From: Asahi Lina <[email protected]> By analogy to `AlwaysRefCounted` and `ARef`, an `Ownable` type is a (typically C FFI) type that *may* be owned by Rust, but need not be. Unlike `AlwaysRefCounted`, this mechanism expects the reference to be unique within Rust, and does not allow cloning.
Conceptually, this is similar to a `KBox<T>`, except that it delegates resource management to the `T` instead of using a generic allocator. [ om: - Split code into separate file and `pub use` it from types.rs. - Make from_raw() and into_raw() public. - Remove OwnableMut, and make DerefMut dependent on Unpin instead. - Usage example/doctest for Ownable/Owned. - Fixes to documentation and commit message. ] Link: https://lore.kernel.org/all/[email protected]/ Signed-off-by: Asahi Lina <[email protected]> Co-developed-by: Oliver Mangold <[email protected]> Signed-off-by: Oliver Mangold <[email protected]> Reviewed-by: Boqun Feng <[email protected]> Reviewed-by: Daniel Almeida <[email protected]> [ Andreas: Updated documentation, examples, and formatting. Change safety requirements, safety comments. Use a reference for `release`. ] Reviewed-by: Gary Guo <[email protected]> Co-developed-by: Andreas Hindborg <[email protected]> Signed-off-by: Andreas Hindborg <[email protected]> --- rust/kernel/lib.rs | 1 + rust/kernel/owned.rs | 181 +++++++++++++++++++++++++++++++++++++++++++++++ rust/kernel/sync/aref.rs | 5 ++ rust/kernel/types.rs | 11 ++- 4 files changed, 197 insertions(+), 1 deletion(-) diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs index 696f62f85eb5f..a2bec807f03f1 100644 --- a/rust/kernel/lib.rs +++ b/rust/kernel/lib.rs @@ -121,6 +121,7 @@ pub mod of; #[cfg(CONFIG_PM_OPP)] pub mod opp; +pub mod owned; pub mod page; #[cfg(CONFIG_PCI)] pub mod pci; diff --git a/rust/kernel/owned.rs b/rust/kernel/owned.rs new file mode 100644 index 0000000000000..26bc325eee406 --- /dev/null +++ b/rust/kernel/owned.rs @@ -0,0 +1,181 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Unique owned pointer types for objects with custom drop logic. +//! +//! These pointer types are useful for C-allocated objects which by API-contract +//! are owned by Rust, but need to be freed through the C API. + +use core::{ + mem::ManuallyDrop, + ops::{ + Deref, + DerefMut, // + }, + pin::Pin, + ptr::NonNull, // +}; + +/// Types that specify their own way of performing allocation and destruction. Typically, this trait +/// is implemented on types from the C side. +/// +/// Implementing this trait allows types to be referenced via the [`Owned<Self>`] pointer type. This +/// is useful when it is desirable to tie the lifetime of the reference to an owned object, rather +/// than pass around a bare reference. [`Ownable`] types can define custom drop logic that is +/// executed when the owned reference [`Owned<Self>`] pointing to the object is dropped. +/// +/// Note: The underlying object is not required to provide internal reference counting, because it +/// represents a unique, owned reference. If reference counting (on the Rust side) is required, +/// [`AlwaysRefCounted`](crate::types::AlwaysRefCounted) should be implemented. +/// +/// # Examples +/// +/// A minimal example implementation of [`Ownable`] and its usage with [`Owned`] looks like +/// this: +/// +/// ``` +/// # #![expect(clippy::disallowed_names)] +/// # use core::cell::Cell; +/// # use core::ptr::NonNull; +/// # use kernel::sync::global_lock; +/// # use kernel::alloc::{flags, kbox::KBox, AllocError}; +/// # use kernel::types::{Owned, Ownable}; +/// +/// // Let's count the allocations to see if freeing works. +/// kernel::sync::global_lock! { +/// // SAFETY: we call `init()` right below, before doing anything else. +/// unsafe(uninit) static FOO_ALLOC_COUNT: Mutex<usize> = 0; +/// } +/// // SAFETY: We call `init()` only once, here. +/// unsafe { FOO_ALLOC_COUNT.init() }; +/// +/// struct Foo; +/// +/// impl Foo { +/// fn new() -> Result<Owned<Self>> { +/// // We are just using a `KBox` here to handle the actual allocation, as our `Foo` is +/// // not actually a C-allocated object. +/// let result = KBox::new( +/// Foo {}, +/// flags::GFP_KERNEL, +/// )?; +/// let result = KBox::into_nonnull(result); +/// // Count new allocation +/// *FOO_ALLOC_COUNT.lock() += 1; +/// // SAFETY: +/// // - We just allocated the `Self`, thus it is valid and we own it. +/// // - We can transfer this ownership to the `from_raw` method. +/// Ok(unsafe { Owned::from_raw(result) }) +/// } +/// } +/// +/// impl Ownable for Foo { +/// unsafe fn release(&mut self) { +/// // SAFETY: The [`KBox<Self>`] is still alive. We can pass ownership to the [`KBox`], as +/// // by requirement on calling this function. +/// drop(unsafe { KBox::from_raw(self) }); +/// // Count released allocation +/// *FOO_ALLOC_COUNT.lock() -= 1; +/// } +/// } +/// +/// { +/// let foo = Foo::new()?; +/// assert!