linux-kernel - Re: [PATCH v3 3/4] rust: block: convert `block::mq` to use `Refcount`

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Message-ID: <CAJ-ks9mEqo72Cwq5_BMtdLPjGSb_sQbm_p+TV_u=iNuYSnuPKQ@mail.gmail.com>
Date: Wed, 19 Feb 2025 17:53:43 -0500
From: Tamir Duberstein <tamird@...il.com>
To: Gary Guo <gary@...yguo.net>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>, 
	Boqun Feng <boqun.feng@...il.com>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Benno Lossin <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, 
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, 
	Danilo Krummrich <dakr@...nel.org>, Will Deacon <will@...nel.org>, Peter Zijlstra <peterz@...radead.org>, 
	Mark Rutland <mark.rutland@....com>, Jens Axboe <axboe@...nel.dk>, 
	Francesco Zardi <frazar00@...il.com>, rust-for-linux@...r.kernel.org, 
	linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/4] rust: block: convert `block::mq` to use `Refcount`

On Wed, Feb 19, 2025 at 5:26 PM Tamir Duberstein <tamird@...il.com> wrote:
>
> On Wed, Feb 19, 2025 at 3:17 PM Gary Guo <gary@...yguo.net> wrote:
> >
> > 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.
> >
> > Acked-by: Andreas Hindborg <a.hindborg@...nel.org>
> > Signed-off-by: Gary Guo <gary@...yguo.net>
> > ---
> >  rust/kernel/block/mq/operations.rs |  7 +--
> >  rust/kernel/block/mq/request.rs    | 70 ++++++++++--------------------
> >  rust/kernel/sync/refcount.rs       | 14 ++++++
> >  3 files changed, 40 insertions(+), 51 deletions(-)
> >
> > diff --git a/rust/kernel/block/mq/operations.rs b/rust/kernel/block/mq/operations.rs
> > index 864ff379dc91..c399dcaa6740 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)) };
>
> Could we just make the field pub and remove refcount_ptr? I believe a
> few callers of `wrapper_ptr` could be replaced with `wrapper_ref`.

I took a stab at this to check it was possible:
https://gist.github.com/tamird/c9de7fa6e54529996f433950268f3f87