[PATCH v5 10/10] rust: sync: Add memory barriers

Boqun Feng
2025-06-18T16:49:53Z

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.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
rust/helpers/barrier.c | 18 ++++++++++
rust/helpers/helpers.c | 1 +
rust/kernel/sync.rs | 1 +
rust/kernel/sync/barrier.rs | 67 +++++++++++++++++++++++++++++++++++++
4 files changed, 87 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..36a5c70e6716
--- /dev/null
+++ b/rust/kernel/sync/barrier.rs
@@ -0,0 +1,67 @@
+// 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-mode/
+
+/// A compiler barrier.
+///
+/// An explicic compiler barrier function that prevents the compiler from moving the memory
+/// accesses either side of it to the other side.
+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 function that prevents both the compiler and the CPU from moving the memory accesses
+/// either side of it to the other side.
+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 function that prevents both the compiler and the CPU from moving the memory write
+/// accesses either side of it to the other side.
+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 function that prevents both the compiler and the CPU from moving the memory read
+/// accesses either side of it to the other side.
+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)