rust/kernel/sync/arc.rs - linux - Git at Google

 // 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::{ForeignOwnable, Opaque},
 };
 use alloc::boxed::Box;
 use core::{
     marker::{PhantomData, Unsize},
     mem::{ManuallyDrop, MaybeUninit},
     ops::{Deref, DerefMut},
     pin::Pin,
     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.
 /// ```
 ///
 /// Using `Arc<T>` as the type of `self`:
 ///
 /// ```
 /// use kernel::sync::Arc;
 ///
 /// struct Example {
 ///     a: u32,
 ///     b: u32,
 /// }
 ///
 /// impl Example {
 ///     fn take_over(self: Arc<Self>) {
 ///         // ...
 ///     }
 ///
 ///     fn use_reference(self: &Arc<Self>) {
 ///         // ...
 ///     }
 /// }
 ///
 /// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
 /// obj.use_reference();
 /// obj.take_over();
 /// ```
 ///
 /// Coercion from `Arc<Example>` to `Arc<dyn MyTrait>`:
 ///
 /// ```
 /// use kernel::sync::{Arc, ArcBorrow};
 ///
 /// trait MyTrait {
 ///     // Trait has a function whose `self` type is `Arc<Self>`.
 ///     fn example1(self: Arc<Self>) {}
 ///
 ///     // Trait has a function whose `self` type is `ArcBorrow<'_, Self>`.
 ///     fn example2(self: ArcBorrow<'_, Self>) {}
 /// }
 ///
 /// struct Example;
 /// impl MyTrait for Example {}
 ///
 /// // `obj` has type `Arc<Example>`.
 /// let obj: Arc<Example> = Arc::try_new(Example)?;
 ///
 /// // `coerced` has type `Arc<dyn MyTrait>`.
 /// let coerced: Arc<dyn MyTrait> = obj;
 /// ```
 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,
 }

 // This is to allow [`Arc`] (and variants) to be used as the type of `self`.
 impl<T: ?Sized> core::ops::Receiver for Arc<T> {}

 // This is to allow coercion from `Arc<T>` to `Arc<U>` if `T` can be converted to the
 // dynamically-sized type (DST) `U`.
 impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<Arc<U>> for Arc<T> {}

 // This is to allow `Arc<U>` to be dispatched on when `Arc<T>` can be coerced into `Arc<U>`.
 impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<Arc<U>> for Arc<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,
         }
     }

     /// Returns an [`ArcBorrow`] from the given [`Arc`].
     ///
     /// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
     /// receiver), but we have an [`Arc`] instead. Getting an [`ArcBorrow`] is free when optimised.
     #[inline]
     pub fn as_arc_borrow(&self) -> ArcBorrow<'_, T> {
         // SAFETY: The constraint that the lifetime of the shared reference must outlive that of
         // the returned `ArcBorrow` ensures that the object remains alive and that no mutable
         // reference can be created.
         unsafe { ArcBorrow::new(self.ptr) }
     }
 }

 impl<T: 'static> ForeignOwnable for Arc<T> {
     type Borrowed<'a> = ArcBorrow<'a, T>;

     fn into_foreign(self) -> *const core::ffi::c_void {
         ManuallyDrop::new(self).ptr.as_ptr() as _
     }

     unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
         // SAFETY: By the safety requirement of this function, we know that `ptr` came from
         // a previous call to `Arc::into_foreign`.
         let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();

         // SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
         // for the lifetime of the returned value. Additionally, the safety requirements of
         // `ForeignOwnable::borrow_mut` ensure that no new mutable references are created.
         unsafe { ArcBorrow::new(inner) }
     }

     unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
         // SAFETY: By the safety requirement of this function, we know that `ptr` came from
         // a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
         // holds a reference count increment that is transferrable to us.
         unsafe { Self::from_inner(NonNull::new(ptr as _).unwrap()) }
     }
 }

