Re: [PATCH v2 2/2] locking/mutex: Mark devm_mutex_init() as __must_check

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Feb 04, 2025 at 07:52:51AM +0100, Thomas Weißschuh wrote:
> Even if it's not critical, the avoidance of checking the error code
> from devm_mutex_init() call today diminishes the point of using devm
> variant of it. Tomorrow it may even leak something. Enforce all callers
> checking the return value through the compiler.
> 
> As devm_mutex_init() itself is a macro which can not be annotated,
> annotate __devm_mutex_init() instead.
> Unfortunately __must_check/warn_unused_result don't propagate through
> statement expression. To work around this move the statement expression
> into the argument list of the call to __devm_mutex_init() so
> devm_mutex_init() directly expands to __devm_mutex_init().
> 
> Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
> ---
>  include/linux/mutex.h | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/mutex.h b/include/linux/mutex.h
> index 2bf91b57591b49e4668752e773419ae945f124da..65b28c9e6efc123982d923d1ed171eae471c82c1 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
> @@ -141,13 +141,12 @@ static inline int __devm_mutex_init(struct device *dev, struct mutex *lock)
>  
>  #endif
>  
> -#define devm_mutex_init(dev, mutex)			\
> -({							\
> -	typeof(mutex) mutex_ = (mutex);			\
> -							\
> -	mutex_init(mutex_);				\
> -	__devm_mutex_init(dev, mutex_);			\
> -})
> +#define devm_mutex_init(dev, mutex) __devm_mutex_init(dev, ({	\
> +	typeof(mutex) mutex_ = (mutex);				\
> +								\
> +	mutex_init(mutex_);					\
> +	mutex_;							\
> +}))

Urgh, that's a bit ugly isn't it.

Now we can either write a helper for that like:

#define mutex_init_ret(mutex)                           \
({                                                      \
	typeof(mutex) mutex_ = (mutex);                 \
	mutex_init(mutex_);                             \
	mutex_;                                         \
})

#define devm_mutex_init(dev, mutex)                     \
        __devm_mutex_init(dev, mutex_init_ret(mutex))


Or we can try and make mutex_init() return the pointer itself. I don't
think that will break anything, but its best to feel that to the robots
to make sure.