[PATCH v4 02/10] locking: introduce devm_mutex_init

[Waiman Long]

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 at 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.
> +
> +#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.

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