[PATCH 01/36] rust: sync: Refactor static_lock_class!() macro

[Boqun Feng <boqun.feng@gmail.com>]

From: Alice Ryhl <aliceryhl@google.com>

By introducing a new_static() constructor, the macro does not need to go
through MaybeUninit::uninit().assume_init(), which is a pattern that is
best avoided when possible.

The safety comment not only requires that the value is leaked, but also
that it is stored in the right portion of memory. This is so that the
lockdep static_obj() check will succeed when using this constructor. One
could argue that lockdep detects this scenario, so that safety
requirement isn't needed. However, it simplifies matters to require that
static_obj() will succeed and it's not a burdensome requirement on the
caller.

Suggested-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Daniel Almeida <daniel.almeida@collabora.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://patch.msgid.link/20250811-lock-class-key-cleanup-v3-1-b12967ee1ca2@google.com
---
 rust/kernel/sync.rs | 24 ++++++++++++++++++------
 1 file changed, 18 insertions(+), 6 deletions(-)

diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index 5df87e2bd212..1dfbee8e9d00 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -45,6 +45,21 @@ pub struct LockClassKey {
 unsafe impl Sync for LockClassKey {}
 
 impl LockClassKey {
+    /// Initializes a statically allocated lock class key.
+    ///
+    /// This is usually used indirectly through the [`static_lock_class!`] macro.
+    ///
+    /// # Safety
+    ///
+    /// * Before using the returned value, it must be pinned in a static memory location.
+    /// * The destructor must never run on the returned `LockClassKey`.
+    #[doc(hidden)]
+    pub const unsafe fn new_static() -> Self {
+        LockClassKey {
+            inner: Opaque::uninit(),
+        }
+    }
+
     /// Initializes a dynamically allocated lock class key. In the common case of using a
     /// statically allocated lock class key, the static_lock_class! macro should be used instead.
     ///
@@ -101,12 +116,9 @@ fn drop(self: Pin<&mut Self>) {
 macro_rules! static_lock_class {
     () => {{
         static CLASS: $crate::sync::LockClassKey =
-            // Lockdep expects uninitialized memory when it's handed a statically allocated `struct
-            // lock_class_key`.
-            //
-            // SAFETY: `LockClassKey` transparently wraps `Opaque` which permits uninitialized
-            // memory.
-            unsafe { ::core::mem::MaybeUninit::uninit().assume_init() };
+            // SAFETY: The returned `LockClassKey` is stored in static memory and we pin it. Drop
+            // never runs on a static global.
+            unsafe { $crate::sync::LockClassKey::new_static() };
         $crate::prelude::Pin::static_ref(&CLASS)
     }};
 }
-- 
2.51.0