Introduce the `SetOnce` type, a container that can only be written once.
The container uses an internal atomic to synchronize writes to the internal
value.

Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
---
 rust/kernel/sync.rs          |   2 +
 rust/kernel/sync/set_once.rs | 125 +++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 127 insertions(+)

diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
index cf5b638a097d9..3f957ad3a9f0a 100644
--- a/rust/kernel/sync.rs
+++ b/rust/kernel/sync.rs
@@ -20,6 +20,7 @@
 pub mod poll;
 pub mod rcu;
 mod refcount;
+mod set_once;
 
 pub use arc::{Arc, ArcBorrow, UniqueArc};
 pub use completion::Completion;
@@ -29,6 +30,7 @@
 pub use lock::spinlock::{new_spinlock, SpinLock, SpinLockGuard};
 pub use locked_by::LockedBy;
 pub use refcount::Refcount;
+pub use set_once::SetOnce;
 
 /// Represents a lockdep class. It's a wrapper around C's `lock_class_key`.
 #[repr(transparent)]
diff --git a/rust/kernel/sync/set_once.rs b/rust/kernel/sync/set_once.rs
new file mode 100644
index 0000000000000..bdba601807d8b
--- /dev/null
+++ b/rust/kernel/sync/set_once.rs
@@ -0,0 +1,125 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! A container that can be initialized at most once.
+
+use super::atomic::{
+    ordering::{Acquire, Relaxed, Release},
+    Atomic,
+};
+use core::{cell::UnsafeCell, mem::MaybeUninit};
+
+/// A container that can be populated at most once. Thread safe.
+///
+/// Once the a [`SetOnce`] is populated, it remains populated by the same object for the
+/// lifetime `Self`.
+///
+/// # Invariants
+///
+/// - `init` may only increase in value.
+/// - `init` may only assume values in the range `0..=2`.
+/// - `init == 0` if and only if `value` is uninitialized.
+/// - `init == 1` if and only if there is exactly one thread with exclusive
+///   access to `self.value`.
+/// - `init == 2` if and only if `value` is initialized and valid for shared
+///   access.
+///
+/// # Example
+///
+/// ```
+/// # use kernel::sync::SetOnce;
+/// let value = SetOnce::new();
+/// assert_eq!(None, value.as_ref());
+///
+/// let status = value.populate(42u8);
+/// assert_eq!(true, status);
+/// assert_eq!(Some(&42u8), value.as_ref());
+/// assert_eq!(Some(42u8), value.copy());
+///
+/// let status = value.populate(101u8);
+/// assert_eq!(false, status);
+/// assert_eq!(Some(&42u8), value.as_ref());
+/// assert_eq!(Some(42u8), value.copy());
+/// ```
+pub struct SetOnce<T> {
+    init: Atomic<u32>,
+    value: UnsafeCell<MaybeUninit<T>>,
+}
+
+impl<T> Default for SetOnce<T> {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl<T> SetOnce<T> {
+    /// Create a new [`SetOnce`].
+    ///
+    /// The returned instance will be empty.
+    pub const fn new() -> Self {
+        // INVARIANT: The container is empty and we initialize `init` to `0`.
+        Self {
+            value: UnsafeCell::new(MaybeUninit::uninit()),
+            init: Atomic::new(0),
+        }
+    }
+
+    /// Get a reference to the contained object.
+    ///
+    /// Returns [`None`] if this [`SetOnce`] is empty.
+    pub fn as_ref(&self) -> Option<&T> {
+        if self.init.load(Acquire) == 2 {
+            // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
+            // is initialized and valid for shared access.
+            Some(unsafe { &*self.value.get().cast() })
+        } else {
+            None
+        }
+    }
+
+    /// Populate the [`SetOnce`].
+    ///
+    /// Returns `true` if the [`SetOnce`] was successfully populated.
+    pub fn populate(&self, value: T) -> bool {
+        // INVARIANT: If the swap succeeds:
+        //  - We increase `init`.
+        //  - We write the valid value `1` to `init`.
+        //  - Only one thread can succeed in this write, so we have exclusive access after the
+        //    write.
+        if let Ok(0) = self.init.cmpxchg(0, 1, Relaxed) {
+            // SAFETY: By the type invariants of `Self`, the fact that we succeeded in writing `1`
+            // to `self.init` means we obtained exclusive access to `self.value`.
+            unsafe { core::ptr::write(self.value.get().cast(), value) };
+            // INVARIANT:
+            //  - We increase `init`.
+            //  - We write the valid value `2` to `init`.
+            //  - We release our exclusive access to `self.value` and it is now valid for shared
+            //    access.
+            self.init.store(2, Release);
+            true
+        } else {
+            false
+        }
+    }
+
+    /// Get a copy of the contained object.
+    ///
+    /// Returns [`None`] if the [`SetOnce`] is empty.
+    pub fn copy(&self) -> Option<T>
+    where
+        T: Copy,
+    {
+        self.as_ref().copied()
+    }
+}
+
+impl<T> Drop for SetOnce<T> {
+    fn drop(&mut self) {
+        if *self.init.get_mut() == 2 {
+            let value = self.value.get_mut();
+            // SAFETY: By the type invariants of `Self`, `self.init == 2` means that `self.value`
+            // contains a valid value. We have exclusive access, as we hold a `mut` reference to
+            // `self`.
+            unsafe { value.assume_init_drop() };
+        }
+    }
+}

-- 
2.47.2