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

[Marek Behún]

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@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Christophe Leroy <christophe.leroy@xxxxxxxxxx>
> ---
>  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@xxxxxxxxxx/
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