// SPDX-License-Identifier: GPL-2.0 //! Atomic primitives. //! //! These primitives have the same semantics as their C counterparts: and the precise definitions of //! semantics can be found at [`LKMM`]. Note that Linux Kernel Memory (Consistency) Model is the //! only model for Rust code in kernel, and Rust's own atomics should be avoided. //! //! # Data races //! //! [`LKMM`] atomics have different rules regarding data races: //! //! - A normal write from C side is treated as an atomic write if //! CONFIG_KCSAN_ASSUME_PLAIN_WRITES_ATOMIC=y. //! - Mixed-size atomic accesses don't cause data races. //! //! [`LKMM`]: srctree/tools/memory-model/ mod internal; pub mod ordering; mod predefine; pub use internal::AtomicImpl; pub use ordering::{Acquire, Full, Relaxed, Release}; use crate::build_error; use internal::{AtomicArithmeticOps, AtomicBasicOps, AtomicExchangeOps, 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; } /// Types that support atomic add operations. /// /// # Safety /// // TODO: Properly defines `wrapping_add` in the following comment. /// `wrapping_add` any value of type `Self::Repr::Delta` obtained by [`Self::rhs_into_delta()`] to /// any value of type `Self::Repr` obtained through transmuting a value of type `Self` to must /// yield a value with a bit pattern also valid for `Self`. pub unsafe trait AtomicAdd: AtomicType { /// Converts `Rhs` into the `Delta` type of the atomic implementation. fn rhs_into_delta(rhs: Rhs) -> ::Delta; } #[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"), } } } impl Atomic where T::Repr: AtomicExchangeOps, { /// Atomic exchange. /// /// Atomically updates `*self` to `v` and returns the old value of `*self`. /// /// # Examples /// /// ``` /// use kernel::sync::atomic::{Atomic, Acquire, Relaxed}; /// /// let x = Atomic::new(42); /// /// assert_eq!(42, x.xchg(52, Acquire)); /// assert_eq!(52, x.load(Relaxed)); /// ``` #[doc(alias("atomic_xchg", "atomic64_xchg", "swap"))] #[inline(always)] pub fn xchg(&self, v: T, _: Ordering) -> T { let v = into_repr(v); // INVARIANT: `self.0` is a valid `T` after `atomic_xchg*()` because `v` is transmutable to // `T`. let ret = { match Ordering::TYPE { OrderingType::Full => T::Repr::atomic_xchg(&self.0, v), OrderingType::Acquire => T::Repr::atomic_xchg_acquire(&self.0, v), OrderingType::Release => T::Repr::atomic_xchg_release(&self.0, v), OrderingType::Relaxed => T::Repr::atomic_xchg_relaxed(&self.0, v), } }; // SAFETY: `ret` comes from reading `*self`, which is a valid `T` per type invariants. unsafe { from_repr(ret) } } /// Atomic compare and exchange. /// /// If `*self` == `old`, atomically updates `*self` to `new`. Otherwise, `*self` is not /// modified. /// /// Compare: The comparison is done via the byte level comparison between `*self` and `old`. /// /// Ordering: When succeeds, provides the corresponding ordering as the `Ordering` type /// parameter indicates, and a failed one doesn't provide any ordering, the load part of a /// failed cmpxchg is a [`Relaxed`] load. /// /// Returns `Ok(value)` if cmpxchg succeeds, and `value` is guaranteed to be equal to `old`, /// otherwise returns `Err(value)`, and `value` is the current value of `*self`. /// /// # Examples /// /// ``` /// use kernel::sync::atomic::{Atomic, Full, Relaxed}; /// /// let x = Atomic::new(42); /// /// // Checks whether cmpxchg succeeded. /// let success = x.cmpxchg(52, 64, Relaxed).is_ok(); /// # assert!(!success); /// /// // Checks whether cmpxchg failed. /// let failure = x.cmpxchg(52, 64, Relaxed).is_err(); /// # assert!(failure); /// /// // Uses the old value if failed, probably re-try cmpxchg. /// match x.cmpxchg(52, 64, Relaxed) { /// Ok(_) => { }, /// Err(old) => { /// // do something with `old`. /// # assert_eq!(old, 42); /// } /// } /// /// // Uses the latest value regardlessly, same as atomic_cmpxchg() in C. /// let latest = x.cmpxchg(42, 64, Full).unwrap_or_else(|old| old); /// # assert_eq!(42, latest); /// assert_eq!