From: George Stark <gnstark@salutedevices.com>

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>
---
 include/linux/mutex.h        | 28 ++++++++++++++++++++++------
 kernel/locking/mutex-debug.c | 22 ++++++++++++++++++++++
 2 files changed, 44 insertions(+), 6 deletions(-)

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index a33aa9eb9fc3..db847220ef44 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -81,14 +81,10 @@ 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
 
 /**
@@ -153,8 +149,6 @@ 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)			\
@@ -171,6 +165,28 @@ do {							\
 } while (0)
 #endif /* CONFIG_PREEMPT_RT */
 
+struct device;
+
+/*
+ * devm_mutex_init() registers a function that calls mutex_destroy()
+ * when the ressource is released.
+ *
+ * When mutex_destroy() is a not, there is no need to register that
+ * function.
+ */
+#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);
+	return 0;
+}
+#endif
+
 /*
  * See kernel/locking/mutex.c for detailed documentation of these APIs.
  * Also see Documentation/locking/mutex-design.rst.
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);
-- 
2.41.0

On Fri, Dec 15, 2023 at 8:23 AM Christophe Leroy
<christophe.leroy@csgroup.eu> wrote:
>
> From: George Stark <gnstark@salutedevices.com>
>
> 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()

Missing period.

...

>  } while (0)
>  #endif /* CONFIG_PREEMPT_RT */

^^^ (1)

> +struct device;
> +
> +/*
> + * devm_mutex_init() registers a function that calls mutex_destroy()
> + * when the ressource is released.
> + *
> + * When mutex_destroy() is a not, there is no need to register that
> + * function.
> + */
> +#ifdef CONFIG_DEBUG_MUTEXES

Shouldn't this be

#if defined(CONFIG_DEBUG_MUTEXES) && !defined(CONFIG_PREEMPT_RT)

(see (1) as well)?

> +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);
> +       return 0;
> +}
> +#endif

-- 
With Best Regards,
Andy Shevchenko

On 12/15/23 10:58, Andy Shevchenko wrote:
> On Fri, Dec 15, 2023 at 8:23 AM Christophe Leroy
> <christophe.leroy@csgroup.eu> wrote:
>> From: George Stark <gnstark@salutedevices.com>
>>
>> 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()
> Missing period.
>
> ...
>
>>   } while (0)
>>   #endif /* CONFIG_PREEMPT_RT */
> ^^^ (1)
>
>> +struct device;
>> +
>> +/*
>> + * devm_mutex_init() registers a function that calls mutex_destroy()
>> + * when the ressource is released.
>> + *
>> + * When mutex_destroy() is a not, there is no need to register that
>> + * function.
>> + */
>> +#ifdef CONFIG_DEBUG_MUTEXES
> Shouldn't this be
>
> #if defined(CONFIG_DEBUG_MUTEXES) && !defined(CONFIG_PREEMPT_RT)
>
> (see (1) as well)?

CONFIG_DEBUG_MUTEXES and CONFIG_PREEMPT_RT are mutually exclusive. At 
most one of them can be set.

Cheers,
Longman