Re: [PATCH v5 4/5] rust: block: convert `block::mq` to use `Refcount`

["Benno Lossin" <lossin@kernel.org>]

On Thu Jul 24, 2025 at 1:32 AM CEST, Gary Guo wrote:
> From: Gary Guo <gary@garyguo.net>
>
> Currently there's a custom reference counting in `block::mq`, which uses
> `AtomicU64` Rust atomics, and this type doesn't exist on some 32-bit
> architectures. We cannot just change it to use 32-bit atomics, because
> doing so will make it vulnerable to refcount overflow. So switch it to
> use the kernel refcount `kernel::sync::Refcount` instead.
>
> There is an operation needed by `block::mq`, atomically decreasing
> refcount from 2 to 0, which is not available through refcount.h, so
> I exposed `Refcount::as_atomic` which allows accessing the refcount
> directly.
>
> Tested-by: David Gow <davidgow@google.com>
> Acked-by: Andreas Hindborg <a.hindborg@kernel.org>
> Signed-off-by: Gary Guo <gary@garyguo.net>

Reviewed-by: Benno Lossin <lossin@kernel.org>

> ---
>  rust/kernel/block/mq/operations.rs |  7 ++--
>  rust/kernel/block/mq/request.rs    | 63 ++++++++----------------------
>  rust/kernel/sync/refcount.rs       | 14 +++++++
>  3 files changed, 34 insertions(+), 50 deletions(-)

> diff --git a/rust/kernel/sync/refcount.rs b/rust/kernel/sync/refcount.rs
> index 3ff4585326b41..a9b24c6b2f8a7 100644
> --- a/rust/kernel/sync/refcount.rs
> +++ b/rust/kernel/sync/refcount.rs
> @@ -4,6 +4,8 @@
>  //!
>  //! C header: [`include/linux/refcount.h`](srctree/include/linux/refcount.h)
>  
> +use core::sync::atomic::AtomicI32;
> +
>  use crate::build_assert;
>  use crate::types::Opaque;
>  
> @@ -34,6 +36,18 @@ fn as_ptr(&self) -> *mut bindings::refcount_t {
>          self.0.get()
>      }
>  
> +    /// Get the underlying atomic counter that backs the refcount.
> +    ///
> +    /// NOTE: This will be changed to LKMM atomic in the future.

Can we discourage using this function a bit more in the docs? At least
point people to try other ways before reaching for this, since it allows
overflowing & doesn't warn on saturate etc.

---
Cheers,
Benno

> +    #[inline]
> +    pub fn as_atomic(&self) -> &AtomicI32 {
> +        let ptr = self.0.get().cast();
> +        // SAFETY: `refcount_t` is a transparent wrapper of `atomic_t`, which is an atomic 32-bit
> +        // integer that is layout-wise compatible with `AtomicI32`. All values are valid for
> +        // `refcount_t`, despite some of the values being considered saturated and "bad".
> +        unsafe { &*ptr }
> +    }
> +
>      /// Set a refcount's value.
>      #[inline]
>      pub fn set(&self, value: i32) {