Re: [PATCH v5 02/10] locking/mutex: introduce devm_mutex_init

[Waiman Long <longman@redhat.com>]

On 3/7/24 04:56, Marek Behún wrote:
> On Thu, Mar 07, 2024 at 05:40:26AM +0300, 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>
>> Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
>> ---
>>   Hello Christophe. Hope you don't mind I put you SoB tag because you helped alot
>>   to make this patch happen.
>>
>>   include/linux/mutex.h        | 13 +++++++++++++
>>   kernel/locking/mutex-debug.c | 22 ++++++++++++++++++++++
>>   2 files changed, 35 insertions(+)
>>
>> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
>> index f7611c092db7..9bcf72cb941a 100644
>> --- a/include/linux/mutex.h
>> +++ b/include/linux/mutex.h
>> @@ -22,6 +22,8 @@
>>   #include <linux/cleanup.h>
>>   #include <linux/mutex_types.h>
>>
>> +struct device;
>> +
>>   #ifdef CONFIG_DEBUG_LOCK_ALLOC
>>   # define __DEP_MAP_MUTEX_INITIALIZER(lockname)			\
>>   		, .dep_map = {					\
>> @@ -115,10 +117,21 @@ do {							\
>>
>>   #ifdef CONFIG_DEBUG_MUTEXES
>>
>> +int devm_mutex_init(struct device *dev, struct mutex *lock);
>>   void mutex_destroy(struct mutex *lock);
>>
>>   #else
>>
>> +static inline int devm_mutex_init(struct device *dev, struct mutex *lock)
>> +{
>> +	/*
>> +	 * since mutex_destroy is nop actually there's no need to register it
>> +	 * in devm subsystem.
>> +	 */
>> +	mutex_init(lock);
>> +	return 0;
>> +}
>> +
>>   static inline void mutex_destroy(struct mutex *lock) {}
>>
>>   #endif
>> 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);
> Hi George,
>
> look at
> https://lore.kernel.org/lkml/7013bf9e-2663-4613-ae61-61872e81355b@redhat.com/
> where Matthew and Hans explain that devm_mutex_init needs to be a macro
> because of the static lockdep key.
>
> so this should be something like:
>
> static inline int __devm_mutex_init(struct device *dev, struct mutex *mutex,
> 				    const char *name,
> 				    struct lock_class_key *key)
> {
> 	__mutex_init(mutex, name, key);
> 	return devm_add_action_or_reset(dev, devm_mutex_release, mutex);
> }
>
> #define devm_mutex_init(dev, mutex)				\
> do {								\
> 	static struct lock_class_key __key;			\
> 								\
> 	__devm_mutex_init(dev, (mutex), #mutex, &__key);	\
> } while (0);
>
>
> Marek

Making devm_mutex_init() a function will make all the devm_mutex share 
the same lockdep key. Making it a macro will make each caller of 
devm_mutex_init() have a distinct lockdep key. It all depends on whether 
all the devm_mutexes have the same lock usage pattern or not and whether 
it is possible for one devm_mutex to be nested inside another. So either 
way can be fine depending on the mutex usage pattern. My suggestion is 
to use a function, if possible, unless it will cause a false positive 
lockdep splat as there is a limit on the maximum # of lockdep keys that 
can be used.

Cheers,
Longman