[PATCH 09/14] rust: sync: Add memory barriers

Boqun Feng
2025-09-05T04:42:08Z

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@google.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/all/20250719030827.61357-10-boqun.feng@gmail.com/
---
rust/helpers/barrier.c | 18 +++++++++++
rust/helpers/helpers.c | 1 +
rust/kernel/sync.rs | 1 +
rust/kernel/sync/barrier.rs | 61 +++++++++++++++++++++++++++++++++++++
4 files changed, 81 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 7053f9245759..85ad14b81925 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 7e962e5429d2..bf8943c88a89 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -12,6 +12,7 @@
mod arc;
pub mod aref;
pub mod atomic;
+pub mod barrier;
pub mod completion;
mod condvar;
pub mod lock;
diff --git a/rust/kernel/sync/barrier.rs b/rust/kernel/sync/barrier.rs
new file mode 100644
index 000000000000..8f2d435fcd94
--- /dev/null
+++ b/rust/kernel/sync/barrier.rs
@@ -0,0 +1,61 @@
+// 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.
+#[inline(always)]
+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.
+ unsafe { core::arch::asm!("") };
+}
+
+/// A full memory barrier.
+///
+/// A barrier that prevents compiler and CPU from reordering memory accesses across the barrier.
+#[inline(always)]
+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.
+#[inline(always)]
+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.
+#[inline(always)]
+pub fn smp_rmb() {
+ if cfg!(CONFIG_SMP) {
+ // SAFETY: `smp_rmb()` is safe to call.
+ unsafe { bindings::smp_rmb() };
+ } else {
+ barrier();
+ }
+}
--
2.51.0