rust/kernel: remove config #ifdef in mutex.rs file

The use of `#ifdef CONFIG_` statements in .c/.rs files should be
avoided: it makes the code much more unmaintainable over time. See:
https://lore.kernel.org/lkml/[email protected]/

Use a Rust-C helper instead to leverage automatic `CONFIG` selection
in C kernel headers.

Signed-off-by: Sven Van Asbroeck <[email protected]>

Author: Sven Van Asbroeck

rust/helpers.c

@@ -8,6 +8,7 @@
 #include <linux/highmem.h>
 #include <linux/uio.h>
 #include <linux/errname.h>
+#include <linux/mutex.h>
 
 void rust_helper_BUG(void)
 {
@@ -123,6 +124,12 @@ const char *rust_helper_errname(int err)
 	return errname(err);
 }
 
+void rust_helper_mutex_lock(struct mutex *lock)
+{
+	mutex_lock(lock);
+}
+EXPORT_SYMBOL_GPL(rust_helper_mutex_lock);
+
 /* We use bindgen's --size_t-is-usize option to bind the C size_t type
  * as the Rust usize type, so we can use it in contexts where Rust
  * expects a usize like slice (array) indices. usize is defined to be

rust/kernel/sync/mutex.rs

@@ -77,19 +77,18 @@ impl<T: ?Sized> NeedsLockClass for Mutex<T> {
     }
 }
 
+extern "C" {
+    fn rust_helper_mutex_lock(mutex: *mut bindings::mutex);
+}
+
 impl<T: ?Sized> Lock for Mutex<T> {
     type Inner = T;
 
-    #[cfg(not(CONFIG_DEBUG_LOCK_ALLOC))]
     fn lock_noguard(&self) {
         // SAFETY: `mutex` points to valid memory.
-        unsafe { bindings::mutex_lock(self.mutex.get()) };
-    }
-
-    #[cfg(CONFIG_DEBUG_LOCK_ALLOC)]
-    fn lock_noguard(&self) {
-        // SAFETY: `mutex` points to valid memory.
-        unsafe { bindings::mutex_lock_nested(self.mutex.get(), 0) };
+        unsafe {
+            rust_helper_mutex_lock(self.mutex.get());
+        }
     }
 
     unsafe fn unlock(&self) {