From: Boqun Feng Date: Fri, 5 Sep 2025 04:41:31 +0000 (-0700) Subject: rust: sync: atomic: Add generic atomics X-Git-Tag: v6.18-rc1~195^2~10 X-Git-Url: http://git.ipfire.org/?a=commitdiff_plain;h=29c32c405e53605dfd24054a4460516f7f6e3938;p=thirdparty%2Fkernel%2Fstable.git rust: sync: atomic: Add generic atomics To provide using LKMM atomics for Rust code, a generic `Atomic` 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 Signed-off-by: Peter Zijlstra (Intel) Reviewed-by: Alice Ryhl Reviewed-by: Benno Lossin Reviewed-by: Elle Rhumsaa Link: https://lore.kernel.org/all/20250719030827.61357-5-boqun.feng@gmail.com/ --- diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs index 2302e6d51fe2b..ea5782b6ee959 100644 --- a/rust/kernel/sync/atomic.rs +++ b/rust/kernel/sync/atomic.rs @@ -19,6 +19,277 @@ #[allow(dead_code, unreachable_pub)] mod internal; pub mod ordering; +mod predefine; pub use internal::AtomicImpl; pub use ordering::{Acquire, Full, Relaxed, Release}; + +use crate::build_error; +use internal::{AtomicBasicOps, AtomicRepr}; +use ordering::OrderingType; + +/// A memory location which can be safely modified from multiple execution contexts. +/// +/// This has the same size, alignment and bit validity as the underlying type `T`. And it disables +/// niche optimization for the same reason as [`UnsafeCell`]. +/// +/// The atomic operations are implemented in a way that is fully compatible with the [Linux Kernel +/// Memory (Consistency) Model][LKMM], hence they should be modeled as the corresponding +/// [`LKMM`][LKMM] atomic primitives. With the help of [`Atomic::from_ptr()`] and +/// [`Atomic::as_ptr()`], this provides a way to interact with [C-side atomic operations] +/// (including those without the `atomic` prefix, e.g. `READ_ONCE()`, `WRITE_ONCE()`, +/// `smp_load_acquire()` and `smp_store_release()`). +/// +/// # Invariants +/// +/// `self.0` is a valid `T`. +/// +/// [`UnsafeCell`]: core::cell::UnsafeCell +/// [LKMM]: srctree/tools/memory-model/ +/// [C-side atomic operations]: srctree/Documentation/atomic_t.txt +#[repr(transparent)] +pub struct Atomic(AtomicRepr); + +// SAFETY: `Atomic` is safe to share among execution contexts because all accesses are atomic. +unsafe impl Sync for Atomic {} + +/// Types that support basic atomic operations. +/// +/// # Round-trip transmutability +/// +/// `T` is round-trip transmutable to `U` if and only if both of these properties hold: +/// +/// - Any valid bit pattern for `T` is also a valid bit pattern for `U`. +/// - Transmuting (e.g. using [`transmute()`]) a value of type `T` to `U` and then to `T` again +/// yields a value that is in all aspects equivalent to the original value. +/// +/// # Safety +/// +/// - [`Self`] must have the same size and alignment as [`Self::Repr`]. +/// - [`Self`] must be [round-trip transmutable] to [`Self::Repr`]. +/// +/// Note that this is more relaxed than requiring the bi-directional transmutability (i.e. +/// [`transmute()`] is always sound between `U` and `T`) because of the support for atomic +/// variables over unit-only enums, see [Examples]. +/// +/// # Limitations +/// +/// Because C primitives are used to implement the atomic operations, and a C function requires a +/// valid object of a type to operate on (i.e. no `MaybeUninit<_>`), hence at the Rust <-> C +/// surface, only types with all the bits initialized can be passed. As a result, types like `(u8, +/// u16)` (padding bytes are uninitialized) are currently not supported. +/// +/// # Examples +/// +/// A unit-only enum that implements [`AtomicType`]: +/// +/// ``` +/// use kernel::sync::atomic::{AtomicType, Atomic, Relaxed}; +/// +/// #[derive(Clone, Copy, PartialEq, Eq)] +/// #[repr(i32)] +/// enum State { +/// Uninit = 0, +/// Working = 1, +/// Done = 2, +/// }; +/// +/// // SAFETY: `State` and `i32` has the same size and alignment, and it's round-trip +/// // transmutable to `i32`. +/// unsafe impl AtomicType for State { +/// type Repr = i32; +/// } +/// +/// let s = Atomic::new(State::Uninit); +/// +/// assert_eq!