[WIP 3/3] rust: atomic: Add fetch_sub_release()

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

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 rust/kernel/sync/atomic.rs            | 23 +++++++++++++++++++++++
 rust/kernel/sync/atomic/arch/arm64.rs | 20 ++++++++++++++++++++
 rust/kernel/sync/atomic/arch/x86.rs   |  5 +++++
 3 files changed, 48 insertions(+)

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index 280040705fb0..c3cae0d25e88 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -39,4 +39,27 @@ pub fn new(v: i32) -> Self {
     pub fn fetch_add_relaxed(&self, i: i32) -> i32 {
         arch::i32_fetch_add_relaxed(&self.0, i)
     }
+
+    /// Subs `i` to the atomic variable with RELEASE ordering.
+    ///
+    /// Returns the old value before the sub.
+    ///
+    /// # Example
+    ///
+    /// ```rust
+    /// use kernel::sync::atomic::AtomicI32;
+    ///
+    /// let a = AtomicI32::new(1);
+    /// let b = a.fetch_sub_release(1);
+    /// let c = a.fetch_sub_release(2);
+    /// let d = a.fetch_sub_release(3);
+    /// let e = a.fetch_sub_release(core::i32::MIN);
+    ///
+    /// assert_eq!(b, 1);
+    /// assert_eq!(c, 0);
+    /// assert_eq!(d, -2);
+    /// ```
+    pub fn fetch_sub_release(&self, i: i32) -> i32 {
+        arch::i32_fetch_sub_release(&self.0, i)
+    }
 }
diff --git a/rust/kernel/sync/atomic/arch/arm64.rs b/rust/kernel/sync/atomic/arch/arm64.rs
index 438f37cf7df6..beea77ecdb20 100644
--- a/rust/kernel/sync/atomic/arch/arm64.rs
+++ b/rust/kernel/sync/atomic/arch/arm64.rs
@@ -24,3 +24,23 @@ pub(crate) fn i32_fetch_add_relaxed(v: &UnsafeCell<i32>, i: i32) -> i32 {
 
     result
 }
+
+pub(crate) fn i32_fetch_sub_release(v: &UnsafeCell<i32>, i: i32) -> i32 {
+    let mut result;
+    unsafe {
+        asm!(
+            "prfm    pstl1strm, [{v}]",
+            "1: ldxr {result:w}, [{v}]",
+            "sub {val:w}, {result:w}, {i:w}",
+            "stlxr {tmp:w}, {val:w}, [{v}]",
+            "cbnz {tmp:w}, 1b",
+            result = out(reg) result,
+            tmp = out(reg) _,
+            val = out(reg) _,
+            v = in(reg) v.get(),
+            i = in(reg) i,
+        )
+    }
+
+    result
+}
diff --git a/rust/kernel/sync/atomic/arch/x86.rs b/rust/kernel/sync/atomic/arch/x86.rs
index 2d715f740b22..7f764cde4576 100644
--- a/rust/kernel/sync/atomic/arch/x86.rs
+++ b/rust/kernel/sync/atomic/arch/x86.rs
@@ -41,3 +41,8 @@ pub(crate) fn i32_fetch_add_relaxed(v: &UnsafeCell<i32>, i: i32) -> i32 {
     // SAFETY: `v.get()` points to a valid `i32`.
     unsafe { i32_xadd(v.get(), i) }
 }
+
+pub(crate) fn i32_fetch_sub_release(v: &UnsafeCell<i32>, i: i32) -> i32 {
+    // SAFETY: `v.get()` points to a valid `i32`.
+    unsafe { i32_xadd(v.get(), i.wrapping_neg()) }
+}
-- 
2.44.0