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

[Boqun Feng]

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@gmail.com/
wip: https://lore.kernel.org/rust-for-linux/20240322233838.868874-1-boqun.feng@gmail.com/

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@mail.gmail.com/

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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