On Thu Jul 24, 2025 at 8:32 AM JST, Gary Guo wrote:
> From: Gary Guo <gary@xxxxxxxxxxx>
>
> 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@xxxxxxxxxx>
> Acked-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
> Signed-off-by: Gary Guo <gary@xxxxxxxxxxx>
> ---
> 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?
