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

[Andreas Hindborg]

"Boqun Feng" <boqun.feng@xxxxxxxxx> 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@xxxxxxxxx>
> ---
>  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