On Wed Jul 9, 2025 at 7:52 PM CEST, Andreas Hindborg wrote:
> 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@xxxxxxxxxx>
> Signed-off-by: Andreas Hindborg <a.hindborg@xxxxxxxxxx>
A couple notes on safety documentation below. Also one pretty subtle
functionality change from last version. With everything fixed:
Reviewed-by: Benno Lossin <lossin@xxxxxxxxxx>
> ---
> rust/kernel/sync.rs | 2 +
> rust/kernel/sync/set_once.rs | 122 +++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 124 insertions(+)
>
> diff --git a/rust/kernel/sync.rs b/rust/kernel/sync.rs
> index 81e3a806e57e2..13e6bc7fa87ac 100644
> --- a/rust/kernel/sync.rs
> +++ b/rust/kernel/sync.rs
> @@ -18,6 +18,7 @@
> mod locked_by;
> pub mod poll;
> pub mod rcu;
> +mod set_once;
>
> pub use arc::{Arc, ArcBorrow, UniqueArc};
> pub use completion::Completion;
> @@ -26,6 +27,7 @@
> pub use lock::mutex::{new_mutex, Mutex, MutexGuard};
> pub use lock::spinlock::{new_spinlock, SpinLock, SpinLockGuard};
> pub use locked_by::LockedBy;
> +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..73706abfe9991
> --- /dev/null
> +++ b/rust/kernel/sync/set_once.rs
> @@ -0,0 +1,122 @@
> +// 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, ptr::drop_in_place};
> +
> +/// 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 the container is empty.
> +/// - `init == 1` if and only if being initialized.
> +/// - `init == 2` if and only if the container is populated and valid for shared access.
I think I have a better idea for the last three invariants:
- `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`
> + // contains a valid value.
s/contains a valid 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 the contained object.
s/to the contained object/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 the contained object and the object 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 {
> + // 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 { drop_in_place(self.value.get()) };
This is sadly doing the wrong thing now since you changed the type of
`value`: `self.value.get()` is of type `MaybeUninit<T>` and dropping
that has (obviously) no effect. So we probably need to do
let value = unsafe { &mut *self.value.get() };
unsafe { value.assume_init_drop() };
I almost overlooked this :)
---
Cheers,
Benno
> + }
> + }
> +}