 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()) };
         }
     }
 }

 impl<T: ?Sized> From<UniqueArc<T>> for Arc<T> {
     fn from(item: UniqueArc<T>) -> Self {
         item.inner
     }
 }

 impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
     fn from(item: Pin<UniqueArc<T>>) -> Self {
         // SAFETY: The type invariants of `Arc` guarantee that the data is pinned.
         unsafe { Pin::into_inner_unchecked(item).inner }
     }
 }

 /// A borrowed reference to an [`Arc`] instance.
 ///
 /// For cases when one doesn't ever need to increment the refcount on the allocation, it is simpler
 /// to use just `&T`, which we can trivially get from an `Arc<T>` instance.
 ///
 /// However, when one may need to increment the refcount, it is preferable to use an `ArcBorrow<T>`
 /// over `&Arc<T>` because the latter results in a double-indirection: a pointer (shared reference)
 /// to a pointer (`Arc<T>`) to the object (`T`). An [`ArcBorrow`] eliminates this double
 /// indirection while still allowing one to increment the refcount and getting an `Arc<T>` when/if
 /// needed.
 ///
 /// # Invariants
 ///
 /// There are no mutable references to the underlying [`Arc`], and it remains valid for the
 /// lifetime of the [`ArcBorrow`] instance.
 ///
 /// # Example
 ///
 /// ```
 /// use crate::sync::{Arc, ArcBorrow};
 ///
 /// struct Example;
 ///
 /// fn do_something(e: ArcBorrow<'_, Example>) -> Arc<Example> {
 ///     e.into()
 /// }
 ///
 /// let obj = Arc::try_new(Example)?;
 /// let cloned = do_something(obj.as_arc_borrow());
 ///
 /// // Assert that both `obj` and `cloned` point to the same underlying object.
 /// assert!(core::ptr::eq(&*obj, &*cloned));
 /// ```
 ///
 /// Using `ArcBorrow<T>` as the type of `self`:
 ///
 /// ```
 /// use crate::sync::{Arc, ArcBorrow};
 ///
 /// struct Example {
 ///     a: u32,
 ///     b: u32,
 /// }
 ///
 /// impl Example {
 ///     fn use_reference(self: ArcBorrow<'_, Self>) {
 ///         // ...
 ///     }
 /// }
 ///
 /// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
 /// obj.as_arc_borrow().use_reference();
 /// ```
 pub struct ArcBorrow<'a, T: ?Sized + 'a> {
     inner: NonNull<ArcInner<T>>,
     _p: PhantomData<&'a ()>,
 }

 // This is to allow [`ArcBorrow`] (and variants) to be used as the type of `self`.
 impl<T: ?Sized> core::ops::Receiver for ArcBorrow<'_, T> {}

 // This is to allow `ArcBorrow<U>` to be dispatched on when `ArcBorrow<T>` can be coerced into
 // `ArcBorrow<U>`.
 impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
     for ArcBorrow<'_, T>
 {
 }

 impl<T: ?Sized> Clone for ArcBorrow<'_, T> {
     fn clone(&self) -> Self {
         *self
     }
 }

 impl<T: ?Sized> Copy for ArcBorrow<'_, T> {}

 impl<T: ?Sized> ArcBorrow<'_, T> {
     /// Creates a new [`ArcBorrow`] instance.
     ///
     /// # Safety
     ///
     /// Callers must ensure the following for the lifetime of the returned [`ArcBorrow`] instance:
     /// 1. That `inner` remains valid;
     /// 2. That no mutable references to `inner` are created.
     unsafe fn new(inner: NonNull<ArcInner<T>>) -> Self {
         // INVARIANT: The safety requirements guarantee the invariants.
         Self {
             inner,
             _p: PhantomData,
         }
     }
 }

