On Tue, 12 Aug 2025 10:17:44 +0200
"Benno Lossin" <lossin@xxxxxxxxxx> wrote:
> On Thu Jul 24, 2025 at 1:32 AM CEST, 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>
>
> Reviewed-by: Benno Lossin <lossin@xxxxxxxxxx>
>
> > ---
> > 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) {
>
