// 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::{self, Error}, init::{self, InPlaceInit, Init, PinInit}, try_init, types::{ForeignOwnable, Opaque},};use alloc::boxed::Box;use core::{ alloc::AllocError, fmt, marker::{PhantomData, Unsize}, mem::{ManuallyDrop, MaybeUninit}, ops::{Deref, DerefMut}, pin::Pin, ptr::NonNull,};use macros::pin_data; mod std_vendor; /// 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>>,} #[pin_data]#[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` using a// mutable reference 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`// 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 a `&Arc<T>` may clone it and get an// `Arc<T>` on that thread, so the thread may ultimately access `T` using a mutable reference when// the reference count reaches zero and `T` is dropped.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, AllocError> { // 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()) }) } /// Use the given initializer to in-place initialize a `T`. /// /// If `T: !Unpin` it will not be able to move afterwards. #[inline] pub fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Self> where Error: From<E>, { UniqueArc::pin_init(init).map(|u| u.into()) } /// Use the given initializer to in-place initialize a `T`. /// /// This is equivalent to [`Arc<T>::pin_init`], since an [`Arc`] is always pinned. #[inline] pub fn init<E>(init: impl Init<T, E>) -> error::Result<Self> where Error: From<E>, { UniqueArc::init(init).map(|u| u.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) } } /// Compare whether two [`Arc`] pointers reference the same underlying object. pub fn ptr_eq(this: &Self, other: &Self) -> bool { core::ptr::eq(this.ptr.as_ptr(), other.ptr.as_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. 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> AsRef<T> for Arc<T> { fn as_ref(&self) -> &T { self.deref() }} 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 })?);Lines 1–500 of 632. Load more
// 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::{self, Error}, init::{self, InPlaceInit, Init, PinInit}, try_init, types::{ForeignOwnable, Opaque}, }; use alloc::boxed::Box; use core::{ alloc::AllocError, fmt, marker::{PhantomData, Unsize}, mem::{ManuallyDrop, MaybeUninit}, ops::{Deref, DerefMut}, pin::Pin, ptr::NonNull, }; use macros::pin_data; mod std_vendor; /// 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>>, } #[pin_data] #[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` using a // mutable reference 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` // 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 a `&Arc<T>` may clone it and get an // `Arc<T>` on that thread, so the thread may ultimately access `T` using a mutable reference when // the reference count reaches zero and `T` is dropped. 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, AllocError> { // 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()) }) } /// Use the given initializer to in-place initialize a `T`. /// /// If `T: !Unpin` it will not be able to move afterwards. #[inline] pub fn pin_init<E>(init: impl PinInit<T, E>) -> error::Result<Self> where Error: From<E>, { UniqueArc::pin_init(init).map(|u| u.into()) } /// Use the given initializer to in-place initialize a `T`. /// /// This is equivalent to [`Arc<T>::pin_init`], since an [`Arc`] is always pinned. #[inline] pub fn init<E>(init: impl Init<T, E>) -> error::Result<Self> where Error: From<E>, { UniqueArc::init(init).map(|u| u.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) } } /// Compare whether two [`Arc`] pointers reference the same underlying object. pub fn ptr_eq(this: &Self, other: &Self) -> bool { core::ptr::eq(this.ptr.as_ptr(), other.ptr.as_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. 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> AsRef<T> for Arc<T> { fn as_ref(&self) -> &T { self.deref() } } 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 })?);