 impl<T: ?Sized> From<ArcBorrow<'_, T>> for Arc<T> {
     fn from(b: ArcBorrow<'_, T>) -> Self {
         // SAFETY: The existence of `b` guarantees that the refcount is non-zero. `ManuallyDrop`
         // guarantees that `drop` isn't called, so it's ok that the temporary `Arc` doesn't own the
         // increment.
         ManuallyDrop::new(unsafe { Arc::from_inner(b.inner) })
             .deref()
             .clone()
     }
 }

 impl<T: ?Sized> Deref for ArcBorrow<'_, T> {
     type Target = T;

     fn deref(&self) -> &Self::Target {
         // SAFETY: By the type invariant, the underlying object is still alive with no mutable
         // references to it, so it is safe to create a shared reference.
         unsafe { &self.inner.as_ref().data }
     }
 }

 /// A refcounted object that is known to have a refcount of 1.
 ///
 /// It is mutable and can be converted to an [`Arc`] so that it can be shared.
 ///
 /// # Invariants
 ///
 /// `inner` always has a reference count of 1.
 ///
 /// # Examples
 ///
 /// In the following example, we make changes to the inner object before turning it into an
 /// `Arc<Test>` object (after which point, it cannot be mutated directly). Note that `x.into()`
 /// cannot fail.
 ///
 /// ```
 /// use kernel::sync::{Arc, UniqueArc};
 ///
 /// struct Example {
 ///     a: u32,
 ///     b: u32,
 /// }
 ///
 /// fn test() -> Result<Arc<Example>> {
 ///     let mut x = UniqueArc::try_new(Example { a: 10, b: 20 })?;
 ///     x.a += 1;
 ///     x.b += 1;
 ///     Ok(x.into())
 /// }
 ///
 /// # test().unwrap();
 /// ```
 ///
 /// In the following example we first allocate memory for a ref-counted `Example` but we don't
 /// initialise it on allocation. We do initialise it later with a call to [`UniqueArc::write`],
 /// followed by a conversion to `Arc<Example>`. This is particularly useful when allocation happens
 /// in one context (e.g., sleepable) and initialisation in another (e.g., atomic):
 ///
 /// ```
 /// use kernel::sync::{Arc, UniqueArc};
 ///
 /// struct Example {
 ///     a: u32,
 ///     b: u32,
 /// }
 ///
 /// fn test() -> Result<Arc<Example>> {
 ///     let x = UniqueArc::try_new_uninit()?;
 ///     Ok(x.write(Example { a: 10, b: 20 }).into())
 /// }
 ///
 /// # test().unwrap();
 /// ```
 ///
 /// In the last example below, the caller gets a pinned instance of `Example` while converting to
 /// `Arc<Example>`; this is useful in scenarios where one needs a pinned reference during
 /// initialisation, for example, when initialising fields that are wrapped in locks.
 ///
 /// ```
 /// use kernel::sync::{Arc, UniqueArc};
 ///
 /// struct Example {
 ///     a: u32,
 ///     b: u32,
 /// }
 ///
 /// fn test() -> Result<Arc<Example>> {
 ///     let mut pinned = Pin::from(UniqueArc::try_new(Example { a: 10, b: 20 })?);
 ///     // We can modify `pinned` because it is `Unpin`.
 ///     pinned.as_mut().a += 1;
 ///     Ok(pinned.into())
 /// }
 ///
 /// # test().unwrap();
 /// ```
 pub struct UniqueArc<T: ?Sized> {
     inner: Arc<T>,
 }

 impl<T> UniqueArc<T> {
     /// Tries to allocate a new [`UniqueArc`] instance.
     pub fn try_new(value: T) -> Result<Self> {
         Ok(Self {
             // INVARIANT: The newly-created object has a ref-count of 1.
             inner: Arc::try_new(value)?,
         })
     }

