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Diffstat (limited to 'rust/kernel/sync/arc.rs')
| -rw-r--r-- | rust/kernel/sync/arc.rs | 503 |
1 files changed, 503 insertions, 0 deletions
diff --git a/rust/kernel/sync/arc.rs b/rust/kernel/sync/arc.rs new file mode 100644 index 000000000000..056d2bae632a --- /dev/null +++ b/rust/kernel/sync/arc.rs @@ -0,0 +1,503 @@ +// 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::code::*, Error, Opaque, Result}; +use alloc::{ + alloc::{alloc, dealloc}, + vec::Vec, +}; +use core::{ + alloc::Layout, + convert::{AsRef, TryFrom}, + marker::{PhantomData, Unsize}, + mem::{ManuallyDrop, MaybeUninit}, + ops::{Deref, DerefMut}, + pin::Pin, + ptr::{self, NonNull}, +}; + +/// A reference-counted pointer to an instance of `T`. +/// +/// The reference count is incremented when new instances of [`Ref`] 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 [`Ref`] is always non-zero. +/// The object pointed to by [`Ref`] is always pinned. +pub struct Ref<T: ?Sized> { + ptr: NonNull<RefInner<T>>, + _p: PhantomData<RefInner<T>>, +} + +#[repr(C)] +struct RefInner<T: ?Sized> { + refcount: Opaque<bindings::refcount_t>, + data: T, +} + +// This is to allow [`Ref`] (and variants) to be used as the type of `self`. +impl<T: ?Sized> core::ops::Receiver for Ref<T> {} + +// This is to allow [`RefBorrow`] (and variants) to be used as the type of `self`. +impl<T: ?Sized> core::ops::Receiver for RefBorrow<'_, T> {} + +// This is to allow coercion from `Ref<T>` to `Ref<U>` if `T` can be converted to the +// dynamically-sized type (DST) `U`. +impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::CoerceUnsized<Ref<U>> for Ref<T> {} + +// This is to allow `Ref<U>` to be dispatched on when `Ref<T>` can be coerced into `Ref<U>`. +impl<T: ?Sized + Unsize<U>, U: ?Sized> core::ops::DispatchFromDyn<Ref<U>> for Ref<T> {} + +// SAFETY: It is safe to send `Ref<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 `Ref<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 Ref<T> {} + +// SAFETY: It is safe to send `&Ref<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 a `&Ref<T>` +// into a `Ref<T>`, which may lead to `T` being accessed by the same reasoning as above. +unsafe impl<T: ?Sized + Sync + Send> Sync for Ref<T> {} + +impl<T> Ref<T> { + /// Constructs a new reference counted instance of `T`. + pub fn try_new(contents: T) -> Result<Self> { + let layout = Layout::new::<RefInner<T>>(); + // SAFETY: The layout size is guaranteed to be non-zero because `RefInner` contains the + // reference count. + let inner = NonNull::new(unsafe { alloc(layout) }) + .ok_or(ENOMEM)? + .cast::<RefInner<T>>(); + + // INVARIANT: The refcount is initialised to a non-zero value. + let value = RefInner { + // SAFETY: Just an FFI call that returns a `refcount_t` initialised to 1. + refcount: Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }), + data: contents, + }; + // SAFETY: `inner` is writable and properly aligned. + unsafe { inner.as_ptr().write(value) }; + + // SAFETY: We just created `inner` with a reference count of 1, which is owned by the new + // `Ref` object. + Ok(unsafe { Self::from_inner(inner) }) + } + + /// Deconstructs a [`Ref`] object into a `usize`. + /// + /// It can be reconstructed once via [`Ref::from_usize`]. + pub fn into_usize(obj: Self) -> usize { + ManuallyDrop::new(obj).ptr.as_ptr() as _ + } + + /// Borrows a [`Ref`] instance previously deconstructed via [`Ref::into_usize`]. + /// + /// # Safety + /// + /// `encoded` must have been returned by a previous call to [`Ref::into_usize`]. Additionally, + /// [`Ref::from_usize`] can only be called after *all* instances of [`RefBorrow`] have been + /// dropped. + pub unsafe fn borrow_usize<'a>(encoded: usize) -> RefBorrow<'a, T> { + // SAFETY: By the safety requirement of this function, we know that `encoded` came from + // a previous call to `Ref::into_usize`. + let inner = NonNull::new(encoded as *mut RefInner<T>).unwrap(); + + // SAFETY: The safety requirements ensure that the object remains alive for the lifetime of + // the returned value. There is no way to create mutable references to the object. + unsafe { RefBorrow::new(inner) } + } + + /// Recreates a [`Ref`] instance previously deconstructed via [`Ref::into_usize`]. + /// + /// # Safety + /// + /// `encoded` must have been returned by a previous call to [`Ref::into_usize`]. Additionally, + /// it can only be called once for each previous call to [`Ref::into_usize`]. + pub unsafe fn from_usize(encoded: usize) -> Self { + // SAFETY: By the safety invariants we know that `encoded` came from `Ref::into_usize`, so + // the reference count held then will be owned by the new `Ref` object. + unsafe { Self::from_inner(NonNull::new(encoded as _).unwrap()) } + } +} + +impl<T: ?Sized> Ref<T> { + /// Constructs a new [`Ref`] from an existing [`RefInner`]. + /// + /// # 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 [`Ref`] instance. + unsafe fn from_inner(inner: NonNull<RefInner<T>>) -> Self { + // INVARIANT: By the safety requirements, the invariants hold. + Ref { + ptr: inner, + _p: PhantomData, + } + } + + /// Determines if two reference-counted pointers point to the same underlying instance of `T`. + pub fn ptr_eq(a: &Self, b: &Self) -> bool { + ptr::eq(a.ptr.as_ptr(), b.ptr.as_ptr()) + } + + /// Deconstructs a [`Ref`] object into a raw pointer. + /// + /// It can be reconstructed once via [`Ref::from_raw`]. + pub fn into_raw(obj: Self) -> *const T { + let ret = &*obj as *const T; + core::mem::forget(obj); + ret + } + + /// Recreates a [`Ref`] instance previously deconstructed via [`Ref::into_raw`]. + /// + /// This code relies on the `repr(C)` layout of structs as described in + /// <https://doc.rust-lang.org/reference/type-layout.html#reprc-structs>. + /// + /// # Safety + /// + /// `ptr` must have been returned by a previous call to [`Ref::into_raw`]. Additionally, it + /// can only be called once for each previous call to [`Ref::into_raw`]. + pub unsafe fn from_raw(ptr: *const T) -> Self { + // SAFETY: The safety requirement ensures that the pointer is valid. + let align = core::mem::align_of_val(unsafe { &*ptr }); + let offset = Layout::new::<RefInner<()>>() + .align_to(align) + .unwrap() + .pad_to_align() + .size(); + // SAFETY: The pointer is in bounds because by the safety requirements `ptr` came from + // `Ref::into_raw`, so it is a pointer `offset` bytes from the beginning of the allocation. + let data = unsafe { (ptr as *const u8).sub(offset) }; + let metadata = ptr::metadata(ptr as *const RefInner<T>); + let ptr = ptr::from_raw_parts_mut(data as _, metadata); + // SAFETY: By the safety requirements we know that `ptr` came from `Ref::into_raw`, so the + // reference count held then will be owned by the new `Ref` object. + unsafe { Self::from_inner(NonNull::new(ptr).unwrap()) } + } + + /// Returns a [`RefBorrow`] from the given [`Ref`]. + /// + /// This is useful when the argument of a function call is a [`RefBorrow`] (e.g., in a method + /// receiver), but we have a [`Ref`] instead. Getting a [`RefBorrow`] is free when optimised. + #[inline] + pub fn as_ref_borrow(&self) -> RefBorrow<'_, T> { + // SAFETY: The constraint that lifetime of the shared reference must outlive that of + // the returned `RefBorrow` ensures that the object remains alive. + unsafe { RefBorrow::new(self.ptr) } + } +} + +impl<T: ?Sized> Deref for Ref<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 Ref<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 `Ref`. + unsafe { Self::from_inner(self.ptr) } + } +} + +impl<T: ?Sized> AsRef<T> for Ref<T> { + fn as_ref(&self) -> &T { + // 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> Drop for Ref<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 `Ref`, 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: This thread holds the only remaining reference to `self`, so it is safe to + // get a mutable reference to it. + let inner = unsafe { self.ptr.as_mut() }; + let layout = Layout::for_value(inner); + // SAFETY: The value stored in inner is valid. + unsafe { core::ptr::drop_in_place(inner) }; + // SAFETY: The pointer was initialised from the result of a call to `alloc`. + unsafe { dealloc(self.ptr.cast().as_ptr(), layout) }; + } + } +} + +impl<T> TryFrom<Vec<T>> for Ref<[T]> { + type Error = Error; + + fn try_from(mut v: Vec<T>) -> Result<Self> { + let value_layout = Layout::array::<T>(v.len())?; + let layout = Layout::new::<RefInner<()>>() + .extend(value_layout)? + .0 + .pad_to_align(); + // SAFETY: The layout size is guaranteed to be non-zero because `RefInner` contains the + // reference count. + let ptr = NonNull::new(unsafe { alloc(layout) }).ok_or(ENOMEM)?; + let inner = + core::ptr::slice_from_raw_parts_mut(ptr.as_ptr() as _, v.len()) as *mut RefInner<[T]>; + + // SAFETY: Just an FFI call that returns a `refcount_t` initialised to 1. + let count = Opaque::new(unsafe { bindings::REFCOUNT_INIT(1) }); + // SAFETY: `inner.refcount` is writable and properly aligned. + unsafe { core::ptr::addr_of_mut!((*inner).refcount).write(count) }; + // SAFETY: The contents of `v` as readable and properly aligned; `inner.data` is writable + // and properly aligned. There is no overlap between the two because `inner` is a new + // allocation. + unsafe { + core::ptr::copy_nonoverlapping( + v.as_ptr(), + core::ptr::addr_of_mut!((*inner).data) as *mut [T] as *mut T, + v.len(), + ) + }; + // SAFETY: We're setting the new length to zero, so it is <= to capacity, and old_len..0 is + // an empty range (so satisfies vacuously the requirement of being initialised). + unsafe { v.set_len(0) }; + // SAFETY: We just created `inner` with a reference count of 1, which is owned by the new + // `Ref` object. + Ok(unsafe { Self::from_inner(NonNull::new(inner).unwrap()) }) + } +} + +impl<T: ?Sized> From<UniqueRef<T>> for Ref<T> { + fn from(item: UniqueRef<T>) -> Self { + item.inner + } +} + +impl<T: ?Sized> From<UniqueRef<T>> for Pin<UniqueRef<T>> { + fn from(obj: UniqueRef<T>) -> Self { + // SAFETY: It is not possible to move/replace `T` inside a `Pin<UniqueRef<T>>` (unless `T` + // is `Unpin`), so it is ok to convert it to `Pin<UniqueRef<T>>`. + unsafe { Pin::new_unchecked(obj) } + } +} + +impl<T: ?Sized> From<Pin<UniqueRef<T>>> for Ref<T> { + fn from(item: Pin<UniqueRef<T>>) -> Self { + // SAFETY: The type invariants of `Ref` guarantee that the data is pinned. + unsafe { Pin::into_inner_unchecked(item).inner } + } +} + +/// A borrowed [`Ref`] with manually-managed lifetime. +/// +/// # Invariants +/// +/// There are no mutable references to the underlying [`Ref`], and it remains valid for the lifetime +/// of the [`RefBorrow`] instance. +pub struct RefBorrow<'a, T: ?Sized + 'a> { + inner: NonNull<RefInner<T>>, + _p: PhantomData<&'a ()>, +} + +impl<T: ?Sized> Clone for RefBorrow<'_, T> { + fn clone(&self) -> Self { + *self + } +} + +impl<T: ?Sized> Copy for RefBorrow<'_, T> {} + +impl<T: ?Sized> RefBorrow<'_, T> { + /// Creates a new [`RefBorrow`] instance. + /// + /// # Safety + /// + /// Callers must ensure the