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

[Christophe Leroy <christophe.leroy@xxxxxxxxxx>]

Le 14/12/2023 à 18:36, George Stark a écrit :
> [Vous ne recevez pas souvent de courriers de gnstark@xxxxxxxxxxxxxxxxx. Découvrez pourquoi ceci est important à https://aka.ms/LearnAboutSenderIdentification ]
> 
> 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>
> ---
>   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);
> +
> +#else
> +
> +static inline int devm_mutex_init(struct device *dev, struct mutex *lock)
> +{
> +       mutex_init(lock);
> +       return 0;
> +}
> +
> +#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);
> --
> 2.25.1
> 

I think it would make sense to keep mutex_destroy() and 
devm_mutex_init() together, see exemple below:

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index ebd03ff1ef66..c620759ff85b 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -83,14 +83,10 @@ struct mutex {
  #define __DEBUG_MUTEX_INITIALIZER(lockname)				\
  	, .magic = &lockname

-extern void mutex_destroy(struct mutex *lock);
-
  #else

  # define __DEBUG_MUTEX_INITIALIZER(lockname)

-static inline void mutex_destroy(struct mutex *lock) {}
-
  #endif

  /**
@@ -131,10 +127,13 @@ extern bool mutex_is_locked(struct mutex *lock);

  #ifdef CONFIG_DEBUG_MUTEXES

+void mutex_destroy(struct mutex *lock);
  int devm_mutex_init(struct device *dev, struct mutex *lock);

  #else

+static inline void mutex_destroy(struct mutex *lock) {}
+
  static inline int devm_mutex_init(struct device *dev, struct mutex *lock)
  {
  	mutex_init(lock);
@@ -169,8 +168,6 @@ extern void __mutex_rt_init(struct mutex *lock, 
const char *name,
  			    struct lock_class_key *key);
  extern int mutex_trylock(struct mutex *lock);

-static inline void mutex_destroy(struct mutex *lock) { }
-
  #define mutex_is_locked(l)	rt_mutex_base_is_locked(&(l)->rtmutex)

  #define __mutex_init(mutex, name, key)			\
@@ -186,6 +183,8 @@ do {							\
  	__mutex_init((mutex), #mutex, &__key);		\
  } while (0)

+static inline void mutex_destroy(struct mutex *lock) { }
+
  static inline int devm_mutex_init(struct device *dev, struct mutex *lock)
  {
  	mutex_init(lock);
---

Would also be nice to have a comment explaining that when 
mutex_destroy() is a nop, devm_mutext_init() doesn't need to register a 
release function.

Either way,

Reviewed-by: christophe.leroy@xxxxxxxxxx