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				In case CONFIG_PREEMPT_RT is enabled __mutex_init() becomes a macro
instead of an extern function (simplified from
include/linux/mutex.h):
    #ifndef CONFIG_PREEMPT_RT
    extern void __mutex_init(struct mutex *lock, const char *name,
    			 struct lock_class_key *key);
    #else
    #define __mutex_init(mutex, name, key)		\
    do {						\
	rt_mutex_base_init(&(mutex)->rtmutex);		\
    	__mutex_rt_init((mutex), name, key);		\
    } while (0)
    #endif
The macro isn't resolved by bindgen, then. What results in a build
error:
error[E0425]: cannot find function `__mutex_init` in crate `bindings`
     --> rust/kernel/sync/lock/mutex.rs:104:28
      |
104   |           unsafe { bindings::__mutex_init(ptr, name, key) }
      |                              ^^^^^^^^^^^^ help: a function with a similar name exists: `__mutex_rt_init`
      |
     ::: rust/bindings/bindings_generated.rs:23722:5
      |
23722 | /     pub fn __mutex_rt_init(
23723 | |         lock: *mut mutex,
23724 | |         name: *const core::ffi::c_char,
23725 | |         key: *mut lock_class_key,
23726 | |     );
      | |_____- similarly named function `__mutex_rt_init` defined here
Fix this by adding a helper.
As explained by Gary Guo in [1] no #ifdef CONFIG_PREEMPT_RT
is needed here as rust/bindings/lib.rs prefers externed function to
helpers if an externed function exists.
Reported-by: Conor Dooley <conor@kernel.org>
Link: https://lore.kernel.org/rust-for-linux/20240913-shack-estate-b376a65921b1@spud/
Link: https://lore.kernel.org/rust-for-linux/20240915123626.1a170103.gary@garyguo.net/ [1]
Fixes: 
		
	
					 | 
			||
|---|---|---|
| .. | ||
| blk.c | ||
| bug.c | ||
| build_assert.c | ||
| build_bug.c | ||
| err.c | ||
| helpers.c | ||
| kunit.c | ||
| mutex.c | ||
| page.c | ||
| rbtree.c | ||
| refcount.c | ||
| signal.c | ||
| slab.c | ||
| spinlock.c | ||
| task.c | ||
| uaccess.c | ||
| wait.c | ||
| workqueue.c | ||