     /// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
     pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>> {
         Ok(UniqueArc::<MaybeUninit<T>> {
             // INVARIANT: The newly-created object has a ref-count of 1.
             inner: Arc::try_new(MaybeUninit::uninit())?,
         })
     }
 }

 impl<T> UniqueArc<MaybeUninit<T>> {
     /// Converts a `UniqueArc<MaybeUninit<T>>` into a `UniqueArc<T>` by writing a value into it.
     pub fn write(mut self, value: T) -> UniqueArc<T> {
         self.deref_mut().write(value);
         let inner = ManuallyDrop::new(self).inner.ptr;
         UniqueArc {
             // SAFETY: The new `Arc` is taking over `ptr` from `self.inner` (which won't be
             // dropped). The types are compatible because `MaybeUninit<T>` is compatible with `T`.
             inner: unsafe { Arc::from_inner(inner.cast()) },
         }
     }
 }

 impl<T: ?Sized> From<UniqueArc<T>> for Pin<UniqueArc<T>> {
     fn from(obj: UniqueArc<T>) -> Self {
         // SAFETY: It is not possible to move/replace `T` inside a `Pin<UniqueArc<T>>` (unless `T`
         // is `Unpin`), so it is ok to convert it to `Pin<UniqueArc<T>>`.
         unsafe { Pin::new_unchecked(obj) }
     }
 }

 impl<T: ?Sized> Deref for UniqueArc<T> {
     type Target = T;

     fn deref(&self) -> &Self::Target {
         self.inner.deref()
     }
 }

 impl<T: ?Sized> DerefMut for UniqueArc<T> {
     fn deref_mut(&mut self) -> &mut Self::Target {
         // SAFETY: By the `Arc` type invariant, there is necessarily a reference to the object, so
         // it is safe to dereference it. Additionally, we know there is only one reference when
         // it's inside a `UniqueArc`, so it is safe to get a mutable reference.
         unsafe { &mut self.inner.ptr.as_mut().data }
     }
 }
	// 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::{ForeignOwnable, Opaque},
	};
	use alloc::boxed::Box;
	use core::{
	marker::{PhantomData, Unsize},
	mem::{ManuallyDrop, MaybeUninit},
	ops::{Deref, DerefMut},
	pin::Pin,
	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.
	/// ```
	///
	/// Using `Arc<T>` as the type of `self`:
	///
	/// ```
	/// use kernel::sync::Arc;
	///
	/// struct Example {
	/// a: u32,
	/// b: u32,
	/// }
	///
	/// impl Example {
	/// fn take_over(self: Arc<Self>) {
	/// // ...
	/// }
	///
	/// fn use_reference(self: &Arc<Self>) {
	/// // ...
	/// }
	/// }
	///
	/// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
	/// obj.use_reference();
	/// obj.take_over();
	/// ```
	///
	/// Coercion from `Arc<Example>` to `Arc<dyn MyTrait>`:
	///
	/// ```
	/// use kernel::sync::{Arc, ArcBorrow};
	///
	/// trait MyTrait {
	/// // Trait has a function whose `self` type is `Arc<Self>`.
	/// fn example1(self: Arc<Self>) {}
	///
	/// // Trait has a function whose `self` type is `ArcBorrow<'_, Self>`.
	/// fn example2(self: ArcBorrow<'_, Self>) {}
	/// }
	///
	/// struct Example;
	/// impl MyTrait for Example {}
	///
	/// // `obj` has type `Arc<Example>`.
	/// let obj: Arc<Example> = Arc::try_new(Example)?;
	///
	/// // `coerced` has type `Arc<dyn MyTrait>`.
	/// let coerced: Arc<dyn MyTrait> = obj;
	/// ```
	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,
	}

	// This is to allow [`Arc`] (and variants) to be used as the type of `self`.
	impl<T: ?Sized> core::ops::Receiver for Arc<T> {}

	// This is to allow coercion from `Arc<T>` to `Arc<U>` if `T` can be converted to the
	// dynamically-sized type (DST) `U`.
	impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<Arc<U>> for Arc<T> {}

	// This is to allow `Arc<U>` to be dispatched on when `Arc<T>` can be coerced into `Arc<U>`.
	impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<Arc<U>> for Arc<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,
	}
	}

