linux-kernel - [PATCH v6 8/9] rust: sync: Add memory barriers

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Message-Id: <20250710060052.11955-9-boqun.feng@gmail.com>
Date: Wed,  9 Jul 2025 23:00:51 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: linux-kernel@...r.kernel.org,
	rust-for-linux@...r.kernel.org,
	lkmm@...ts.linux.dev,
	linux-arch@...r.kernel.org
Cc: "Miguel Ojeda" <ojeda@...nel.org>,
	"Alex Gaynor" <alex.gaynor@...il.com>,
	"Boqun Feng" <boqun.feng@...il.com>,
	"Gary Guo" <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	"Benno Lossin" <lossin@...nel.org>,
	"Andreas Hindborg" <a.hindborg@...nel.org>,
	"Alice Ryhl" <aliceryhl@...gle.com>,
	"Trevor Gross" <tmgross@...ch.edu>,
	"Danilo Krummrich" <dakr@...nel.org>,
	"Will Deacon" <will@...nel.org>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"Mark Rutland" <mark.rutland@....com>,
	"Wedson Almeida Filho" <wedsonaf@...il.com>,
	"Viresh Kumar" <viresh.kumar@...aro.org>,
	"Lyude Paul" <lyude@...hat.com>,
	"Ingo Molnar" <mingo@...nel.org>,
	"Mitchell Levy" <levymitchell0@...il.com>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	"Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	Alan Stern <stern@...land.harvard.edu>
Subject: [PATCH v6 8/9] rust: sync: Add memory barriers

Memory barriers are building blocks for concurrent code, hence provide
a minimal set of them.

The compiler barrier, barrier(), is implemented in inline asm instead of
using core::sync::atomic::compiler_fence() because memory models are
different: kernel's atomics are implemented in inline asm therefore the
compiler barrier should be implemented in inline asm as well. Also it's
currently only public to the kernel crate until there's a reasonable
driver usage.

Reviewed-by: Alice Ryhl <aliceryhl@...gle.com>
Signed-off-by: Boqun Feng <boqun.feng@...il.com>
---
 rust/helpers/barrier.c      | 18 ++++++++++
 rust/helpers/helpers.c      |  1 +
 rust/kernel/sync.rs         |  1 +
 rust/kernel/sync/barrier.rs | 65 +++++++++++++++++++++++++++++++++++++
 4 files changed, 85 insertions(+)
 create mode 100644 rust/helpers/barrier.c
 create mode 100644 rust/kernel/sync/barrier.rs

diff --git a/rust/helpers/barrier.c b/rust/helpers/barrier.c
new file mode 100644
index 000000000000..cdf28ce8e511
--- /dev/null
+++ b/rust/helpers/barrier.c
@@ -0,0 +1,18 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <asm/barrier.h>
+
+void rust_helper_smp_mb(void)
+{
+	smp_mb();
+}
+
+void rust_helper_smp_wmb(void)
+{
+	smp_wmb();
+}
+
+void rust_helper_smp_rmb(void)
+{
+	smp_rmb();
+}
diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
index 83e89f6a68fb..8ddfc8f84e87 100644
--- a/rust/helpers/helpers.c
+++ b/rust/helpers/helpers.c
@@ -9,6 +9,7 @@
 
 #include "atomic.c"
 #include "auxiliary.c"
+#include "barrier.c"
 #include "blk.c"
 #include "bug.c"
 #include "build_assert.c"
diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index b620027e0641..c7c0e552bafe 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -11,6 +11,7 @@
 
 mod arc;
 pub mod atomic;
+pub mod barrier;
 mod condvar;
 pub mod lock;
 mod locked_by;
diff --git a/rust/kernel/sync/barrier.rs b/rust/kernel/sync/barrier.rs
new file mode 100644
index 000000000000..df4015221503
--- /dev/null
+++ b/rust/kernel/sync/barrier.rs
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Memory barriers.
+//!
+//! These primitives have the same semantics as their C counterparts: and the precise definitions
+//! of semantics can be found at [`LKMM`].
+//!
+//! [`LKMM`]: srctree/tools/memory-model/
+
+/// A compiler barrier.
+///
+/// A barrier that prevents compiler from reordering memory accesses across the barrier.
+pub(crate) fn barrier() {
+    // By default, Rust inline asms are treated as being able to access any memory or flags, hence
+    // it suffices as a compiler barrier.
+    //
+    // SAFETY: An empty asm block should be safe.
+    unsafe {
+        core::arch::asm!("");
+    }
+}
+
+/// A full memory barrier.
+///
+/// A barrier that prevents compiler and CPU from reordering memory accesses across the barrier.
+pub fn smp_mb() {
+    if cfg!(CONFIG_SMP) {
+        // SAFETY: `smp_mb()` is safe to call.
+        unsafe {
+            bindings::smp_mb();
+        }
+    } else {
+        barrier();
+    }
+}
+
+/// A write-write memory barrier.
+///
+/// A barrier that prevents compiler and CPU from reordering memory write accesses across the
+/// barrier.
+pub fn smp_wmb() {
+    if cfg!(CONFIG_SMP) {
+        // SAFETY: `smp_wmb()` is safe to call.
+        unsafe {
+            bindings::smp_wmb();
+        }
+    } else {
+        barrier();
+    }
+}
+
+/// A read-read memory barrier.
+///
+/// A barrier that prevents compiler and CPU from reordering memory read accesses across the
+/// barrier.
+pub fn smp_rmb() {
+    if cfg!(CONFIG_SMP) {
+        // SAFETY: `smp_rmb()` is safe to call.
+        unsafe {
+            bindings::smp_rmb();
+        }
+    } else {
+        barrier();
+    }
+}
-- 
2.39.5 (Apple Git-154)