[PATCH 10/14] rust: implement `kernel::sync::Refcount`

Boqun Feng
2025-09-05T04:42:10Z

From: Gary Guo <gary@garyguo.net>

This is a wrapping layer of `include/linux/refcount.h`. Currently the
kernel refcount has already been used in `Arc`, however it calls into
FFI directly.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Reviewed-by: Fiona Behrens <me@kloenk.dev>
Signed-off-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Benno Lossin <lossin@kernel.org>
[boqun: Add the missing <> for the link in comment]
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Link: https://lore.kernel.org/r/20250723233312.3304339-2-gary@kernel.org
---
rust/helpers/refcount.c | 10 ++++
rust/kernel/sync.rs | 2 +
rust/kernel/sync/refcount.rs | 98 ++++++++++++++++++++++++++++++++++++
3 files changed, 110 insertions(+)
create mode 100644 rust/kernel/sync/refcount.rs

diff --git a/rust/helpers/refcount.c b/rust/helpers/refcount.c
index d6adbd2e45a1..d175898ad7b8 100644
--- a/rust/helpers/refcount.c
+++ b/rust/helpers/refcount.c
@@ -7,11 +7,21 @@ refcount_t rust_helper_REFCOUNT_INIT(int n)
return (refcount_t)REFCOUNT_INIT(n);
}

+void rust_helper_refcount_set(refcount_t *r, int n)
+{
+ refcount_set(r, n);
+}
+
void rust_helper_refcount_inc(refcount_t *r)
{
refcount_inc(r);
}

+void rust_helper_refcount_dec(refcount_t *r)
+{
+ refcount_dec(r);
+}
+
bool rust_helper_refcount_dec_and_test(refcount_t *r)
{
return refcount_dec_and_test(r);
diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index bf8943c88a89..cf5b638a097d 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -19,6 +19,7 @@
mod locked_by;
pub mod poll;
pub mod rcu;
+mod refcount;

pub use arc::{Arc, ArcBorrow, UniqueArc};
pub use completion::Completion;
@@ -27,6 +28,7 @@
pub use lock::mutex::{new_mutex, Mutex, MutexGuard};
pub use lock::spinlock::{new_spinlock, SpinLock, SpinLockGuard};
pub use locked_by::LockedBy;
+pub use refcount::Refcount;

/// Represents a lockdep class. It's a wrapper around C's `lock_class_key`.
#[repr(transparent)]
diff --git a/rust/kernel/sync/refcount.rs b/rust/kernel/sync/refcount.rs
new file mode 100644
index 000000000000..cc1a80ae7ae9
--- /dev/null
+++ b/rust/kernel/sync/refcount.rs
@@ -0,0 +1,98 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Atomic reference counting.
+//!
+//! C header: [`include/linux/refcount.h`](srctree/include/linux/refcount.h)
+
+use crate::build_assert;
+use crate::types::Opaque;
+
+/// Atomic reference counter.
+///
+/// This type is conceptually an atomic integer, but provides saturation semantics compared to
+/// normal atomic integers. Values in the negative range when viewed as a signed integer are
+/// saturation (bad) values. For details about the saturation semantics, please refer to top of
+/// [`include/linux/refcount.h`](srctree/include/linux/refcount.h).
+///
+/// Wraps the kernel's C `refcount_t`.
+#[repr(transparent)]
+pub struct Refcount(Opaque<bindings::refcount_t>);
+
+impl Refcount {
+ /// Construct a new [`Refcount`] from an initial value.
+ ///
+ /// The initial value should be non-saturated.
+ #[inline]
+ pub fn new(value: i32) -> Self {
+ build_assert!(value >= 0, "initial value saturated");
+ // SAFETY: There are no safety requirements for this FFI call.
+ Self(Opaque::new(unsafe { bindings::REFCOUNT_INIT(value) }))
+ }
+
+ #[inline]
+ fn as_ptr(&self) -> *mut bindings::refcount_t {
+ self.0.get()
+ }
+
+ /// Set a refcount's value.
+ #[inline]
+ pub fn set(&self, value: i32) {
+ // SAFETY: `self.as_ptr()` is valid.
+ unsafe { bindings::refcount_set(self.as_ptr(), value) }
+ }
+
+ /// Increment a refcount.
+ ///
+ /// It will saturate if overflows and `WARN`. It will also `WARN` if the refcount is 0, as this
+ /// represents a possible use-after-free condition.
+ ///
+ /// Provides no memory ordering, it is assumed that caller already has a reference on the
+ /// object.
+ #[inline]
+ pub fn inc(&self) {
+ // SAFETY: self is valid.
+ unsafe { bindings::refcount_inc(self.as_ptr()) }
+ }
+
+ /// Decrement a refcount.
+ ///
+ /// It will `WARN` on underflow and fail to decrement when saturated.
+ ///
+ /// Provides release memory ordering, such that prior loads and stores are done
+ /// before.
+ #[inline]
+ pub fn dec(&self) {
+ // SAFETY: `self.as_ptr()` is valid.
+ unsafe { bindings::refcount_dec(self.as_ptr()) }
+ }
+
+ /// Decrement a refcount and test if it is 0.
+ ///
+ /// It will `WARN` on underflow and fail to decrement when saturated.
+ ///
+ /// Provides release memory ordering, such that prior loads and stores are done
+ /// before, and provides an acquire ordering on success such that memory deallocation
+ /// must come after.
+ ///
+ /// Returns true if the resulting refcount is 0, false otherwise.
+ ///
+ /// # Notes
+ ///
+ /// A common pattern of using `Refcount` is to free memory when the reference count reaches
+ /// zero. This means that the reference to `Refcount` could become invalid after calling this
+ /// function. This is fine as long as the reference to `Refcount` is no longer used when this
+ /// function returns `false`. It is not necessary to use raw pointers in this scenario, see
+ /// <https://github.com/rust-lang/rust/issues/55005>.
+ #[inline]
+ #[must_use = "use `dec` instead if you do not need to test if it is 0"]
+ pub fn dec_and_test(&self) -> bool {
+ // SAFETY: `self.as_ptr()` is valid.
+ unsafe { bindings::refcount_dec_and_test(self.as_ptr()) }
+ }
+}
+
+// SAFETY: `refcount_t` is thread-safe.
+unsafe impl Send for Refcount {}
+
+// SAFETY: `refcount_t` is thread-safe.
+unsafe impl Sync for Refcount {}
--
2.51.0