	/// Returns an [`ArcBorrow`] from the given [`Arc`].
	///
	/// This is useful when the argument of a function call is an [`ArcBorrow`] (e.g., in a method
	/// receiver), but we have an [`Arc`] instead. Getting an [`ArcBorrow`] is free when optimised.
	#[inline]
	pub fn as_arc_borrow(&self) -> ArcBorrow<'_, T> {
	// SAFETY: The constraint that the lifetime of the shared reference must outlive that of
	// the returned `ArcBorrow` ensures that the object remains alive and that no mutable
	// reference can be created.
	unsafe { ArcBorrow::new(self.ptr) }
	}
	}

	impl<T: 'static> ForeignOwnable for Arc<T> {
	type Borrowed<'a> = ArcBorrow<'a, T>;

	fn into_foreign(self) -> *const core::ffi::c_void {
	ManuallyDrop::new(self).ptr.as_ptr() as _
	}

	unsafe fn borrow<'a>(ptr: *const core::ffi::c_void) -> ArcBorrow<'a, T> {
	// SAFETY: By the safety requirement of this function, we know that `ptr` came from
	// a previous call to `Arc::into_foreign`.
	let inner = NonNull::new(ptr as *mut ArcInner<T>).unwrap();

	// SAFETY: The safety requirements of `from_foreign` ensure that the object remains alive
	// for the lifetime of the returned value. Additionally, the safety requirements of
	// `ForeignOwnable::borrow_mut` ensure that no new mutable references are created.
	unsafe { ArcBorrow::new(inner) }
	}

	unsafe fn from_foreign(ptr: *const core::ffi::c_void) -> Self {
	// SAFETY: By the safety requirement of this function, we know that `ptr` came from
	// a previous call to `Arc::into_foreign`, which guarantees that `ptr` is valid and
	// holds a reference count increment that is transferrable to us.
	unsafe { Self::from_inner(NonNull::new(ptr as _).unwrap()) }
	}
	}

	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()) };
	}
	}
	}

	impl<T: ?Sized> From<UniqueArc<T>> for Arc<T> {
	fn from(item: UniqueArc<T>) -> Self {
	item.inner
	}
	}

	impl<T: ?Sized> From<Pin<UniqueArc<T>>> for Arc<T> {
	fn from(item: Pin<UniqueArc<T>>) -> Self {
	// SAFETY: The type invariants of `Arc` guarantee that the data is pinned.
	unsafe { Pin::into_inner_unchecked(item).inner }
	}
	}

	/// A borrowed reference to an [`Arc`] instance.
	///
	/// For cases when one doesn't ever need to increment the refcount on the allocation, it is simpler
	/// to use just `&T`, which we can trivially get from an `Arc<T>` instance.
	///
	/// However, when one may need to increment the refcount, it is preferable to use an `ArcBorrow<T>`
	/// over `&Arc<T>` because the latter results in a double-indirection: a pointer (shared reference)
	/// to a pointer (`Arc<T>`) to the object (`T`). An [`ArcBorrow`] eliminates this double
	/// indirection while still allowing one to increment the refcount and getting an `Arc<T>` when/if
	/// needed.
	///
	/// # Invariants
	///
	/// There are no mutable references to the underlying [`Arc`], and it remains valid for the
	/// lifetime of the [`ArcBorrow`] instance.
	///
	/// # Example
	///
	/// ```
	/// use crate::sync::{Arc, ArcBorrow};
	///
	/// struct Example;
	///
	/// fn do_something(e: ArcBorrow<'_, Example>) -> Arc<Example> {
	/// e.into()
	/// }
	///
	/// let obj = Arc::try_new(Example)?;
	/// let cloned = do_something(obj.as_arc_borrow());
	///
	/// // Assert that both `obj` and `cloned` point to the same underlying object.
	/// assert!(core::ptr::eq(&obj, &cloned));
	/// ```
	///
	/// Using `ArcBorrow<T>` as the type of `self`:
	///
	/// ```
	/// use crate::sync::{Arc, ArcBorrow};
	///
	/// struct Example {
	/// a: u32,
	/// b: u32,
	/// }
	///
	/// impl Example {
	/// fn use_reference(self: ArcBorrow<'_, Self>) {
	/// // ...
	/// }
	/// }
	///
	/// let obj = Arc::try_new(Example { a: 10, b: 20 })?;
	/// obj.as_arc_borrow().use_reference();
	/// ```
	pub struct ArcBorrow<'a, T: ?Sized + 'a> {
	inner: NonNull<ArcInner<T>>,
	_p: PhantomData<&'a ()>,
	}

