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

[Elle Rhumsaa <elle@weathered-steel.dev>]

On Thu, Sep 04, 2025 at 09:41:36PM -0700, Boqun Feng wrote:
> 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
> 
> 

Reviewed-by: Elle Rhumsaa <elle@weathered-steel.dev>