[PATCH v4] locking/mutex: Mark devm_mutex_init() as __must_check

[PATCH v4] locking/mutex: Mark devm_mutex_init() as __must_check

[Thomas Weißschuh]

Even if it's not critical, the avoidance of checking the error code
from devm_mutex_init() call today diminishes the point of using the 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, 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>
---
Changes in v4:
- Drop already applied leds-1202 driver patch
- Rebase on v6.15-rc1
- Link to v3: https://lore.kernel.org/r/20250208-must_check-devm_mutex_init-v3-0-245e417dcc9e@weissschuh.net

Changes in v3:
- Introduce and use helper macro __mutex_init_ret()
- Link to v2: https://lore.kernel.org/r/20250204-must_check-devm_mutex_init-v2-0-7b6271c4b7e6@weissschuh.net

Changes in v2:
- Rebase on 6.14-rc1
- Fix up leds-1202 driver
- Link to v1: https://lore.kernel.org/r/20241202-must_check-devm_mutex_init-v1-1-e60eb97b8c72@weissschuh.net
---
 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 2143d05116be1b0ac239951a9d5d0b90ad99062c..d9342341c13fc036dd9257537c85ff61f0f03da0 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.

---
base-commit: 0af2f6be1b4281385b618cb86ad946eded089ac8
change-id: 20241031-must_check-devm_mutex_init-cac583bda8fe

Best regards,
-- 
Thomas Weißschuh <linux@weissschuh.net>

Re: [PATCH v4] locking/mutex: Mark devm_mutex_init() as __must_check

[Andy Shevchenko]

On Mon, Apr 07, 2025 at 09:46:47AM +0200, 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 the 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, 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.

FWIW,
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

-- 
With Best Regards,
Andy Shevchenko