[PATCH v2 1/2] rust: sync: atomic: Clarify the need of CONFIG_ARCH_SUPPORTS_ATOMIC_RMW

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

Currently, since all the architectures that support Rust all have
CONFIG_ARCH_SUPPORTS_ATOMIC_RMW selected, the helpers of atomic
load/store on i8 and i16 relies on CONFIG_ARCH_SUPPORTS_ATOMIC_RMW=y.
It's generally fine since most of architectures support that.

The plan for CONFIG_ARCH_SUPPORTS_ATOMIC_RMW=n architectures is adding
their (probably lock-based) atomic load/store for i8 and i16 as their
atomic_{read,set}() and atomic64_{read,set}() counterpart when they
plans to support Rust.

Hence use a statis_assert!() to check this and remind the future us the
need of the helpers. This is more clear than the #[cfg] on impl blocks
of i8 and i16.

Suggested-by: Dirk Behme <dirk.behme@gmail.com>
Suggested-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 rust/kernel/sync/atomic/internal.rs | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/rust/kernel/sync/atomic/internal.rs b/rust/kernel/sync/atomic/internal.rs
index 0dac58bca2b3..ef516bcb02ee 100644
--- a/rust/kernel/sync/atomic/internal.rs
+++ b/rust/kernel/sync/atomic/internal.rs
@@ -37,16 +37,23 @@ pub trait AtomicImpl: Sized + Send + Copy + private::Sealed {
     type Delta;
 }
 
-// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
-// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
-#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
+// The current helpers of load/store of atomic `i8` and `i16` use `{WRITE,READ}_ONCE()` hence the
+// atomicity is only guaranteed against read-modify-write operations if the architecture supports
+// native atomic RmW.
+//
+// In the future when a CONFIG_ARCH_SUPPORTS_ATOMIC_RMW=n architecture plans to support Rust, the
+// load/store helpers that guarantee atomicity against RmW operations (usually via a lock) need to
+// be added.
+crate::static_assert!(
+    cfg!(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW),
+    "The current implementation of atomic i8/i16/ptr relies on the architecure being \
+    ARCH_SUPPORTS_ATOMIC_RMW"
+);
+
 impl AtomicImpl for i8 {
     type Delta = Self;
 }
 
-// The current helpers of load/store uses `{WRITE,READ}_ONCE()` hence the atomicity is only
-// guaranteed against read-modify-write operations if the architecture supports native atomic RmW.
-#[cfg(CONFIG_ARCH_SUPPORTS_ATOMIC_RMW)]
 impl AtomicImpl for i16 {
     type Delta = Self;
 }
-- 
2.51.0