(State::Uninit, s.load(Relaxed)); +/// ``` +/// [`transmute()`]: core::mem::transmute +/// [round-trip transmutable]: AtomicType#round-trip-transmutability +/// [Examples]: AtomicType#examples +pub unsafe trait AtomicType: Sized + Send + Copy { + /// The backing atomic implementation type. + type Repr: AtomicImpl; +} + +#[inline(always)] +const fn into_repr(v: T) -> T::Repr { + // SAFETY: Per the safety requirement of `AtomicType`, `T` is round-trip transmutable to + // `T::Repr`, therefore the transmute operation is sound. + unsafe { core::mem::transmute_copy(&v) } +} + +/// # Safety +/// +/// `r` must be a valid bit pattern of `T`. +#[inline(always)] +const unsafe fn from_repr(r: T::Repr) -> T { + // SAFETY: Per the safety requirement of the function, the transmute operation is sound. + unsafe { core::mem::transmute_copy(&r) } +} + +impl Atomic { + /// Creates a new atomic `T`. + pub const fn new(v: T) -> Self { + // INVARIANT: Per the safety requirement of `AtomicType`, `into_repr(v)` is a valid `T`. + Self(AtomicRepr::new(into_repr(v))) + } + + /// Creates a reference to an atomic `T` from a pointer of `T`. + /// + /// This usually is used when communicating with C side or manipulating a C struct, see + /// examples below. + /// + /// # Safety + /// + /// - `ptr` is aligned to `align_of::()`. + /// - `ptr` is valid for reads and writes for `'a`. + /// - For the duration of `'a`, other accesses to `*ptr` must not cause data races (defined + /// by [`LKMM`]) against atomic operations on the returned reference. Note that if all other + /// accesses are atomic, then this safety requirement is trivially fulfilled. + /// + /// [`LKMM`]: srctree/tools/memory-model + /// + /// # Examples + /// + /// Using [`Atomic::from_ptr()`] combined with [`Atomic::load()`] or [`Atomic::store()`] can + /// achieve the same functionality as `READ_ONCE()`/`smp_load_acquire()` or + /// `WRITE_ONCE()`/`smp_store_release()` in C side: + /// + /// ``` + /// # use kernel::types::Opaque; + /// use kernel::sync::atomic::{Atomic, Relaxed, Release}; + /// + /// // Assume there is a C struct `foo`. + /// mod cbindings { + /// #[repr(C)] + /// pub(crate) struct foo { + /// pub(crate) a: i32, + /// pub(crate) b: i32 + /// } + /// } + /// + /// let tmp = Opaque::new(cbindings::foo { a: 1, b: 2 }); + /// + /// // struct foo *foo_ptr = ..; + /// let foo_ptr = tmp.get(); + /// + /// // SAFETY: `foo_ptr` is valid, and `.a` is in bounds. + /// let foo_a_ptr = unsafe { &raw mut (*foo_ptr).a }; + /// + /// // a = READ_ONCE(foo_ptr->a); + /// // + /// // SAFETY: `foo_a_ptr` is valid for read, and all other accesses on it is atomic, so no + /// // data race. + /// let a = unsafe { Atomic::from_ptr(foo_a_ptr) }.load(Relaxed); + /// # assert_eq!