1 files changed, 113 insertions, 0 deletions
diff --git a/rust/kernel/sync/refcount.rs b/rust/kernel/sync/refcount.rs
new file mode 100644
index 000000000000..19236a5bccde
--- /dev/null
+++ b/rust/kernel/sync/refcount.rs
@@ -0,0 +1,113 @@
+// 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::sync::atomic::Atomic;
+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()
+    }
+
+    /// Get the underlying atomic counter that backs the refcount.
+    ///
+    /// NOTE: Usage of this function is discouraged as it can circumvent the protections offered by
+    /// `refcount.h`. If there is no way to achieve the result using APIs in `refcount.h`, then
+    /// this function can be used. Otherwise consider adding a binding for the required API.
+    #[inline]
+    pub fn as_atomic(&self) -> &Atomic<i32> {
+        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 `Atomic<i32>`. 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) {
+        // 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 {}