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

[Boqun Feng <boqun.feng@gmail.com>]

On Sat, Aug 09, 2025 at 05:21:49PM +0900, Alexandre Courbot wrote:
> 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

Hmm.. a refcount being 0 is not unsafe I would say, it means no one
references the object (usually in a particular type of referencing). In
general, one should use `Arc` and be done with that, but if you were to
have different types of referencing to a same object, then one of the
refcounts may end up being 0.

> `Refcount::new(1)` immediately followed by a `set(0)` so other users are
> not tricked into creating an invalid Refcount?

This I will call bad code, as it would introduce further confusion
because the `new(1)` is pretty pointless imo.

Regards,
Boqun

> 
>