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

["Alexandre Courbot" <acourbot@nvidia.com>]

On Thu Jul 24, 2025 at 8:32 AM JST, 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>
> ---
>  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/block/mq/operations.rs b/rust/kernel/block/mq/operations.rs
> index c2b98f507bcbd..c0f95a9419c4e 100644
> --- a/rust/kernel/block/mq/operations.rs
> +++ b/rust/kernel/block/mq/operations.rs
> @@ -10,9 +10,10 @@
>      block::mq::Request,
>      error::{from_result, Result},
>      prelude::*,
> +    sync::Refcount,
>      types::ARef,
>  };
> -use core::{marker::PhantomData, sync::atomic::AtomicU64, sync::atomic::Ordering};
> +use core::marker::PhantomData;
>  
>  /// Implement this trait to interface blk-mq as block devices.
>  ///
> @@ -78,7 +79,7 @@ impl<T: Operations> OperationsVTable<T> {
>          let request = unsafe { &*(*bd).rq.cast::<Request<T>>() };
>  
>          // One refcount for the ARef, one for being in flight
> -        request.wrapper_ref().refcount().store(2, Ordering::Relaxed);
> +        request.wrapper_ref().refcount().set(2);
>  
>          // SAFETY:
>          //  - We own a refcount that we took above. We pass that to `ARef`.
> @@ -187,7 +188,7 @@ impl<T: Operations> OperationsVTable<T> {
>  
>              // SAFETY: The refcount field is allocated but not initialized, so
>              // it is valid for writes.
> -            unsafe { RequestDataWrapper::refcount_ptr(pdu.as_ptr()).write(AtomicU64::new(0)) };
> +            unsafe { RequestDataWrapper::refcount_ptr(pdu.as_ptr()).write(Refcount::new(0)) };

Ah, so that's why `0` is allowed as a valid value for `Refcount::new`.
Seeing the use that is made of atomics here, I wonder if `Refcount` is a
good choice, or if we could adapt the code to use them as expected. I am
not familiar enough with this part of the code to give informed advice
unfortunately.

But at the very least, I think the constructor should not be made unsafe
due to account for one particular user. How about doing a
`Refcount::new(1)` immediately followed by a `set(0)` so other users are
not tricked into creating an invalid Refcount?