[RFC v2 02/13] rust: sync: Add basic atomic operation mapping framework

[Alice Ryhl]

On Fri, Nov 1, 2024 at 7:03 AM Boqun Feng <boqun.feng at gmail.com> wrote:
>
> Preparation for generic atomic implementation. To unify the
> ipmlementation of a generic method over `i32` and `i64`, the C side
> atomic methods need to be grouped so that in a generic method, they can
> be referred as <type>::<method>, otherwise their parameters and return
> value are different between `i32` and `i64`, which would require using
> `transmute()` to unify the type into a `T`.
>
> Introduce `AtomicIpml` to represent a basic type in Rust that has the
> direct mapping to an atomic implementation from C. This trait is sealed,
> and currently only `i32` and `i64` ipml this.

There seems to be quite a few instances of "impl" spelled as "ipml" here.

> Further, different methods are put into different `*Ops` trait groups,
> and this is for the future when smaller types like `i8`/`i16` are
> supported but only with a limited set of API (e.g. only set(), load(),
> xchg() and cmpxchg(), no add() or sub() etc).
>
> While the atomic mod is introduced, documentation is also added for
> memory models and data races.
>
> Also bump my role to the maintainer of ATOMIC INFRASTRUCTURE to reflect
> my responsiblity on the Rust atomic mod.
>
> Signed-off-by: Boqun Feng <boqun.feng at gmail.com>
> ---
>  MAINTAINERS                    |   4 +-
>  rust/kernel/sync.rs            |   1 +
>  rust/kernel/sync/atomic.rs     |  19 ++++
>  rust/kernel/sync/atomic/ops.rs | 199 +++++++++++++++++++++++++++++++++
>  4 files changed, 222 insertions(+), 1 deletion(-)
>  create mode 100644 rust/kernel/sync/atomic.rs
>  create mode 100644 rust/kernel/sync/atomic/ops.rs
>
> diff --git a/MAINTAINERS b/MAINTAINERS
> index b77f4495dcf4..e09471027a63 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -3635,7 +3635,7 @@ F:        drivers/input/touchscreen/atmel_mxt_ts.c
>  ATOMIC INFRASTRUCTURE
>  M:     Will Deacon <will at kernel.org>
>  M:     Peter Zijlstra <peterz at infradead.org>
> -R:     Boqun Feng <boqun.feng at gmail.com>
> +M:     Boqun Feng <boqun.feng at gmail.com>
>  R:     Mark Rutland <mark.rutland at arm.com>
>  L:     linux-kernel at vger.kernel.org
>  S:     Maintained
> @@ -3644,6 +3644,8 @@ F:        arch/*/include/asm/atomic*.h
>  F:     include/*/atomic*.h
>  F:     include/linux/refcount.h
>  F:     scripts/atomic/
> +F:     rust/kernel/sync/atomic.rs
> +F:     rust/kernel/sync/atomic/

This is why mod.rs files are superior :)

> @@ -0,0 +1,19 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Atomic primitives.
> +//!
> +//! These primitives have the same semantics as their C counterparts: and the precise definitions of
> +//! semantics can be found at [`LKMM`]. Note that Linux Kernel Memory (Consistency) Model is the
> +//! only model for Rust code in kernel, and Rust's own atomics should be avoided.
> +//!
> +//! # Data races
> +//!
> +//! [`LKMM`] atomics have different rules regarding data races:
> +//!
> +//! - A normal read doesn't data-race with an atomic read.

This was fixed:
https://github.com/rust-lang/rust/pull/128778

> +mod private {
> +    /// Sealed trait marker to disable customized impls on atomic implementation traits.
> +    pub trait Sealed {}
> +}

Just make the trait unsafe?

Alice