rust: sync: add `Arc` for ref-counted allocations

This is a basic implementation of `Arc` backed by C's `refcount_t`. It
allows Rust code to idiomatically allocate memory that is ref-counted.

Cc: Will Deacon <will@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Wedson Almeida Filho <wedsonaf@gmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Reviewed-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

3 files changed, 168 insertions, 0 deletions

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index 53040fa9e897..ace064a3702a 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -31,6 +31,7 @@ mod static_assert;
 #[doc(hidden)]
 pub mod std_vendor;
 pub mod str;
+pub mod sync;
 pub mod types;
 
 #[doc(hidden)]
diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
new file mode 100644
index 000000000000..39b379dd548f
--- /dev/null
+++ b/rust/kernel/sync.rs
@@ -0,0 +1,10 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Synchronisation primitives.
+//!
+//! This module contains the kernel APIs related to synchronisation that have been ported or
+//! wrapped for usage by Rust code in the kernel.
+
+mod arc;
+
+pub use arc::Arc;
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs
new file mode 100644
index 000000000000..22290eb5ab9b
--- /dev/null
+++ b/rust/kernel/sync/arc.rs
@@ -0,0 +1,157 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! A reference-counted pointer.
+//!
+//! This module implements a way for users to create reference-counted objects and pointers to
+//! them. Such a pointer automatically increments and decrements the count, and drops the
+//! underlying object when it reaches zero. It is also safe to use concurrently from multiple
+//! threads.
+//!
+//! It is different from the standard library's [`Arc`] in a few ways:
+//! 1. It is backed by the kernel's `refcount_t` type.
+//! 2. It does not support weak references, which allows it to be half the size.
+//! 3. It saturates the reference count instead of aborting when it goes over a threshold.
+//! 4. It does not provide a `get_mut` method, so the ref counted object is pinned.
+//!
+//! [`Arc`]: https://doc.rust-lang.org/std/sync/struct.Arc.html
+
+use crate::{bindings, error::Result, types::Opaque};
+use alloc::boxed::Box;
+use core::{marker::PhantomData, ops::Deref, ptr::NonNull};
+
+/// A reference-counted pointer to an instance of `T`.
+///
+/// The reference count is incremented when new instances of [`Arc`] are created, and decremented
+/// when they are dropped. When the count reaches zero, the underlying `T` is also dropped.
+///
+/// # Invariants
+///
+/// The reference count on an instance of [`Arc`] is always non-zero.
+/// The object pointed to by [`Arc`] is always pinned.
+///
+/// # Examples
+///
+/// ```
+/// use kernel::sync::Arc;
+///
+/// struct Example {
+///     a: u32,
+///     b: u32,
+/// }
+///
+/// // Create a ref-counted instance of `Example`.
+/// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
+///
+/// // Get a new pointer to `obj` and increment the refcount.
+/// let cloned = obj.clone();
+///
+/// // Assert that both `obj` and `cloned` point to the same underlying object.
+/// assert!(core::ptr::eq(&*obj, &*cloned));
+///
+/// // Destroy `obj` and decrement its refcount.
+/// drop(obj);
+///
+/// // Check that the values are still accessible through `cloned`.
+/// assert_eq!(cloned.a, 10);
+/// assert_eq!(cloned.b, 20);
+///
+/// // The refcount drops to zero when `cloned` goes out of scope, and the memory is freed.
+/// ```
+pub struct Arc<T: ?Sized> {
+    ptr: NonNull<ArcInner<T>>,
+    _p: PhantomData<ArcInner<T>>,
+}
+
+#[repr(C)]
+struct ArcInner<T: ?Sized> {
+    refcount: Opaque<bindings::refcount_t>,
+    data: T,
+}
+
+// SAFETY: It is safe to send `Arc<T>` to another thread when the underlying `T` is `Sync` because
+// it effectively means sharing `&T` (which is safe because `T` is `Sync`); additionally, it needs
+// `T` to be `Send` because any thread that has an `Arc<T>` may ultimately access `T` directly, for
+// example, when the reference count reaches zero and `T` is dropped.
+unsafe impl<T: ?Sized + Sync + Send> Send for Arc<T> {}
+
+// SAFETY: It is safe to send `&Arc<T>` to another thread when the underlying `T` is `Sync` for the
+// same reason as above. `T` needs to be `Send` as well because a thread can clone an `&Arc<T>`
+// into an `Arc<T>`, which may lead to `T` being accessed by the same reasoning as above.
+unsafe impl<T: ?Sized + Sync + Send> Sync for Arc<T> {}
+
+impl<T> Arc<T> {
+    /// Constructs a new reference counted instance of `T`.
+    pub fn try_new(contents: T) -> Result<Self> {
+        // INVARIANT: The refcount is initialised to a non-zero value.
+        let value = ArcInner {
+            // SAFETY: There are no safety requirements for this FFI call.
+            refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }),
+            data: contents,
+        };
+
+        let inner = Box::try_new(value)?;
+
+        // SAFETY: We just created `inner` with a reference count of 1, which is owned by the new
+        // `Arc` object.
+        Ok(unsafe { Self::from_inner(Box::leak(inner).into()) })
+    }
+}
+
+impl<T: ?Sized> Arc<T> {
+    /// Constructs a new [`Arc`] from an existing [`ArcInner`].
+    ///
+    /// # Safety
+    ///
+    /// The caller must ensure that `inner` points to a valid location and has a non-zero reference
+    /// count, one of which will be owned by the new [`Arc`] instance.
+    unsafe fn from_inner(inner: NonNull<ArcInner<T>>) -> Self {
+        // INVARIANT: By the safety requirements, the invariants hold.
+        Arc {
+            ptr: inner,
+            _p: PhantomData,
+        }
+    }
+}
+
+impl<T: ?Sized> Deref for Arc<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
+        // safe to dereference it.
+        unsafe { &self.ptr.as_ref().data }
+    }
+}
+
+impl<T: ?Sized> Clone for Arc<T> {
+    fn clone(&self) -> Self {
+        // INVARIANT: C `refcount_inc` saturates the refcount, so it cannot overflow to zero.
+        // SAFETY: By the type invariant, there is necessarily a reference to the object, so it is
+        // safe to increment the refcount.
+        unsafe { bindings::refcount_inc(self.ptr.as_ref().refcount.get()) };
+
+        // SAFETY: We just incremented the refcount. This increment is now owned by the new `Arc`.
+        unsafe { Self::from_inner(self.ptr) }
+    }
+}
+
+impl<T: ?Sized> Drop for Arc<T> {
+    fn drop(&mut self) {
+        // SAFETY: By the type invariant, there is necessarily a reference to the object. We cannot
+        // touch `refcount` after it's decremented to a non-zero value because another thread/CPU
+        // may concurrently decrement it to zero and free it. It is ok to have a raw pointer to
+        // freed/invalid memory as long as it is never dereferenced.
+        let refcount = unsafe { self.ptr.as_ref() }.refcount.get();
+
+        // INVARIANT: If the refcount reaches zero, there are no other instances of `Arc`, and
+        // this instance is being dropped, so the broken invariant is not observable.
+        // SAFETY: Also by the type invariant, we are allowed to decrement the refcount.
+        let is_zero = unsafe { bindings::refcount_dec_and_test(refcount) };
+        if is_zero {
+            // The count reached zero, we must free the memory.
+            //
+            // SAFETY: The pointer was initialised from the result of `Box::leak`.
+            unsafe { Box::from_raw(self.ptr.as_ptr()) };
+        }
+    }
+}