linux-kernel - [WIP 3/3] rust: atomic: Add fetch_sub

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Date: Fri, 22 Mar 2024 16:38:38 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: rust-for-linux@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org,
	llvm@...ts.linux.dev
Cc: Miguel Ojeda <ojeda@...nel.org>,	Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Boqun Feng <boqun.feng@...il.com>,	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Alan Stern <stern@...land.harvard.edu>,
	Andrea Parri <parri.andrea@...il.com>,	Will Deacon <will@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Nicholas Piggin <npiggin@...il.com>,	David Howells <dhowells@...hat.com>,
	Jade Alglave <j.alglave@....ac.uk>,	Luc Maranget <luc.maranget@...ia.fr>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Akira Yokosawa <akiyks@...il.com>,	Daniel Lustig <dlustig@...dia.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nick Desaulniers <ndesaulniers@...gle.com>,	kent.overstreet@...il.com,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,	elver@...gle.com,
	Mark Rutland <mark.rutland@....com>,
	Thomas Gleixner <tglx@...utronix.de>,	Ingo Molnar <mingo@...hat.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,	x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>,
	Catalin Marinas <catalin.marinas@....com>,	torvalds@...ux-foundation.org,
	linux-arm-kernel@...ts.infradead.org,	linux-fsdevel@...r.kernel.org
Subject: [WIP 3/3] rust: atomic: Add fetch_sub_release()

Signed-off-by: Boqun Feng <boqun.feng@...il.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