[PATCH v6 4/9] rust: sync: atomic: Add generic atomics

[Boqun Feng <boqun.feng@gmail.com>]

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 `AllowAtomic` for `i32` and `i64`, and so far only basic
operations load() and store() are introduced.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
---
 rust/kernel/sync/atomic.rs         |  14 ++
 rust/kernel/sync/atomic/generic.rs | 289 +++++++++++++++++++++++++++++
 2 files changed, 303 insertions(+)
 create mode 100644 rust/kernel/sync/atomic/generic.rs

diff --git a/rust/kernel/sync/atomic.rs b/rust/kernel/sync/atomic.rs
index e80ac049f36b..c5193c1c90fe 100644
--- a/rust/kernel/sync/atomic.rs
+++ b/rust/kernel/sync/atomic.rs
@@ -16,7 +16,21 @@
 //!
 //! [`LKMM`]: srctree/tools/memory-model/
 
+pub mod generic;
 pub mod ops;
 pub mod ordering;
 
+pub use generic::Atomic;
 pub use ordering::{Acquire, Full, Relaxed, Release};
+
+// SAFETY: `i32` has the same size and alignment with itself, and is round-trip transmutable to
+// itself.
+unsafe impl generic::AllowAtomic for i32 {
+    type Repr = i32;
+}
+
+// SAFETY: `i64` has the same size and alignment with itself, and is round-trip transmutable to
+// itself.
+unsafe impl generic::AllowAtomic for i64 {
+    type Repr = i64;
+}
diff --git a/rust/kernel/sync/atomic/generic.rs b/rust/kernel/sync/atomic/generic.rs
new file mode 100644
index 000000000000..e044fe21b128
--- /dev/null
+++ b/rust/kernel/sync/atomic/generic.rs
@@ -0,0 +1,289 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Generic atomic primitives.
+
+use super::ops::*;
+use super::ordering::*;
+use crate::build_error;
+use core::cell::UnsafeCell;
+
+/// A generic atomic variable.
+///
+/// `T` must impl [`AllowAtomic`], that is, an [`AtomicImpl`] has to be chosen.
+///
+/// # Examples
+///
+/// A customized type stored in [`Atomic`]:
+///
+/// ```rust
+/// use kernel::sync::atomic::{generic::AllowAtomic, 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 AllowAtomic for State {
+///     type Repr = i32;
+/// }
+///
+/// let s = Atomic::new(State::Uninit);
+///
+/// assert_eq!(State::Uninit, s.load(Relaxed));
+/// ```
+///
+/// # Guarantees
+///
+/// Doing an atomic operation while holding a reference of [`Self`] won't cause a data race,
+/// this is guaranteed by the safety requirement of [`Self::from_ptr()`] and the extra safety
+/// requirement of the usage on pointers returned by [`Self::as_ptr()`].
+#[repr(transparent)]
+pub struct Atomic<T: AllowAtomic>(UnsafeCell<T>);
+
+// SAFETY: `Atomic<T>` is safe to share among execution contexts because all accesses are atomic.
+unsafe impl<T: AllowAtomic> Sync for Atomic<T> {}
+
+/// Types that support basic atomic operations.
+///
+/// # Round-trip transmutability
+///
+/// Implementing [`AllowAtomic`] requires that the type is round-trip transmutable to its
+/// representation:
+///
+/// - Any value of [`Self`] must be sound to [`transmute()`] to a [`Self::Repr`], and this also
+///   means that a pointer to [`Self`] can be treated as a pointer to [`Self::Repr`] for reading.
+/// - If a value of [`Self::Repr`] is a result a [`transmute()`] from a [`Self`], it must be
+///   sound to [`transmute()`] the value back to a [`Self`].
+///
+/// This essentially means a valid bit pattern of `T: AllowAtomic` has to be a valid bit pattern
+/// of `T::Repr`. This is needed because [`Atomic<T: AllowAtomic>`] operates on `T::Repr` to
+/// implement atomic operations on `T`.
+///
+/// Note that this is more relaxed than bidirectional transmutability (i.e. [`transmute()`] is
+/// always sound between `T` and `T::Repr`) because of the support for atomic variables over
+/// unit-only enums:
+///
+/// ```
+/// #[repr(i32)]
+/// enum State { Init = 0, Working = 1, Done = 2, }
+/// ```
+///
+/// # Safety
+///
+/// - [`Self`] must have the same size and alignment as [`Self::Repr`].
+/// - [`Self`] and [`Self::Repr`] must have the [round-trip transmutability].
+///
+/// # 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 no uninitialized bits can be passed. As a result, types like `(u8,
+/// u16)` (a tuple with a `MaybeUninit` hole in it) are currently not supported. Note that
+/// technically these types can be supported if some APIs are removed for them and the inner
+/// implementation is tweaked, but the justification of support such a type is not strong enough at
+/// the moment. This should be resolved if there is an implementation for `MaybeUninit<i32>` as
+/// `AtomicImpl`.
+///
+/// [`transmute()`]: core::mem::transmute
+/// [round-trip transmutability]: AllowAtomic#round-trip-transmutability
+pub unsafe trait AllowAtomic: Sized + Send + Copy {
+    /// The backing atomic implementation type.
+    type Repr: AtomicImpl;
+}
+
+#[inline(always)]
+const fn into_repr<T: AllowAtomic>(v: T) -> T::Repr {
+    // SAFETY: Per the safety requirement of `AllowAtomic`, 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<T: AllowAtomic>(r: T::Repr) -> T {
+    // SAFETY: Per the safety requirement of the function, the transmute operation is sound.