(a, 1); + /// + /// // smp_store_release(&foo_ptr->a, 2); + /// // + /// // SAFETY: `foo_a_ptr` is valid for writes, and all other accesses on it is atomic, so + /// // no data race. + /// unsafe { Atomic::from_ptr(foo_a_ptr) }.store(2, Release); + /// ``` + pub unsafe fn from_ptr<'a>(ptr: *mut T) -> &'a Self + where + T: Sync, + { + // CAST: `T` and `Atomic` have the same size, alignment and bit validity. + // SAFETY: Per function safety requirement, `ptr` is a valid pointer and the object will + // live long enough. It's safe to return a `&Atomic` because function safety requirement + // guarantees other accesses won't cause data races. + unsafe { &*ptr.cast::() } + } + + /// Returns a pointer to the underlying atomic `T`. + /// + /// Note that use of the return pointer must not cause data races defined by [`LKMM`]. + /// + /// # Guarantees + /// + /// The returned pointer is valid and properly aligned (i.e. aligned to [`align_of::()`]). + /// + /// [`LKMM`]: srctree/tools/memory-model + /// [`align_of::()`]: core::mem::align_of + pub const fn as_ptr(&self) -> *mut T { + // GUARANTEE: Per the function guarantee of `AtomicRepr::as_ptr()`, the `self.0.as_ptr()` + // must be a valid and properly aligned pointer for `T::Repr`, and per the safety guarantee + // of `AtomicType`, it's a valid and properly aligned pointer of `T`. + self.0.as_ptr().cast() + } + + /// Returns a mutable reference to the underlying atomic `T`. + /// + /// This is safe because the mutable reference of the atomic `T` guarantees exclusive access. + pub fn get_mut(&mut self) -> &mut T { + // CAST: `T` and `T::Repr` has the same size and alignment per the safety requirement of + // `AtomicType`, and per the type invariants `self.0` is a valid `T`, therefore the casting + // result is a valid pointer of `T`. + // SAFETY: The pointer is valid per the CAST comment above, and the mutable reference + // guarantees exclusive access. + unsafe { &mut *self.0.as_ptr().cast() } + } +} + +impl Atomic +where + T::Repr: AtomicBasicOps, +{ + /// Loads the value from the atomic `T`. + /// + /// # Examples + /// + /// ``` + /// use kernel::sync::atomic::{Atomic, Relaxed}; + /// + /// let x = Atomic::new(42i32); + /// + /// assert_eq!(42, x.load(Relaxed)); + /// + /// let x = Atomic::new(42i64); + /// + /// assert_eq!(42, x.load(Relaxed)); + /// ``` + #[doc(alias("atomic_read", "atomic64_read"))] + #[inline(always)] + pub fn load(&self, _: Ordering) -> T { + let v = { + match Ordering::TYPE { + OrderingType::Relaxed => T::Repr::atomic_read(&self.0), + OrderingType::Acquire => T::Repr::atomic_read_acquire(&self.0), + _ => build_error!("Wrong ordering"), + } + }; + + // SAFETY: `v` comes from reading `self.0`, which is a valid `T` per the type invariants. + unsafe { from_repr(v) } + } + + /// Stores a value to the atomic `T`. + /// + /// # Examples + /// + /// ``` + /// use kernel::sync::atomic::{Atomic, Relaxed}; + /// + /// let x = Atomic::new(42i32); + /// + /// assert_eq!(42, x.load(Relaxed)); + /// + /// x.store(43, Relaxed); + /// + /// assert_eq!(43, x.load(Relaxed)); + /// ``` + #[doc(alias("atomic_set", "atomic64_set"))] + #[inline(always)] + pub fn store(&self, v: T, _: Ordering) { + let v = into_repr(v); + + // INVARIANT: `v` is a valid `T`, and is stored to `self.0` by `atomic_set*()`. + match Ordering::TYPE { + OrderingType::Relaxed => T::Repr::atomic_set(&self.0, v), + OrderingType::Release => T::Repr::atomic_set_release(&self.0, v), + _ => build_error!("Wrong ordering"), + } + } +} diff --git a/rust/kernel/sync/atomic/predefine.rs b/rust/kernel/sync/atomic/predefine.rs new file mode 100644 index 0000000000000..33356deee9524 --- /dev/null +++ b/rust/kernel/sync/atomic/predefine.rs @@ -0,0 +1,15 @@ +// SPDX-License-Identifier: GPL-2.0 + +//! Pre-defined atomic types + +// SAFETY: `i32` has the same size and alignment with itself, and is round-trip transmutable to +// itself. +unsafe impl super::AtomicType for i32 { + type Repr = i32; +} + +// SAFETY: `i64` has the same size and alignment with itself, and is round-trip transmutable to +// itself. +unsafe impl super::AtomicType for i64 { + type Repr = i64; +}