	// This is to allow [`ArcBorrow`] (and variants) to be used as the type of `self`.
	impl<T: ?Sized> core::ops::Receiver for ArcBorrow<'_, T> {}

	// This is to allow `ArcBorrow<U>` to be dispatched on when `ArcBorrow<T>` can be coerced into
	// `ArcBorrow<U>`.
	impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<ArcBorrow<'_, U>>
	for ArcBorrow<'_, T>
	{
	}

	impl<T: ?Sized> Clone for ArcBorrow<'_, T> {
	fn clone(&self) -> Self {
	*self
	}
	}

	impl<T: ?Sized> Copy for ArcBorrow<'_, T> {}

	impl<T: ?Sized> ArcBorrow<'_, T> {
	/// Creates a new [`ArcBorrow`] instance.
	///
	/// # Safety
	///
	/// Callers must ensure the following for the lifetime of the returned [`ArcBorrow`] instance:
	/// 1. That `inner` remains valid;
	/// 2. That no mutable references to `inner` are created.
	unsafe fn new(inner: NonNull<ArcInner<T>>) -> Self {
	// INVARIANT: The safety requirements guarantee the invariants.
	Self {
	inner,
	_p: PhantomData,
	}
	}
	}

	impl<T: ?Sized> From<ArcBorrow<'_, T>> for Arc<T> {
	fn from(b: ArcBorrow<'_, T>) -> Self {
	// SAFETY: The existence of `b` guarantees that the refcount is non-zero. `ManuallyDrop`
	// guarantees that `drop` isn't called, so it's ok that the temporary `Arc` doesn't own the
	// increment.
	ManuallyDrop::new(unsafe { Arc::from_inner(b.inner) })
	.deref()
	.clone()
	}
	}

	impl<T: ?Sized> Deref for ArcBorrow<'_, T> {
	type Target = T;

	fn deref(&self) -> &Self::Target {
	// SAFETY: By the type invariant, the underlying object is still alive with no mutable
	// references to it, so it is safe to create a shared reference.
	unsafe { &self.inner.as_ref().data }
	}
	}

