[RFC 2/2] rust: sync: Add atomic support

[Gary Guo]

On Wed, 12 Jun 2024 15:30:25 -0700
Boqun Feng <boqun.feng at gmail.com> wrote:

> Provide two atomic types: AtomicI32 and AtomicI64 with the existing
> implemenation of C atomics. These atomics have the same semantics of the
> corresponding LKMM C atomics, and using one memory (ordering) model
> certainly reduces the reasoning difficulty and potential bugs from the
> interaction of two different memory models.
> 
> Also bump my role to the maintainer of ATOMIC INFRASTRUCTURE to reflect
> my responsiblity on these Rust APIs.
> 
> Note that `Atomic*::new()`s are implemented vi open coding on struct
> atomic*_t. This allows `new()` being a `const` function, so that it can
> be used in constant contexts.
> 
> Signed-off-by: Boqun Feng <boqun.feng at gmail.com>
> ---
>  MAINTAINERS                       |    4 +-
>  arch/arm64/kernel/cpufeature.c    |    2 +
>  rust/kernel/sync.rs               |    1 +
>  rust/kernel/sync/atomic.rs        |   63 ++
>  rust/kernel/sync/atomic/impl.rs   | 1375 +++++++++++++++++++++++++++++
>  scripts/atomic/gen-atomics.sh     |    1 +
>  scripts/atomic/gen-rust-atomic.sh |  136 +++
>  7 files changed, 1581 insertions(+), 1 deletion(-)
>  create mode 100644 rust/kernel/sync/atomic.rs
>  create mode 100644 rust/kernel/sync/atomic/impl.rs
>  create mode 100755 scripts/atomic/gen-rust-atomic.sh
> 
> diff --git a/MAINTAINERS b/MAINTAINERS
> index d6c90161c7bf..a8528d27b260 100644
> --- a/MAINTAINERS
> +++ b/MAINTAINERS
> @@ -3458,7 +3458,7 @@ 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
> @@ -3467,6 +3467,8 @@ 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/
>  
>  ATTO EXPRESSSAS SAS/SATA RAID SCSI DRIVER
>  M:	Bradley Grove <linuxdrivers at attotech.com>
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 48e7029f1054..99e6e2b2867f 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1601,6 +1601,8 @@ static bool
>  has_cpuid_feature(const struct arm64_cpu_capabilities *entry, int scope)
>  {
>  	u64 val = read_scoped_sysreg(entry, scope);
> +	if (entry->capability == ARM64_HAS_LSE_ATOMICS)
> +		return false;
>  	return feature_matches(val, entry);
>  }
>  
> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> index 0ab20975a3b5..66ac3752ca71 100644
> --- a/rust/kernel/sync.rs
> +++ b/rust/kernel/sync.rs
> @@ -8,6 +8,7 @@
>  use crate::types::Opaque;
>  
>  mod arc;
> +pub mod atomic;
>  mod condvar;
>  pub mod lock;
>  mod locked_by;
> diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
> new file mode 100644
> index 000000000000..b0f852cf1741
> --- /dev/null
> +++ b/rust/kernel/sync/atomic.rs
> @@ -0,0 +1,63 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +//! Atomic primitives.
> +//!
> +//! These primitives have the same semantics as their C counterparts, for precise definitions of
> +//! the semantics, please refer to tools/memory-model. Note that Linux Kernel Memory (Consistency)
> +//! Model is the only model for Rust development in kernel right now, please avoid to use Rust's
> +//! own atomics.
> +
> +use crate::bindings::{atomic64_t, atomic_t};
> +use crate::types::Opaque;
> +
> +mod r#impl;
> +
> +/// Atomic 32bit signed integers.
> +pub struct AtomicI32(Opaque<atomic_t>);
> +
> +/// Atomic 64bit signed integers.
> +pub struct AtomicI64(Opaque<atomic64_t>);


Can we avoid two types and use a generic `Atomic<T>` and then implement
on `Atomic<i32>` and `Atomic<i64>` instead? Like the recent
generic NonZero in Rust standard library or the atomic crate
(https://docs.rs/atomic/).

I think this is better for ergonomics. The impl do need some extra
casting though.

Best,
Gary