Re: [PATCH v5 02/10] rust: sync: Add basic atomic operation mapping framework

[Boqun Feng <boqun.feng@gmail.com>]

On Thu, Jun 26, 2025 at 12:17:14PM +0200, Andreas Hindborg wrote:
> "Boqun Feng" <boqun.feng@gmail.com> writes:
> 
> > Preparation for generic atomic implementation. To unify the
> > implementation 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`.
> 
> I can't follow this, could you expand a bit?
> 

So let's say I want to implement a generic `Atomic::load()`, without the
unification, what I can use are:

    pub fn atomic_read(ptr: *mut i32) -> i32

and

    pub fn atomic64_read(ptr: *mut i64) -> i64

and the implementation of `Atomic::load()` would be:

    impl<T:...> Atomic<T> {
        pub fn load(&self) -> T {
	    if size_of::<T> == 4 {
	        unsafe { transmute(atomic_read(self.0.get())) }
	    } else {
	        unsafe { transmute(atomic64_read(self.0.get())) }
	    }
	}
    }

because although load() is function of a generic struct, "if ... else
..." expression requires each branch has the same return type, so the
`transmute()` would be needed. What I meant was a trait method was
provided to `i32` and `i64`:

    impl AtomicImpl for i32 {
        fn atomic_read(ptr: *mut Self) -> Self;
    }

    impl AtomicImpl for i64 {
        fn atomic_read(ptr: *mut Self) -> Self;
    }

so that I could do:

    impl<T:...> Atomic<T> {
        pub fn load(&self) -> T {
	    T::atomic_read(self.0.get())
	}
    }

> >
> > Introduce `AtomicImpl` 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` impl this.
> >
> > 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@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 0c1d245bf7b8..5eef524975ca 100644
> > --- a/MAINTAINERS
> > +++ b/MAINTAINERS
> > @@ -3894,7 +3894,7 @@ F:	drivers/input/touchscreen/atmel_mxt_ts.c
> >  ATOMIC INFRASTRUCTURE
> >  M:	Will Deacon <will@kernel.org>
> >  M:	Peter Zijlstra <peterz@infradead.org>
> > -R:	Boqun Feng <boqun.feng@gmail.com>
> > +M:	Boqun Feng <boqun.feng@gmail.com>
> >  R:	Mark Rutland <mark.rutland@arm.com>
> >  L:	linux-kernel@vger.kernel.org
> >  S:	Maintained
> > @@ -3903,6 +3903,8 @@ 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@attotech.com>
> > diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> > index 36a719015583..b620027e0641 100644
> > --- a/rust/kernel/sync.rs
> > +++ b/rust/kernel/sync.rs
> > @@ -10,6 +10,7 @@
> >  use pin_init;
> >
> >  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..65e41dba97b7
> > --- /dev/null
> > +++ b/rust/kernel/sync/atomic.rs
> > @@ -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 write from C side is treated as an atomic write if
> > +//!   CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y.
> > +//! - Mixed-size atomic accesses don't cause data races.
> > +//!
> > +//! [`LKMM`]: srctree/tools/memory-mode/
> > +
> > +pub mod ops;
> > diff --git a/rust/kernel/sync/atomic/ops.rs b/rust/kernel/sync/atomic/ops.rs
> > new file mode 100644
> > index 000000000000..f8825f7c84f0
> > --- /dev/null
> > +++ b/rust/kernel/sync/atomic/ops.rs
> > @@ -0,0 +1,199 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +
> > +//! Atomic implementations.
> > +//!
> > +//! Provides 1:1 mapping of atomic implementations.
> > +
> > +use crate::bindings::*;
> > +use crate::macros::paste;
> > +
> > +mod private {
> > +    /// Sealed trait marker to disable customized impls on atomic implementation traits.
> > +    pub trait Sealed {}
> > +}
> > +
> > +// `i32` and `i64` are only supported atomic implementations.
> > +impl private::Sealed for i32 {}
> > +impl private::Sealed for i64 {}
> > +
> > +/// A marker trait for types that implement atomic operations with C side primitives.
> > +///
> > +/// This trait is sealed, and only types that have directly mapping to the C side atomics should
> > +/// impl this:
> > +///
> > +/// - `i32` maps to `atomic_t`.
> > +/// - `i64` maps to `atomic64_t`.
> > +pub trait AtomicImpl: Sized + Send + Copy + private::Sealed {}
> > +
> > +// `atomic_t` implements atomic operations on `i32`.
> > +impl AtomicImpl for i32 {}
> > +
> > +// `atomic64_t` implements atomic operations on `i64`.
> > +impl AtomicImpl for i64 {}
> > +
> > +// This macro generates the function signature with given argument list and return type.
> 
> Perhaps could we add an example expansion to make the macro easier for
> people to parse the first time:
> 

That might be a good idea, I will see what I can. However, note that
these macros are only for internal usage (i.e. not an export macro)
similar to impl_item_type!() in configfs.rs, so you can just expand the
file to see the result.

Actually, I could use scripts (similar to patch #1) to generate these,
but was suggested to use macros instead.

The current hesitation from me is because I need to change these macros
and I would probably look into adding expansion examples when things are
stable (i.e. I don't need to change) again. So maybe not the next
version.

> declare_atomic_method!(
>     read[acquire](ptr: *mut Self) -> Self
> );
> 
> ->
> 
> #[doc = "Atomic read_acquire"]
> ..
> unsafe fn atomic_read_acquire(ptr: *mut Self) -> Self;
> 
> #[doc = "Atomic read"]
> ..
> unsafe fn atomic_read(ptr: *mut Self) -> Self;
> 
> 
[..]
> 
> Lastly, perhaps we should do `ptr.cast()` rather than `as *mut _` ?
> 

Sure, that makes sense. I missed that because clippy cannot work on
macro'd code?

Regards,
Boqun

> 
> Best regards,
> Andreas Hindborg
> 
>