[RFC v2 00/13] LKMM *generic* atomics in Rust

From: Boqun Feng
Date: Fri Nov 01 2024 - 02:04:11 EST


Hi,

This is another RFC version of LKMM atomics in Rust, you can find the
previous versions:

v1: https://lore.kernel.org/rust-for-linux/20240612223025.1158537-1-boqun.feng@xxxxxxxxx/
wip: https://lore.kernel.org/rust-for-linux/20240322233838.868874-1-boqun.feng@xxxxxxxxx/

I add a few more people Cced this time, so if you're curious about why
we choose to implement LKMM atomics instead of using the Rust ones, you
can find some explanation:

* In my Kangrejos talk: https://lwn.net/Articles/993785/
* In Linus' email: https://lore.kernel.org/rust-for-linux/CAHk-=whkQk=zq5XiMcaU3xj4v69+jyoP-y6Sywhq-TvxSSvfEA@xxxxxxxxxxxxxx/

This time, I try implementing a generic atomic type `Atomic<T>`, since
Benno and Gary suggested last time, and also Rust standard library is
also going to that direction [1].

Honestly, a generic atomic type is still not quite necessary for myself,
but here are a few reasons that it's useful:

* It's useful for type alias, for example, if you have:

type c_long = isize;

Then `Atomic<c_clong>` and `Atomic<isize>` is the same type,
this may make FFI code (mapping a C type to a Rust type or vice
versa) more readable.

* In kernel, we sometimes switch atomic to percpu for better
scalabity, percpu is naturally a generic type, because it can
have data that is larger than machine word size. Making atomic
generic ease the potential switching/refactoring.

* Generic atomics provide all the functionalities that non-generic
atomics could have.

That said, currently "generic" is limited to a few types: the type must
be the same size as atomic_t or atomic64_t, other than basic types, only
#[repr(<basic types>)] struct can be used in a `Atomic<T>`.

Also this is not a full feature set patchset, things like different
arithmetic operations and bit operations are still missing, they can be
either future work or for future versions.

I included an RCU pointer implementation as one example of the usage, so
a patch from Danilo is added, but I will drop it once his patch merged.

This is based on today's rust-next, and I've run all kunit tests from
the doc test on x86, arm64 and riscv.

Feedbacks and comments are welcome! Thanks.

Regards,
Boqun

[1]: https://github.com/rust-lang/rust/issues/130539

Boqun Feng (12):
rust: Introduce atomic API helpers
rust: sync: Add basic atomic operation mapping framework
rust: sync: atomic: Add ordering annotation types
rust: sync: atomic: Add generic atomics
rust: sync: atomic: Add atomic {cmp,}xchg operations
rust: sync: atomic: Add the framework of arithmetic operations
rust: sync: atomic: Add Atomic<u{32,64}>
rust: sync: atomic: Add Atomic<{usize,isize}>
rust: sync: atomic: Add Atomic<*mut T>
rust: sync: atomic: Add arithmetic ops for Atomic<*mut T>
rust: sync: Add memory barriers
rust: sync: rcu: Add RCU protected pointer

Wedson Almeida Filho (1):
rust: add rcu abstraction

MAINTAINERS | 4 +-
rust/helpers/atomic.c | 1038 +++++++++++++++++++++
rust/helpers/helpers.c | 3 +
rust/helpers/rcu.c | 13 +
rust/kernel/sync.rs | 3 +
rust/kernel/sync/atomic.rs | 228 +++++
rust/kernel/sync/atomic/generic.rs | 516 ++++++++++
rust/kernel/sync/atomic/ops.rs | 199 ++++
rust/kernel/sync/atomic/ordering.rs | 94 ++
rust/kernel/sync/barrier.rs | 67 ++
rust/kernel/sync/rcu.rs | 319 +++++++
scripts/atomic/gen-atomics.sh | 1 +
scripts/atomic/gen-rust-atomic-helpers.sh | 65 ++
13 files changed, 2549 insertions(+), 1 deletion(-)
create mode 100644 rust/helpers/atomic.c
create mode 100644 rust/helpers/rcu.c
create mode 100644 rust/kernel/sync/atomic.rs
create mode 100644 rust/kernel/sync/atomic/generic.rs
create mode 100644 rust/kernel/sync/atomic/ops.rs
create mode 100644 rust/kernel/sync/atomic/ordering.rs
create mode 100644 rust/kernel/sync/barrier.rs
create mode 100644 rust/kernel/sync/rcu.rs
create mode 100755 scripts/atomic/gen-rust-atomic-helpers.sh

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