following for the lifetime of the returned [`RefBorrow`] instance: + /// 1. That `obj` remains valid; + /// 2. That no mutable references to `obj` are created. + unsafe fn new(inner: NonNull<RefInner<T>>) -> Self { + // INVARIANT: The safety requirements guarantee the invariants. + Self { + inner, + _p: PhantomData, + } + } +} + +impl<T: ?Sized> From<RefBorrow<'_, T>> for Ref<T> { + fn from(b: RefBorrow<'_, 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 `Ref` doesn't own the + // increment. + ManuallyDrop::new(unsafe { Ref::from_inner(b.inner) }) + .deref() + .clone() + } +} + +impl<T: ?Sized> Deref for RefBorrow<'_, 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 a [`Ref`] 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 a +/// `Ref<Test>` object (after which point, it cannot be mutated directly). Note that `x.into()` +/// cannot fail. +/// +/// ``` +/// use kernel::sync::{Ref, UniqueRef}; +/// +/// struct Example { +/// a: u32, +/// b: u32, +/// } +/// +/// fn test() -> Result<Ref<Example>> { +/// let mut x = UniqueRef::try_new(Example { a: 10, b: 20 })?; +/// x.a += 1; +/// x.b += 1; +/// Ok(x.into()) +/// } +/// +/// # test(); +/// ``` +/// +/// 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 [`UniqueRef::write`], +/// followed by a conversion to `Ref<Example>`. This is particularly useful when allocation happens +/// in one context (e.g., sleepable) and initialisation in another (e.g., atomic): +/// +/// ``` +/// use kernel::sync::{Ref, UniqueRef}; +/// +/// struct Example { +/// a: u32, +/// b: u32, +/// } +/// +/// fn test() -> Result<Ref<Example>> { +/// let x = UniqueRef::try_new_uninit()?; +/// Ok(x.write(Example { a: 10, b: 20 }).into()) +/// } +/// +/// # test(); +/// ``` +/// +/// In the last example below, the caller gets a pinned instance of `Example` while converting to +/// `Ref<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::{Ref, UniqueRef}; +/// +/// struct Example { +/// a: u32, +/// b: u32, +/// } +/// +/// fn test() -> Result<Ref<Example>> { +/// let mut pinned = Pin::from(UniqueRef::try_new(Example { a: 10, b: 20 })?); +/// // We can modify `pinned` because it is `Unpin`. +/// pinned.as_mut().a += 1; +/// Ok(pinned.into()) +/// } +/// +/// # test(); +/// ``` +pub struct UniqueRef<T: ?Sized> { + inner: Ref<T>, +} + +impl<T> UniqueRef<T> { + /// Tries to allocate a new [`UniqueRef`] instance. + pub fn try_new(value: T) -> Result<Self> { + Ok(Self { + // INVARIANT: The newly-created object has a ref-count of 1. + inner: Ref::try_new(value)?, + }) + } + + /// Tries to allocate a new [`UniqueRef`] instance whose contents are not initialised yet. + pub fn try_new_uninit() -> Result<UniqueRef<MaybeUninit<T>>> { + Ok(UniqueRef::<MaybeUninit<T>> { + // INVARIANT: The newly-created object has a ref-count of 1. + inner: Ref::try_new(MaybeUninit::uninit())?, + }) + } +} + +impl<T> UniqueRef<MaybeUninit<T>> { + /// Converts a `UniqueRef<MaybeUninit<T>>` into a `UniqueRef<T>` by writing a value into it. + pub fn write(mut self, value: T) -> UniqueRef<T> { + self.deref_mut().write(value); + let inner = ManuallyDrop::new(self).inner.ptr; + UniqueRef { + // SAFETY: The new `Ref` 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 { Ref::from_inner(inner.cast()) }, + } + } +} + +impl<T: ?Sized> Deref for UniqueRef<T> { + type Target = T; + + fn deref(&self) -> &Self::Target { + self.inner.deref() + } +} + +impl<T: ?Sized> DerefMut for UniqueRef<T> { + fn deref_mut(&mut self) -> &mut Self::Target { + // SAFETY: By the `Ref` 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 `UniqueRef`, so it is safe to get a mutable reference. + unsafe { &mut self.inner.ptr.as_mut().data } + } +} |