Re: [PATCH v2 2/4] rust: sync: generic memory barriers

[Boqun Feng]

On Tue, Jun 09, 2026 at 04:38:38PM +0100, Gary Guo wrote:
> Implement a generic interface for memory barriers (full system/DMA/SMP).
> The interface uses a parameter to force user to specify their intent with
> barriers.
> 
> Provide `Read`, `Write`, `Full` orderings which map to the existing
> `rmb()`, `wmb()` and `mb()`. Generic is used here instead of providing
> individual standalone functions to reduce code duplication; for example,
> the `CONFIG_SMP` check in `smp_mb` is uniformly implemented for all SMP
> barriers. This could extend to `virt_mb`'s if they're introduced in the
> future. It would also make it easier if new ordering types are introduced
> in the future (e.g. `Acquire`, `Release`).
> 

Queued with minor changes:

* Added "Add" into the commit title.
* Applied vertical import style.

But I do want to get more memory model reviews on this patch series, so
please do take a look if you're interested ;-)

Regards,
Boqun

> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
> ---
>  rust/kernel/sync/atomic/ordering.rs |   2 +-
>  rust/kernel/sync/barrier.rs         | 123 ++++++++++++++++++++++++++++--------
>  2 files changed, 96 insertions(+), 29 deletions(-)
> 
> diff --git a/rust/kernel/sync/atomic/ordering.rs b/rust/kernel/sync/atomic/ordering.rs
> index 3f103aa8db99..c4e732e7212f 100644
> --- a/rust/kernel/sync/atomic/ordering.rs
> +++ b/rust/kernel/sync/atomic/ordering.rs
> @@ -15,7 +15,7 @@
>  //!   - It provides ordering between the annotated operation and all the following memory accesses.
>  //!   - It provides ordering between all the preceding memory accesses and all the following memory
>  //!     accesses.
> -//!   - All the orderings are the same strength as a full memory barrier (i.e. `smp_mb()`).
> +//!   - All the orderings are the same strength as a full memory barrier (i.e. `smp_mb(Full)`).
>  //! - [`Relaxed`] provides no ordering except the dependency orderings. Dependency orderings are
>  //!   described in "DEPENDENCY RELATIONS" in [`LKMM`]'s [`explanation`].
>  //!
> diff --git a/rust/kernel/sync/barrier.rs b/rust/kernel/sync/barrier.rs
> index 8f2d435fcd94..54c527fdb760 100644
> --- a/rust/kernel/sync/barrier.rs
> +++ b/rust/kernel/sync/barrier.rs
> @@ -7,6 +7,34 @@
>  //!
>  //! [`LKMM`]: srctree/tools/memory-model/
>  
> +#![expect(private_bounds, reason = "sealed implementation")]
> +
> +/// Memory barrier orderings.
> +///
> +/// The semantics of these orderings follows the [`LKMM`] definitions and rules.
> +///
> +/// - [`Read`] provides ordering between preceding load operations and succeeding load operations.
> +/// - [`Write`] provides ordering between preceding store operations and succeeding store
> +///   operations.
> +/// - [`Full`] provides ordering between all the preceding memory accesses and succeeding memory
> +///   accesses.
> +///
> +/// [`LKMM`]: srctree/tools/memory-model/
> +pub mod ordering {
> +    pub use crate::sync::atomic::ordering::Full;
> +
> +    /// The annotation type for read-read barrier ordering.
> +    pub struct Read;
> +
> +    /// The annotation type for write-write barrier ordering.
> +    pub struct Write;
> +}
> +
> +pub use ordering::{Full, Read, Write};
> +
> +struct Smp;
> +struct Dma;
> +
>  /// A compiler barrier.
>  ///
>  /// A barrier that prevents compiler from reordering memory accesses across the barrier.
> @@ -19,43 +47,82 @@ pub(crate) fn barrier() {
>      unsafe { core::arch::asm!("") };
>  }
>  
> -/// A full memory barrier.
> +trait MemoryBarrier<Flavour = ()> {
> +    fn run();
> +}
> +
> +macro_rules! define_barrier {
> +    ($([$flavour:ident])? $ordering:ident, $binding:ident) => {
> +        impl MemoryBarrier$(<$flavour>)? for $ordering {
> +            #[inline]
> +            fn run() {
> +                // SAFETY: barrier methods are safe to call.
> +                unsafe { bindings::$binding() };
> +            }
> +        }
> +    };
> +}
> +
> +define_barrier!(Full, mb);
> +define_barrier!(Read, rmb);
> +define_barrier!(Write, wmb);
> +define_barrier!([Dma] Full, dma_mb);
> +define_barrier!([Dma] Read, dma_rmb);
> +define_barrier!([Dma] Write, dma_wmb);
> +define_barrier!([Smp] Full, smp_mb);
> +define_barrier!([Smp] Read, smp_rmb);
> +define_barrier!([Smp] Write, smp_wmb);
> +
> +/// 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();
> -    }
> +///
> +/// The specific forms of reordering can be specified using the parameter.
> +/// - `mb(Read)` provides a read-read barrier.
> +/// - `mb(Write)` provides a write-write barrier.
> +/// - `mb(Full)` provides a full barrier.
> +///
> +/// # Examples
> +///
> +/// ```
> +/// # use kernel::sync::barrier::*;
> +/// mb(Read);
> +/// mb(Write);
> +/// mb(Full);
> +/// ```
> +#[inline]
> +#[doc(alias = "rmb")]
> +#[doc(alias = "wmb")]
> +pub fn mb<T: MemoryBarrier>(_: T) {
> +    T::run()
>  }
>  
> -/// A write-write memory barrier.
> +/// Memory barrier between CPUs.
>  ///
> -/// A barrier that prevents compiler and CPU from reordering memory write accesses across the
> -/// barrier.
> -#[inline(always)]
> -pub fn smp_wmb() {
> +/// A barrier that prevents compiler and CPU from reordering memory accesses across the barrier.
> +/// Does not prevent re-ordering with respect to other bus-mastering devices.
> +///
> +/// See [`mb`] for usage.
> +#[inline]
> +#[doc(alias = "smp_rmb")]
> +#[doc(alias = "smp_wmb")]
> +pub fn smp_mb<T: MemoryBarrier<Smp>>(_: T) {
>      if cfg!(CONFIG_SMP) {
> -        // SAFETY: `smp_wmb()` is safe to call.
> -        unsafe { bindings::smp_wmb() };
> +        T::run()
>      } else {
> -        barrier();
> +        barrier()
>      }
>  }
>  
> -/// A read-read memory barrier.
> +/// Memory barrier between local CPU and bus-mastering devices.
>  ///
> -/// 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();
> -    }
> +/// A barrier that prevents compiler and CPU from reordering memory accesses across the barrier.
> +/// Does not prevent re-ordering with respect to other CPUs.
> +///
> +/// See [`mb`] for usage.
> +#[inline]
> +#[doc(alias = "dma_rmb")]
> +#[doc(alias = "dma_wmb")]
> +pub fn dma_mb<T: MemoryBarrier<Dma>>(_: T) {
> +    T::run()
>  }
> 
> -- 
> 2.54.0
>