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

Andreas Hindborg
2025-06-26T13:36:36Z

"Boqun Feng" <boqun.feng@gmail.com> writes:

> 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/

Typo in link target.

> +
> +/// 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.

Typo in "explicit".

How about:

A compiler barrier. Prevents the compiler from reordering
memory access instructions 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 function that prevents both the compiler and the CPU from moving the memory accesses
> +/// either side of it to the other side.

A barrier that prevents compiler and CPU from reordering memory access
instructions 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 function that prevents both the compiler and the CPU from moving the memory write
> +/// accesses either side of it to the other side.

A barrier that prevents compiler and CPU from reordering memory write
instructions 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 function that prevents both the compiler and the CPU from moving the memory read
> +/// accesses either side of it to the other side.

A barrier that prevents compiler and CPU from reordering memory read
instructions across the barrier.

> +pub fn smp_rmb() {
> + if cfg!(CONFIG_SMP) {
> + // SAFETY: `smp_rmb()` is safe to call.
> + unsafe {
> + bindings::smp_rmb();
> + }
> + } else {
> + barrier();
> + }
> +}

Best regards,
Andreas Hindborg