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

[Gary Guo <gary@garyguo.net>]

On Tue, 12 Aug 2025 10:17:44 +0200
"Benno Lossin" <lossin@kernel.org> wrote:

> 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.

Would this additional doc comment be good enough for you?

/// NOTE: usage of this function is discouraged unless there is no way
/// to achieve the desired result using APIs in `refcount.h`. If an API
/// in `refcount.h` does not currently contain a binding, please
/// consider adding a binding for it instead.

Best,
Gary


> 
> ---
> 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) {  
>