	/// A refcounted object that is known to have a refcount of 1.
	///
	/// It is mutable and can be converted to an [`Arc`] so that it can be shared.
	///
	/// # Invariants
	///
	/// `inner` always has a reference count of 1.
	///
	/// # Examples
	///
	/// In the following example, we make changes to the inner object before turning it into an
	/// `Arc<Test>` object (after which point, it cannot be mutated directly). Note that `x.into()`
	/// cannot fail.
	///
	/// ```
	/// use kernel::sync::{Arc, UniqueArc};
	///
	/// struct Example {
	/// a: u32,
	/// b: u32,
	/// }
	///
	/// fn test() -> Result<Arc<Example>> {
	/// let mut x = UniqueArc::try_new(Example { a: 10, b: 20 })?;
	/// x.a += 1;
	/// x.b += 1;
	/// Ok(x.into())
	/// }
	///
	/// # test().unwrap();
	/// ```
	///
	/// In the following example we first allocate memory for a ref-counted `Example` but we don't
	/// initialise it on allocation. We do initialise it later with a call to [`UniqueArc::write`],
	/// followed by a conversion to `Arc<Example>`. This is particularly useful when allocation happens
	/// in one context (e.g., sleepable) and initialisation in another (e.g., atomic):
	///
	/// ```
	/// use kernel::sync::{Arc, UniqueArc};
	///
	/// struct Example {
	/// a: u32,
	/// b: u32,
	/// }
	///
	/// fn test() -> Result<Arc<Example>> {
	/// let x = UniqueArc::try_new_uninit()?;
	/// Ok(x.write(Example { a: 10, b: 20 }).into())
	/// }
	///
	/// # test().unwrap();
	/// ```
	///
	/// In the last example below, the caller gets a pinned instance of `Example` while converting to
	/// `Arc<Example>`; this is useful in scenarios where one needs a pinned reference during
	/// initialisation, for example, when initialising fields that are wrapped in locks.
	///
	/// ```
	/// use kernel::sync::{Arc, UniqueArc};
	///
	/// struct Example {
	/// a: u32,
	/// b: u32,
	/// }
	///
	/// fn test() -> Result<Arc<Example>> {
	/// let mut pinned = Pin::from(UniqueArc::try_new(Example { a: 10, b: 20 })?);
	/// // We can modify `pinned` because it is `Unpin`.
	/// pinned.as_mut().a += 1;
	/// Ok(pinned.into())
	/// }
	///
	/// # test().unwrap();
	/// ```
	pub struct UniqueArc<T: ?Sized> {
	inner: Arc<T>,
	}

	impl<T> UniqueArc<T> {
	/// Tries to allocate a new [`UniqueArc`] instance.
	pub fn try_new(value: T) -> Result<Self> {
	Ok(Self {
	// INVARIANT: The newly-created object has a ref-count of 1.
	inner: Arc::try_new(value)?,
	})
	}

	/// Tries to allocate a new [`UniqueArc`] instance whose contents are not initialised yet.
	pub fn try_new_uninit() -> Result<UniqueArc<MaybeUninit<T>>> {
	Ok(UniqueArc::<MaybeUninit<T>> {
	// INVARIANT: The newly-created object has a ref-count of 1.
	inner: Arc::try_new(MaybeUninit::uninit())?,
	})
	}
	}

	impl<T> UniqueArc<MaybeUninit<T>> {
	/// Converts a `UniqueArc<MaybeUninit<T>>` into a `UniqueArc<T>` by writing a value into it.
	pub fn write(mut self, value: T) -> UniqueArc<T> {
	self.deref_mut().write(value);
	let inner = ManuallyDrop::new(self).inner.ptr;
	UniqueArc {
	// SAFETY: The new `Arc` is taking over `ptr` from `self.inner` (which won't be
	// dropped). The types are compatible because `MaybeUninit<T>` is compatible with `T`.
	inner: unsafe { Arc::from_inner(inner.cast()) },
	}
	}
	}

	impl<T: ?Sized> From<UniqueArc<T>> for Pin<UniqueArc<T>> {
	fn from(obj: UniqueArc<T>) -> Self {
	// SAFETY: It is not possible to move/replace `T` inside a `Pin<UniqueArc<T>>` (unless `T`
	// is `Unpin`), so it is ok to convert it to `Pin<UniqueArc<T>>`.
	unsafe { Pin::new_unchecked(obj) }
	}
	}

	impl<T: ?Sized> Deref for UniqueArc<T> {
	type Target = T;

	fn deref(&self) -> &Self::Target {
	self.inner.deref()
	}
	}

	impl<T: ?Sized> DerefMut for UniqueArc<T> {
	fn deref_mut(&mut self) -> &mut Self::Target {
	// SAFETY: By the `Arc` type invariant, there is necessarily a reference to the object, so
	// it is safe to dereference it. Additionally, we know there is only one reference when
	// it's inside a `UniqueArc`, so it is safe to get a mutable reference.
	unsafe { &mut self.inner.ptr.as_mut().data }
	}
	}