"Benno Lossin" <lossin@xxxxxxxxxx> writes:
> 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.
Sounds good to me.
>
>> +///
>> +/// # 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/
OK.
>
>> + 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`/
OK.
>
>> + 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 :)
Oops.
Best regards,
Andreas Hindborg
