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rust: sync: atomic: Add generic atomics
To provide using LKMM atomics for Rust code, a generic `Atomic<T>` is added, currently `T` needs to be Send + Copy because these are the straightforward usages and all basic types support this. Implement `AtomicType` for `i32` and `i64`, and so far only basic operations load() and store() are introduced. Signed-off-by: Boqun Feng <boqun.feng@gmail.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Reviewed-by: Alice Ryhl <aliceryhl@google.com> Reviewed-by: Benno Lossin <lossin@kernel.org> Reviewed-by: Elle Rhumsaa <elle@weathered-steel.dev> Link: https://lore.kernel.org/all/20250719030827.61357-5-boqun.feng@gmail.com/
This commit is contained in:
Boqun Feng
Peter Zijlstra
parent
b638c9bc47
commit
29c32c405e
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@ -19,6 +19,277 @@
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#[allow(dead_code, unreachable_pub)]
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mod internal;
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pub mod ordering;
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mod predefine;
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pub use internal::AtomicImpl;
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pub use ordering::{Acquire, Full, Relaxed, Release};
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use crate::build_error;
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use internal::{AtomicBasicOps, AtomicRepr};
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use ordering::OrderingType;
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/// A memory location which can be safely modified from multiple execution contexts.
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///
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/// This has the same size, alignment and bit validity as the underlying type `T`. And it disables
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/// niche optimization for the same reason as [`UnsafeCell`].
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///
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/// The atomic operations are implemented in a way that is fully compatible with the [Linux Kernel
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/// Memory (Consistency) Model][LKMM], hence they should be modeled as the corresponding
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/// [`LKMM`][LKMM] atomic primitives. With the help of [`Atomic::from_ptr()`] and
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/// [`Atomic::as_ptr()`], this provides a way to interact with [C-side atomic operations]
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/// (including those without the `atomic` prefix, e.g. `READ_ONCE()`, `WRITE_ONCE()`,
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/// `smp_load_acquire()` and `smp_store_release()`).
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///
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/// # Invariants
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///
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/// `self.0` is a valid `T`.
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///
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/// [`UnsafeCell`]: core::cell::UnsafeCell
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/// [LKMM]: srctree/tools/memory-model/
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/// [C-side atomic operations]: srctree/Documentation/atomic_t.txt
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#[repr(transparent)]
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pub struct Atomic<T: AtomicType>(AtomicRepr<T::Repr>);
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// SAFETY: `Atomic<T>` is safe to share among execution contexts because all accesses are atomic.
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unsafe impl<T: AtomicType> Sync for Atomic<T> {}
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/// Types that support basic atomic operations.
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///
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/// # Round-trip transmutability
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///
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/// `T` is round-trip transmutable to `U` if and only if both of these properties hold:
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///
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/// - Any valid bit pattern for `T` is also a valid bit pattern for `U`.
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/// - Transmuting (e.g. using [`transmute()`]) a value of type `T` to `U` and then to `T` again
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/// yields a value that is in all aspects equivalent to the original value.
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///
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/// # Safety
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///
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/// - [`Self`] must have the same size and alignment as [`Self::Repr`].
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/// - [`Self`] must be [round-trip transmutable] to [`Self::Repr`].
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///
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/// Note that this is more relaxed than requiring the bi-directional transmutability (i.e.
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/// [`transmute()`] is always sound between `U` and `T`) because of the support for atomic
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/// variables over unit-only enums, see [Examples].
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///
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/// # Limitations
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///
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/// Because C primitives are used to implement the atomic operations, and a C function requires a
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/// valid object of a type to operate on (i.e. no `MaybeUninit<_>`), hence at the Rust <-> C
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/// surface, only types with all the bits initialized can be passed. As a result, types like `(u8,
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/// u16)` (padding bytes are uninitialized) are currently not supported.
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///
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/// # Examples
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///
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/// A unit-only enum that implements [`AtomicType`]:
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///
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/// ```
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/// use kernel::sync::atomic::{AtomicType, Atomic, Relaxed};
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///
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/// #[derive(Clone, Copy, PartialEq, Eq)]
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/// #[repr(i32)]
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/// enum State {
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/// Uninit = 0,
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/// Working = 1,
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/// Done = 2,
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/// };
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///
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/// // SAFETY: `State` and `i32` has the same size and alignment, and it's round-trip
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/// // transmutable to `i32`.
