically destroyed when the driver 
>> is detached.
>> + *
>> + * Returns: 0 on success or a negative error code on failure.
>> + */
>> +int devm_mutex_init(struct device *dev, struct mutex *lock)
>> +{
>> +    mutex_init(lock);
>> +    return devm_add_action_or_reset(dev, devm_mutex_release, lock);
>> +}
>> +EXPORT_SYMBOL_GPL(devm_mutex_init);
> 
> The mutex-debug.c change looks fine to me.
> 
> Cheers,
> Longman
> 
>