devm_mutex_init() can fail. With CONFIG_DEBUG_MUTEXES=y the mutex will be
marked as unusable and trigger errors on usage.
Enforce all callers check the return value through the compiler.

As devm_mutex_init() itself is a macro, it can not be annotated
directly. Annotate __devm_mutex_init() instead.
Unfortunately __must_check/warn_unused_result don't propagate through
statement expression. So move the statement expression into the argument
list of the call to __devm_mutex_init() through a helper macro.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
---
 include/linux/mutex.h | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/include/linux/mutex.h b/include/linux/mutex.h
index a039fa8c17807c700d3b61193feac0418cad1243..00afd341d293ddfcc0a427b576efdce044955e38 100644
--- a/include/linux/mutex.h
+++ b/include/linux/mutex.h
@@ -126,11 +126,11 @@ do {							\
 
 #ifdef CONFIG_DEBUG_MUTEXES
 
-int __devm_mutex_init(struct device *dev, struct mutex *lock);
+int __must_check __devm_mutex_init(struct device *dev, struct mutex *lock);
 
 #else
 
-static inline int __devm_mutex_init(struct device *dev, struct mutex *lock)
+static inline int __must_check __devm_mutex_init(struct device *dev, struct mutex *lock)
 {
 	/*
 	 * When CONFIG_DEBUG_MUTEXES is off mutex_destroy() is just a nop so
@@ -141,14 +141,17 @@ static inline int __devm_mutex_init(struct device *dev, struct mutex *lock)
 
 #endif
 
-#define devm_mutex_init(dev, mutex)			\
+#define __mutex_init_ret(mutex)				\
 ({							\
 	typeof(mutex) mutex_ = (mutex);			\
 							\
 	mutex_init(mutex_);				\
-	__devm_mutex_init(dev, mutex_);			\
+	mutex_;						\
 })
 
+#define devm_mutex_init(dev, mutex) \
+	__devm_mutex_init(dev, __mutex_init_ret(mutex))
+
 /*
  * See kernel/locking/mutex.c for detailed documentation of these APIs.
  * Also see Documentation/locking/mutex-design.rst.

-- 
2.50.0

On Tue, Jun 17, 2025 at 07:08:14PM +0200, Thomas Weißschuh wrote:
> devm_mutex_init() can fail. With CONFIG_DEBUG_MUTEXES=y the mutex will be
> marked as unusable and trigger errors on usage.
> Enforce all callers check the return value through the compiler.
> 
> As devm_mutex_init() itself is a macro, it can not be annotated
> directly. Annotate __devm_mutex_init() instead.
> Unfortunately __must_check/warn_unused_result don't propagate through
> statement expression. So move the statement expression into the argument
> list of the call to __devm_mutex_init() through a helper macro.
> 

Given it's Peter's suggestion [1] for __mutex_init_ret(), I think a

Suggested-by: Peter Zijlstra <peterz@infradead.org>

should be added here. No need for a new version just for that. If patch
#1 got some reviews, I would queue this in my lockdep-for-tip branch.
Hopefully these will land v6.17.

[1]: https://lore.kernel.org/all/20250204090106.GP7145@noisy.programming.kicks-ass.net/

Regards,
Boqun

> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
> Reviewed-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
> ---
>  include/linux/mutex.h | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
> index a039fa8c17807c700d3b61193feac0418cad1243..00afd341d293ddfcc0a427b576efdce044955e38 100644
> --- a/include/linux/mutex.h
> +++ b/include/linux/mutex.h
> @@ -126,11 +126,11 @@ do {							\
>  
>  #ifdef CONFIG_DEBUG_MUTEXES
>  
> -int __devm_mutex_init(struct device *dev, struct mutex *lock);
> +int __must_check __devm_mutex_init(struct device *dev, struct mutex *lock);
>  
>  #else
>  
> -static inline int __devm_mutex_init(struct device *dev, struct mutex *lock)
> +static inline int __must_check __devm_mutex_init(struct device *dev, struct mutex *lock)
>  {
>  	/*
>  	 * When CONFIG_DEBUG_MUTEXES is off mutex_destroy() is just a nop so
> @@ -141,14 +141,17 @@ static inline int __devm_mutex_init(struct device *dev, struct mutex *lock)
>  
>  #endif
>  
> -#define devm_mutex_init(dev, mutex)			\
> +#define __mutex_init_ret(mutex)				\
>  ({							\
>  	typeof(mutex) mutex_ = (mutex);			\
>  							\
>  	mutex_init(mutex_);				\
> -	__devm_mutex_init(dev, mutex_);			\
> +	mutex_;						\
>  })
>  
> +#define devm_mutex_init(dev, mutex) \
> +	__devm_mutex_init(dev, __mutex_init_ret(mutex))
> +
>  /*
>   * See kernel/locking/mutex.c for detailed documentation of these APIs.
>   * Also see Documentation/locking/mutex-design.rst.
> 
> -- 
> 2.50.0
>