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/// unsafe impl AtomicType for State {
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/// type Repr = i32;
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/// }
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///
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/// let s = Atomic::new(State::Uninit);
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///
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/// assert_eq!(State::Uninit, s.load(Relaxed));
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/// ```
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/// [`transmute()`]: core::mem::transmute
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/// [round-trip transmutable]: AtomicType#round-trip-transmutability
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/// [Examples]: AtomicType#examples
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pub unsafe trait AtomicType: Sized + Send + Copy {
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/// The backing atomic implementation type.
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type Repr: AtomicImpl;
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}
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#[inline(always)]
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const fn into_repr<T: AtomicType>(v: T) -> T::Repr {
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// SAFETY: Per the safety requirement of `AtomicType`, `T` is round-trip transmutable to
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// `T::Repr`, therefore the transmute operation is sound.
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unsafe { core::mem::transmute_copy(&v) }
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}
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/// # Safety
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///
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/// `r` must be a valid bit pattern of `T`.
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#[inline(always)]
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const unsafe fn from_repr<T: AtomicType>(r: T::Repr) -> T {
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// SAFETY: Per the safety requirement of the function, the transmute operation is sound.
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unsafe { core::mem::transmute_copy(&r) }
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}
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impl<T: AtomicType> Atomic<T> {
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/// Creates a new atomic `T`.
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pub const fn new(v: T) -> Self {
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// INVARIANT: Per the safety requirement of `AtomicType`, `into_repr(v)` is a valid `T`.
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Self(AtomicRepr::new(into_repr(v)))
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}
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/// Creates a reference to an atomic `T` from a pointer of `T`.
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///
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/// This usually is used when communicating with C side or manipulating a C struct, see
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/// examples below.
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///
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/// # Safety
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///
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/// - `ptr` is aligned to `align_of::<T>()`.
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/// - `ptr` is valid for reads and writes for `'a`.
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/// - For the duration of `'a`, other accesses to `*ptr` must not cause data races (defined
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/// by [`LKMM`]) against atomic operations on the returned reference. Note that if all other
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/// accesses are atomic, then this safety requirement is trivially fulfilled.
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///
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/// [`LKMM`]: srctree/tools/memory-model
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///
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/// # Examples
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///
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/// Using [`Atomic::from_ptr()`] combined with [`Atomic::load()`] or [`Atomic::store()`] can
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/// achieve the same functionality as `READ_ONCE()`/`smp_load_acquire()` or
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/// `WRITE_ONCE()`/`smp_store_release()` in C side:
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///
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/// ```
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/// # use kernel::types::Opaque;
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/// use kernel::sync::atomic::{Atomic, Relaxed, Release};
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///
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/// // Assume there is a C struct `foo`.
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/// mod cbindings {
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/// #[repr(C)]
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/// pub(crate) struct foo {
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/// pub(crate) a: i32,
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/// pub(crate) b: i32
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/// }
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/// }
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///
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/// let tmp = Opaque::new(cbindings::foo { a: 1, b: 2 });
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///
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/// // struct foo *foo_ptr = ..;
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/// let foo_ptr = tmp.get();
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///
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/// // SAFETY: `foo_ptr` is valid, and `.a` is in bounds.
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/// let foo_a_ptr = unsafe { &raw mut (*foo_ptr).a };
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///
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/// // a = READ_ONCE(foo_ptr->a);
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/// //
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/// // SAFETY: `foo_a_ptr` is valid for read, and all other accesses on it is atomic, so no
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/// // data race.
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/// let a = unsafe { Atomic::from_ptr(foo_a_ptr) }.load(Relaxed);
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/// # assert_eq!(a, 1);
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///
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/// // smp_store_release(&foo_ptr->a, 2);
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/// //
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/// // SAFETY: `foo_a_ptr` is valid for writes, and all other accesses on it is atomic, so
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/// // no data race.
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/// unsafe { Atomic::from_ptr(foo_a_ptr) }.store(2, Release);
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/// ```
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pub unsafe fn from_ptr<'a>(ptr: *mut T) -> &'a Self
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where
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T: Sync,
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{
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// CAST: `T` and `Atomic<T>` have the same size, alignment and bit validity.
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// SAFETY: Per function safety requirement, `ptr` is a valid pointer and the object will
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// live long enough. It's safe to return a `&Atomic<T>` because function safety requirement
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// guarantees other accesses won't cause data races.
