rust: sync: atomic: Add ordering annotation types

Preparation for atomic primitives. Instead of a suffix like _acquire, a
method parameter along with the corresponding generic parameter will be
used to specify the ordering of an atomic operations. For example,
atomic load() can be defined as:

	impl<T: ...> Atomic<T> {
	    pub fn load<O: AcquireOrRelaxed>(&self, _o: O) -> T { ... }
	}

and acquire users would do:

	let r = x.load(Acquire);

relaxed users:

	let r = x.load(Relaxed);

doing the following:

	let r = x.load(Release);

will cause a compiler error.

Compared to suffixes, it's easier to tell what ordering variants an
operation has, and it also make it easier to unify the implementation of
all ordering variants in one method via generic. The `TYPE` associate
const is for generic function to pick up the particular implementation
specified by an ordering annotation.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Reviewed-by: Elle Rhumsaa <elle@weathered-steel.dev>
Link: https://lore.kernel.org/all/20250719030827.61357-4-boqun.feng@gmail.com/

Author: fbq

rust/kernel/sync/atomic.rs

@@ -18,5 +18,7 @@
 
 #[allow(dead_code, unreachable_pub)]
 mod internal;
+pub mod ordering;
 
 pub use internal::AtomicImpl;
+pub use ordering::{Acquire, Full, Relaxed, Release};

rust/kernel/sync/atomic/ordering.rs

@@ -0,0 +1,104 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Memory orderings.
+//!
+//! The semantics of these orderings follows the [`LKMM`] definitions and rules.
+//!
+//! - [`Acquire`] provides ordering between the load part of the annotated operation and all the
+//!   following memory accesses, and if there is a store part, the store part has the [`Relaxed`]
+//!   ordering.
+//! - [`Release`] provides ordering between all the preceding memory accesses and the store part of
+//!   the annotated operation, and if there is a load part, the load part has the [`Relaxed`]
+//!   ordering.
+//! - [`Full`] means "fully-ordered", that is:
+//!   - It provides ordering between all the preceding memory accesses and the annotated operation.
+//!   - It provides ordering between the annotated operation and all the following memory accesses.
+//!   - It provides ordering between all the preceding memory accesses and all the following memory
+//!     accesses.
+//!   - All the orderings are the same strength as a full memory barrier (i.e. `smp_mb()`).
+//! - [`Relaxed`] provides no ordering except the dependency orderings. Dependency orderings are
+//!   described in "DEPENDENCY RELATIONS" in [`LKMM`]'s [`explanation`].
+//!
+//! [`LKMM`]: srctree/tools/memory-model/
+//! [`explanation`]: srctree/tools/memory-model/Documentation/explanation.txt
+
+/// The annotation type for relaxed memory ordering, for the description of relaxed memory
+/// ordering, see [module-level documentation].
+///
+/// [module-level documentation]: crate::sync::atomic::ordering
+pub struct Relaxed;
+
+/// The annotation type for acquire memory ordering, for the description of acquire memory
+/// ordering, see [module-level documentation].
+///
+/// [module-level documentation]: crate::sync::atomic::ordering
+pub struct Acquire;
+
+/// The annotation type for release memory ordering, for the description of release memory
+/// ordering, see [module-level documentation].
+///
+/// [module-level documentation]: crate::sync::atomic::ordering
+pub struct Release;
+
+/// The annotation type for fully-ordered memory ordering, for the description fully-ordered memory
+/// ordering, see [module-level documentation].
+///
+/// [module-level documentation]: crate::sync::atomic::ordering
+pub struct Full;
+
+/// Describes the exact memory ordering.
+#[doc(hidden)]
+pub enum OrderingType {
+    /// Relaxed ordering.
+    Relaxed,
+    /// Acquire ordering.
+    Acquire,
+    /// Release ordering.
+    Release,
+    /// Fully-ordered.
+    Full,
+}
+
+mod internal {
+    /// Sealed trait, can be only implemented inside atomic mod.
+    pub trait Sealed {}
+
+    impl Sealed for super::Relaxed {}
+    impl Sealed for super::Acquire {}
+    impl Sealed for super::Release {}
+    impl Sealed for super::Full {}
+}
+
+/// The trait bound for annotating operations that support any ordering.
+pub trait Ordering: internal::Sealed {
+    /// Describes the exact memory ordering.
+    const TYPE: OrderingType;
+}
+
+impl Ordering for Relaxed {
+    const TYPE: OrderingType = OrderingType::Relaxed;
+}
+
+impl Ordering for Acquire {
+    const TYPE: OrderingType = OrderingType::Acquire;
+}
+
+impl Ordering for Release {
+    const TYPE: OrderingType = OrderingType::Release;
+}
+
+impl Ordering for Full {
+    const TYPE: OrderingType = OrderingType::Full;
+}
+
+/// The trait bound for operations that only support acquire or relaxed ordering.
+pub trait AcquireOrRelaxed: Ordering {}
+
+impl AcquireOrRelaxed for Acquire {}
+impl AcquireOrRelaxed for Relaxed {}
+
+/// The trait bound for operations that only support release or relaxed ordering.
+pub trait ReleaseOrRelaxed: Ordering {}
+
+impl ReleaseOrRelaxed for Release {}
+impl ReleaseOrRelaxed for Relaxed {}