(64, x.load(Relaxed)); /// ``` /// /// [`Relaxed`]: ordering::Relaxed #[doc(alias( "atomic_cmpxchg", "atomic64_cmpxchg", "atomic_try_cmpxchg", "atomic64_try_cmpxchg", "compare_exchange" ))] #[inline(always)] pub fn cmpxchg( &self, mut old: T, new: T, o: Ordering, ) -> Result { // Note on code generation: // // try_cmpxchg() is used to implement cmpxchg(), and if the helper functions are inlined, // the compiler is able to figure out that branch is not needed if the users don't care // about whether the operation succeeds or not. One exception is on x86, due to commit // 44fe84459faf ("locking/atomic: Fix atomic_try_cmpxchg() semantics"), the // atomic_try_cmpxchg() on x86 has a branch even if the caller doesn't care about the // success of cmpxchg and only wants to use the old value. For example, for code like: // // let latest = x.cmpxchg(42, 64, Full).unwrap_or_else(|old| old); // // It will still generate code: // // movl $0x40, %ecx // movl $0x34, %eax // lock // cmpxchgl %ecx, 0x4(%rsp) // jne 1f // 2: // ... // 1: movl %eax, %ecx // jmp 2b // // This might be "fixed" by introducing a try_cmpxchg_exclusive() that knows the "*old" // location in the C function is always safe to write. if self.try_cmpxchg(&mut old, new, o) { Ok(old) } else { Err(old) } } /// Atomic compare and exchange and returns whether the operation succeeds. /// /// If `*self` == `old`, atomically updates `*self` to `new`. Otherwise, `*self` is not /// modified, `*old` is updated to the current value of `*self`. /// /// "Compare" and "Ordering" part are the same as [`Atomic::cmpxchg()`]. /// /// Returns `true` means the cmpxchg succeeds otherwise returns `false`. #[inline(always)] fn try_cmpxchg(&self, old: &mut T, new: T, _: Ordering) -> bool { let mut tmp = into_repr(*old); let new = into_repr(new); // INVARIANT: `self.0` is a valid `T` after `atomic_try_cmpxchg*()` because `new` is // transmutable to `T`. let ret = { match Ordering::TYPE { OrderingType::Full => T::Repr::atomic_try_cmpxchg(&self.0, &mut tmp, new), OrderingType::Acquire => { T::Repr::atomic_try_cmpxchg_acquire(&self.0, &mut tmp, new) } OrderingType::Release => { T::Repr::atomic_try_cmpxchg_release(&self.0, &mut tmp, new) } OrderingType::Relaxed => { T::Repr::atomic_try_cmpxchg_relaxed(&self.0, &mut tmp, new) } } }; // SAFETY: `tmp` comes from reading `*self`, which is a valid `T` per type invariants. *old = unsafe { from_repr(tmp) }; ret } } impl Atomic where T::Repr: AtomicArithmeticOps, { /// Atomic add. /// /// Atomically updates `*self` to `(*self).wrapping_add(v)`. /// /// # Examples /// /// ``` /// use kernel::sync::atomic::{Atomic, Relaxed}; /// /// let x = Atomic::new(42); /// /// assert_eq!(42, x.load(Relaxed)); /// /// x.add(12, Relaxed); /// /// assert_eq!(54, x.load(Relaxed)); /// ``` #[inline(always)] pub fn add(&self, v: Rhs, _: ordering::Relaxed) where T: AtomicAdd, { let v = T::rhs_into_delta(v); // INVARIANT: `self.0` is a valid `T` after `atomic_add()` due to safety requirement of // `AtomicAdd`. T::Repr::atomic_add(&self.0, v); } /// Atomic fetch and add. /// /// Atomically updates `*self` to `(*self).wrapping_add(v)`, and returns the value of `*self` /// before the update. /// /// # Examples /// /// ``` /// use kernel::sync::atomic::{Atomic, Acquire, Full, Relaxed}; /// /// let x = Atomic::new(42); /// /// assert_eq!(42, x.load(Relaxed)); /// /// assert_eq!(54, { x.fetch_add(12, Acquire); x.load(Relaxed) }); /// /// let x = Atomic::new(42); /// /// assert_eq!(42, x.load(Relaxed)); /// /// assert_eq!(54, { x.fetch_add(12, Full); x.load(Relaxed) } ); /// ``` #[inline(always)] pub fn fetch_add(&self, v: Rhs, _: Ordering) -> T where T: AtomicAdd, { let v = T::rhs_into_delta(v); // INVARIANT: `self.0` is a valid `T` after `atomic_fetch_add*()` due to safety requirement // of `AtomicAdd`. let ret = { match Ordering::TYPE { OrderingType::Full => T::Repr::atomic_fetch_add(&self.0, v), OrderingType::Acquire => T::Repr::atomic_fetch_add_acquire(&self.0, v), OrderingType::Release => T::Repr::atomic_fetch_add_release(&self.0, v), OrderingType::Relaxed => T::Repr::atomic_fetch_add_relaxed(&self.0, v), } }; // SAFETY: `ret` comes from reading `self.0`, which is a valid `T` per type invariants. unsafe { from_repr(ret) } } }