[PATCH v5 0/5] implement `kernel::sync::Refcount` and convert users

[Gary Guo <gary@kernel.org>]

From: Gary Guo <gary@garyguo.net>

Currently there're two refcount usage in rust/, `Arc` and `block::mq`.
`Arc` uses `refcount_t` with FFI calls directly, and `block::mq` use
Rust atomics and custom refcounting.

This series consolidate them to have a single `Refcount` which wraps
`refcount_t` and have it used by both.

With the removal of Rust `AtomicU64` this would also make the code work
for 32-bit systems. See [1] as a previous attempt to fix this issue.

Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Link: https://lore.kernel.org/rust-for-linux/20240905061214.3954271-1-davidgow@google.com/ [1]

Changes in v5:
- Forbid using `Refcount::new()` with already-saturated initial value.
  (`Refcount::set` can still set value to the saturated range, as
  users may want to set it to a runtime value that the compiler cannot
  easily determine to be non-negative).
- Moved a documentation comment from one commit to another following
  Benno's suggestion
- Link to v4: https://lore.kernel.org/rust-for-linux/20250622125802.3224264-1-gary@kernel.org/

Changes in v4:
- Move justification on why `&Refcount` is okay when used for reference
  coutning into the `Refcount::dec_and_test` documentation comment.
- Split `Arc::into_unique_or_drop` signature change into another patch.
- Wording/comment changes
- Link to v3: https://lore.kernel.org/rust-for-linux/20250219201602.1898383-1-gary@garyguo.net/

Changes in v3:
- Drop `dec_not_one` and revert to `dec_and_test`/`set` as the former
  lacks acquire semantics.
- Move a `use` statement from patch 1 to correct place (patch 3).
- Update maintainer entry to include refcount.rs and add myself as a
  reviewer, as suggested by Boqun.
- Link to v2: https://lore.kernel.org/rust-for-linux/20241221183024.3929500-1-gary@garyguo.net

Changes in v2:
- `Refcount::read` method is dropped
- `Refcount::dec_not_one` method is added and `Arc::into_unique_or_drop`
  is converted to use it.
- Link to v1: https://lore.kernel.org/rust-for-linux/20241004155247.2210469-1-gary@garyguo.net

Gary Guo (5):
  rust: implement `kernel::sync::Refcount`
  rust: make `Arc::into_unique_or_drop` associated function
  rust: convert `Arc` to use `Refcount`
  rust: block: convert `block::mq` to use `Refcount`
  MAINTAINERS: update atomic infrastructure entry to include Rust

 MAINTAINERS                        |   2 +
 rust/helpers/refcount.c            |  10 +++
 rust/kernel/block/mq/operations.rs |   7 +-
 rust/kernel/block/mq/request.rs    |  63 +++++-----------
 rust/kernel/sync.rs                |   2 +
 rust/kernel/sync/arc.rs            |  55 +++++---------
 rust/kernel/sync/refcount.rs       | 112 +++++++++++++++++++++++++++++
 7 files changed, 165 insertions(+), 86 deletions(-)
 create mode 100644 rust/kernel/sync/refcount.rs


base-commit: cc84ef3b88f407e8bd5a5f7b6906d1e69851c856
-- 
2.49.0