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unsafe { &*ptr.cast::<Self>() }
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}
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/// Returns a pointer to the underlying atomic `T`.
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///
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/// Note that use of the return pointer must not cause data races defined by [`LKMM`].
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///
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/// # Guarantees
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///
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/// The returned pointer is valid and properly aligned (i.e. aligned to [`align_of::<T>()`]).
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///
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/// [`LKMM`]: srctree/tools/memory-model
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/// [`align_of::<T>()`]: core::mem::align_of
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pub const fn as_ptr(&self) -> *mut T {
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// GUARANTEE: Per the function guarantee of `AtomicRepr::as_ptr()`, the `self.0.as_ptr()`
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// must be a valid and properly aligned pointer for `T::Repr`, and per the safety guarantee
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// of `AtomicType`, it's a valid and properly aligned pointer of `T`.
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self.0.as_ptr().cast()
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}
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/// Returns a mutable reference to the underlying atomic `T`.
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///
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/// This is safe because the mutable reference of the atomic `T` guarantees exclusive access.
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pub fn get_mut(&mut self) -> &mut T {
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// CAST: `T` and `T::Repr` has the same size and alignment per the safety requirement of
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// `AtomicType`, and per the type invariants `self.0` is a valid `T`, therefore the casting
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// result is a valid pointer of `T`.
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// SAFETY: The pointer is valid per the CAST comment above, and the mutable reference
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// guarantees exclusive access.
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unsafe { &mut *self.0.as_ptr().cast() }
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}
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}
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impl<T: AtomicType> Atomic<T>
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where
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T::Repr: AtomicBasicOps,
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{
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/// Loads the value from the atomic `T`.
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///
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/// # Examples
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///
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/// ```
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/// use kernel::sync::atomic::{Atomic, Relaxed};
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///
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/// let x = Atomic::new(42i32);
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///
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/// assert_eq!(42, x.load(Relaxed));
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///
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/// let x = Atomic::new(42i64);
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///
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/// assert_eq!(42, x.load(Relaxed));
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/// ```
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#[doc(alias("atomic_read", "atomic64_read"))]
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#[inline(always)]
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pub fn load<Ordering: ordering::AcquireOrRelaxed>(&self, _: Ordering) -> T {
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let v = {
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match Ordering::TYPE {
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OrderingType::Relaxed => T::Repr::atomic_read(&self.0),
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OrderingType::Acquire => T::Repr::atomic_read_acquire(&self.0),
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_ => build_error!("Wrong ordering"),
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}
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};
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// SAFETY: `v` comes from reading `self.0`, which is a valid `T` per the type invariants.
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unsafe { from_repr(v) }
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}
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/// Stores a value to the atomic `T`.
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///
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/// # Examples
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///
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/// ```
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/// use kernel::sync::atomic::{Atomic, Relaxed};
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///
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/// let x = Atomic::new(42i32);
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///
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/// assert_eq!(42, x.load(Relaxed));
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///
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/// x.store(43, Relaxed);
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///
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/// assert_eq!(43, x.load(Relaxed));
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/// ```
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#[doc(alias("atomic_set", "atomic64_set"))]
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#[inline(always)]
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pub fn store<Ordering: ordering::ReleaseOrRelaxed>(&self, v: T, _: Ordering) {
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let v = into_repr(v);
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// INVARIANT: `v` is a valid `T`, and is stored to `self.0` by `atomic_set*()`.
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match Ordering::TYPE {
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OrderingType::Relaxed => T::Repr::atomic_set(&self.0, v),
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OrderingType::Release => T::Repr::atomic_set_release(&self.0, v),
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_ => build_error!("Wrong ordering"),
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}
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}
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}
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15
rust/kernel/sync/atomic/predefine.rs
Normal file
15
rust/kernel/sync/atomic/predefine.rs
Normal file
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@ -0,0 +1,15 @@
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// SPDX-License-Identifier: GPL-2.0
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//! Pre-defined atomic types
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// SAFETY: `i32` has the same size and alignment with itself, and is round-trip transmutable to
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// itself.
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unsafe impl super::AtomicType for i32 {
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type Repr = i32;
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}
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// SAFETY: `i64` has the same size and alignment with itself, and is round-trip transmutable to
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// itself.
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unsafe impl super::AtomicType for i64 {
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type Repr = i64;
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}
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