+    unsafe { core::mem::transmute_copy(&r) }
+}
+
+impl<T: AllowAtomic> Atomic<T> {
+    /// Creates a new atomic.
+    pub const fn new(v: T) -> Self {
+        Self(UnsafeCell::new(v))
+    }
+
+    /// Creates a reference to [`Self`] from a pointer.
+    ///
+    /// # Safety
+    ///
+    /// - `ptr` has to be a valid pointer.
+    /// - `ptr` has to be valid for both reads and writes for the whole lifetime `'a`.
+    /// - For the duration of `'a`, other accesses to the object cannot 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:
+    ///
+    /// ```rust
+    /// # 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 a valid pointer, 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 a valid pointer for read, and all 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 a valid pointer for write, and all accesses on it is atomic, so
+    /// // no data race.
+    /// unsafe { Atomic::from_ptr(foo_a_ptr) }.store(2, Release);
+    /// ```
+    ///
+    /// However, this should be only used when communicating with C side or manipulating a C struct.
+    pub unsafe fn from_ptr<'a>(ptr: *mut T) -> &'a Self
+    where
+        T: Sync,
+    {
+        // CAST: `T` is transparent to `Atomic<T>`.
+        // SAFETY: Per function safety requirement, `ptr` is a valid pointer and the object will
+        // live long enough. It's safe to return a `&Atomic<T>` because function safety requirement
+        // guarantees other accesses won't cause data races.
+        unsafe { &*ptr.cast::<Self>() }
+    }
+
+    /// Returns a pointer to the underlying atomic variable.
+    ///
+    /// Extra safety requirement on using the return pointer: the operations done via the pointer
+    /// cannot cause data races defined by [`LKMM`].
+    ///
+    /// [`LKMM`]: srctree/tools/memory-model
+    pub const fn as_ptr(&self) -> *mut T {
+        self.0.get()
+    }
+
+    /// Returns a mutable reference to the underlying atomic variable.
+    ///
+    /// This is safe because the mutable reference of the atomic variable guarantees the exclusive
+    /// access.
+    pub fn get_mut(&mut self) -> &mut T {
+        // SAFETY: `self.as_ptr()` is a valid pointer to `T`. `&mut self` guarantees the exclusive
+        // access, so it's safe to reborrow mutably.
+        unsafe { &mut *self.as_ptr() }
+    }
+}
+
+impl<T: AllowAtomic> Atomic<T>
+where
+    T::Repr: AtomicHasBasicOps,
+{
+    /// Loads the value from the atomic variable.
+    ///
+    /// # Examples
+    ///
+    /// Simple usages:
+    ///
+    /// ```rust
+    /// 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<Ordering: AcquireOrRelaxed>(&self, _: Ordering) -> T {
+        // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is also a
+        // valid pointer of `T::Repr`.
+        let a = self.as_ptr().cast::<T::Repr>();
+
+        // SAFETY:
+        // - For calling the atomic_read*() function:
+        //   - `a` is a valid pointer for the function per the CAST justification above.
+        //   - Per the type guarantees, the following atomic operation won't cause data races.
+        // - For extra safety requirement of usage on pointers returned by `self.as_ptr()`:
+        //   - Atomic operations are used here.
+        let v = unsafe {
+            match Ordering::TYPE {
+                OrderingType::Relaxed => T::Repr::atomic_read(a),
+                OrderingType::Acquire => T::Repr::atomic_read_acquire(a),
+                _ => build_error!("Wrong ordering"),
+            }
+        };
+
+        // SAFETY: The atomic variable holds a valid `T`, so `v` is a valid bit pattern of `T`,
+        // therefore it's safe to call `from_repr()`.
+        unsafe { from_repr(v) }
+    }
+
+    /// Stores a value to the atomic variable.
+    ///
+    /// # Examples
+    ///
+    /// ```rust
+    /// 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<Ordering: ReleaseOrRelaxed>(&self, v: T, _: Ordering) {
+        let v = into_repr(v);
+        // CAST: Per the safety requirement of `AllowAtomic`, a valid pointer of `T` is also a
+        // valid pointer of `T::Repr`.
+        let a = self.as_ptr().cast::<T::Repr>();
+
+        // SAFETY:
+        // - For calling the atomic_set*() function:
+        //   - `a` is a valid pointer for the function per the CAST justification above.
+        //   - Per the type guarantees, the following atomic operation won't cause data races.
+        // - For extra safety requirement of usage on pointers returned by `self.as_ptr()`:
+        //   - Atomic operations are used here.
+        // - For the bit validity of `Atomic<T>`:
+        //   - `v` is a valid bit pattern of `T`, so it's sound to store it in an `Atomic<T>`.
+        unsafe {
+            match Ordering::TYPE {
+                OrderingType::Relaxed => T::Repr::atomic_set(a, v),
+                OrderingType::Release => T::Repr::atomic_set_release(a, v),
+                _ => build_error!("Wrong ordering"),
+            }
+        };
+    }
+}
-- 
2.39.5 (Apple Git-154)