linux-kernel - Re: [PATCH v6 8/9] rust: sync: Add memory barriers

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Message-Id: <DB93NWEAK46D.2YW5P9MSAWVCN@kernel.org>
Date: Fri, 11 Jul 2025 10:57:48 +0200
From: "Benno Lossin" <lossin@...nel.org>
To: "Boqun Feng" <boqun.feng@...il.com>, <linux-kernel@...r.kernel.org>,
 <rust-for-linux@...r.kernel.org>, <lkmm@...ts.linux.dev>,
 <linux-arch@...r.kernel.org>
Cc: "Miguel Ojeda" <ojeda@...nel.org>, "Alex Gaynor"
 <alex.gaynor@...il.com>, "Gary Guo" <gary@...yguo.net>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, "Andreas
 Hindborg" <a.hindborg@...nel.org>, "Alice Ryhl" <aliceryhl@...gle.com>,
 "Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>,
 "Will Deacon" <will@...nel.org>, "Peter Zijlstra" <peterz@...radead.org>,
 "Mark Rutland" <mark.rutland@....com>, "Wedson Almeida Filho"
 <wedsonaf@...il.com>, "Viresh Kumar" <viresh.kumar@...aro.org>, "Lyude
 Paul" <lyude@...hat.com>, "Ingo Molnar" <mingo@...nel.org>, "Mitchell Levy"
 <levymitchell0@...il.com>, "Paul E. McKenney" <paulmck@...nel.org>, "Greg
 Kroah-Hartman" <gregkh@...uxfoundation.org>, "Linus Torvalds"
 <torvalds@...ux-foundation.org>, "Thomas Gleixner" <tglx@...utronix.de>,
 "Alan Stern" <stern@...land.harvard.edu>
Subject: Re: [PATCH v6 8/9] rust: sync: Add memory barriers

On Thu Jul 10, 2025 at 8:00 AM CEST, Boqun Feng wrote:
> diff --git a/rust/kernel/sync/barrier.rs b/rust/kernel/sync/barrier.rs
> new file mode 100644
> index 000000000000..df4015221503
> --- /dev/null
> +++ b/rust/kernel/sync/barrier.rs
> @@ -0,0 +1,65 @@
> +// 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.
> +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.

I don't know about this, but it also isn't my area of expertise... I
think I heard Ralf talk about this at Rust Week, but I don't remember...

> +    //
> +    // SAFETY: An empty asm block should be safe.

    // SAFETY: An empty asm block.

> +    unsafe {
> +        core::arch::asm!("");
> +    }

    unsafe { core::arch::asm!("") };

> +}
> +
> +/// A full memory barrier.
> +///
> +/// A barrier that prevents compiler and CPU from reordering memory accesses across the barrier.
> +pub fn smp_mb() {
> +    if cfg!(CONFIG_SMP) {
> +        // SAFETY: `smp_mb()` is safe to call.
> +        unsafe {
> +            bindings::smp_mb();

Does this really work? How does the Rust compiler know this is a memory
barrier?

---
Cheers,
Benno

> +        }
> +    } else {
> +        barrier();
> +    }
> +}