(*FOO_ALLOC_COUNT.lock() == 1); +/// } +/// // `foo` is out of scope now, so we expect no live allocations. +/// assert!(*FOO_ALLOC_COUNT.lock() == 0); +/// # Ok::<(), Error>(()) +/// ``` +pub trait Ownable { + /// Tear down this `Ownable`. + /// + /// Implementers of `Ownable` can use this function to clean up the use of `Self`. This can + /// include freeing the underlying object. + /// + /// # Safety + /// + /// Callers must ensure that the caller has exclusive ownership of `T`, and this ownership can + /// be transferred to the `release` method. + unsafe fn release(&mut self); +} + +/// A mutable reference to an owned `T`. +/// +/// The [`Ownable`] is automatically freed or released when an instance of [`Owned`] is +/// dropped. +/// +/// # Invariants +/// +/// - Until `T::release` is called, this `Owned<T>` exclusively owns the underlying `T`. +/// - The `T` value is pinned. +pub struct Owned<T: Ownable> { + ptr: NonNull<T>, +} + +impl<T: Ownable> Owned<T> { + /// Creates a new instance of [`Owned`]. + /// + /// This function takes over ownership of the underlying object. + /// + /// # Safety + /// + /// Callers must ensure that: + /// - `ptr` points to a valid instance of `T`. + /// - Until `T::release` is called, the returned `Owned<T>` exclusively owns the underlying `T`. + pub unsafe fn from_raw(ptr: NonNull<T>) -> Self { + // INVARIANT: By funvtion safety requirement we satisfy the first invariant of `Self`. + // We treat `T` as pinned from now on. + Self { ptr } + } + + /// Consumes the [`Owned`], returning a raw pointer. + /// + /// This function does not drop the underlying `T`. When this function returns, ownership of the + /// underlying `T` is with the caller. + pub fn into_raw(me: Self) -> NonNull<T> { + ManuallyDrop::new(me).ptr + } + + /// Get a pinned mutable reference to the data owned by this `Owned<T>`. + pub fn as_pin_mut(&mut self) -> Pin<&mut T> { + // SAFETY: The type invariants guarantee that the object is valid, and that we can safely + // return a mutable reference to it. + let unpinned = unsafe { self.ptr.as_mut() }; + + // SAFETY: By type invariant `T` is pinned. + unsafe { Pin::new_unchecked(unpinned) } + } +} + +// SAFETY: It is safe to send an [`Owned<T>`] to another thread when the underlying `T` is [`Send`], +// because of the ownership invariant. Sending an [`Owned<T>`] is equivalent to sending the `T`. +unsafe impl<T: Ownable + Send> Send for Owned<T> {} + +// SAFETY: It is safe to send [`&Owned<T>`] to another thread when the underlying `T` is [`Sync`], +// because of the ownership invariant. Sending an [`&Owned<T>`] is equivalent to sending the `&T`. +unsafe impl<T: Ownable + Sync> Sync for Owned<T> {} + +impl<T: Ownable> Deref for Owned<T> { + type Target = T; + + fn deref(&self) -> &Self::Target { + // SAFETY: The type invariants guarantee that the object is valid. + unsafe { self.ptr.as_ref() } + } +} + +impl<T: Ownable + Unpin> DerefMut for Owned<T> { + fn deref_mut(&mut self) -> &mut Self::Target { + // SAFETY: The type invariants guarantee that the object is valid, and that we can safely + // return a mutable reference to it. + unsafe { self.ptr.as_mut() } + } +} + +impl<T: Ownable> Drop for Owned<T> { + fn drop(&mut self) { + // SAFETY: By existence of `&mut self` we exclusively own `self` and the underlying `T`. As + // we are dropping `self`, we can transfer ownership of the `T` to the `release` method. + unsafe { T::release(self.ptr.as_mut()) }; + } +} diff --git a/rust/kernel/sync/aref.rs b/rust/kernel/sync/aref.rs index 0d24a0432015d..e175aefe86151 100644 --- a/rust/kernel/sync/aref.rs +++ b/rust/kernel/sync/aref.rs @@ -29,6 +29,11 @@ /// Rust code, the recommendation is to use [`Arc`](crate::sync::Arc) to create reference-counted /// instances of a type. /// +/// Note: Implementing this trait allows types to be wrapped in an [`ARef<Self>`]. It requires an +/// internal reference count and provides only shared references. If unique references are required +/// [`Ownable`](crate::types::Ownable) should be implemented which allows types to be wrapped in an +/// [`Owned<Self>`](crate::types::Owned). +/// /// # Safety /// /// Implementers must ensure that increments to the reference count keep the object alive in memory diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs index 9c5e7dbf16323..4aec7b699269a 100644 --- a/rust/kernel/types.rs +++ b/rust/kernel/types.rs @@ -11,7 +11,16 @@ }; use pin_init::{PinInit, Wrapper, Zeroable}; -pub use crate::sync::aref::{ARef, AlwaysRefCounted}; +pub use crate::{ + owned::{ + Ownable, + Owned, // + }, + sync::aref::{ + ARef, + AlwaysRefCounted, // + }, // +}; /// Used to transfer ownership to and from foreign (non-Rust) languages. /// -- 2.51.2
