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Subject: Re: [WIP 0/3] Memory model and atomic API in Rust
From: comex <comexk@gmail.com>
In-Reply-To: <CAHk-=wjwxKD9CxYsf5x+K5fJbJa_JYZh1eKB4PT5cZJq1+foGw@mail.gmail.com>
Date: Wed, 27 Mar 2024 09:16:09 -0700
Cc: "Dr. David Alan Gilbert" <dave@treblig.org>,
 Kent Overstreet <kent.overstreet@linux.dev>,
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To: Linus Torvalds <torvalds@linux-foundation.org>

On Mar 25, 2024, at 8:49=E2=80=AFPM, Linus Torvalds =
<torvalds@linux-foundation.org> wrote:

> But you should _start_ the design of your language memory model around
> the unsafe "raw atomic access operations" model.
>=20
> Then you can use those strictly more powerful operations, and you
> create an object model *around* it.

To some extent Rust does this already, unlike C++.

C++ allows atomics to be implemented using locks.  Partly for this =
reason,
`std::atomic<T>` is documented as not necessarily having the same
representation as `T` [1].  C++ also has strict aliasing, so even if =
those types
do have the same representation, you still can't cast `T *` to
`std::atomic<T> *`.

But Rust atomics are lower-level.  First, they are guaranteed lock-free =
[2].
Second, they are documented as having "the same in-memory representation =
as the
underlying" type [3].  (They also usually have the same alignment, =
except on
x86 where u64 is only 4-byte aligned but AtomicU64 of course needs to be =
8-byte
aligned.)  Meanwhile, Rust intentionally lacks strict aliasing.

Combined, this means it's perfectly legal in Rust to cast e.g. `&mut =
u32` to
`&AtomicU32` and perform atomic accesses on it.  Or the same with =
u64/AtomicU64
if you know the pointer is validly aligned.  This is by design; the =
Atomic
types' methods are considered the official way to perform atomic =
operations on
arbitrary memory, making it unnecessary to also stabilize 'lower-level'
intrinsics.

That said, there *are* currently some holes in Rust's atomics model, =
based on
the fact that it's mostly inherited from C++.  =46rom the documentation:

> Since C++ does not support mixing atomic and non-atomic accesses, or
> non-synchronized different-sized accesses to the same data, Rust does =
not
> support those operations either. Note that both of those restrictions =
only
> apply if the accesses are non-synchronized.
https://doc.rust-lang.org/std/sync/atomic/index.html

There are some open issues around this:

- "How can we allow read-read races between atomic and non-atomic =
accesses?"
  https://github.com/rust-lang/unsafe-code-guidelines/issues/483

  > [..] I do think we should allow such code. However, then we have to =
change
  > the way we document our atomics [..]

- "What about: mixed-size atomic accesses"
  https://github.com/rust-lang/unsafe-code-guidelines/issues/345"

  > Apparently the x86 manual says you "should" not do this [..] It is =
unclear
  > what "should" means (or what anything else here really means, =
operationally
  > speaking...)

[1] https://eel.is/c++draft/atomics#types.generic.general-3
[2] https://doc.rust-lang.org/std/sync/atomic/index.html#portability
[3] =
https://doc.rust-lang.org/nightly/std/sync/atomic/struct.AtomicU64.html