Re: [PATCH v5 05/10] rust: sync: atomic: Add atomic {cmp,}xchg operations

Benno Lossin
2025-06-27T08:58:50Z

On Wed Jun 18, 2025 at 6:49 PM CEST, Boqun Feng wrote:
> +impl<T: AllowAtomic> Atomic<T>
> +where
> + T::Repr: AtomicHasXchgOps,
> +{
> + /// Atomic exchange.
> + ///
> + /// # Examples
> + ///
> + /// ```rust
> + /// 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"))]
> + #[inline(always)]
> + pub fn xchg<Ordering: All>(&self, v: T, _: Ordering) -> T {

Can we name this `exchange`?

> + let v = T::into_repr(v);
> + let a = self.as_ptr().cast::<T::Repr>();
> +
> + // SAFETY:
> + // - For calling the atomic_xchg*() function:
> + // - `self.as_ptr()` is a valid pointer, and per the safety requirement of `AllocAtomic`,
> + // a `*mut T` is a valid `*mut T::Repr`. Therefore `a` is a valid pointer,
> + // - per the type invariants, 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 ret = unsafe {
> + match Ordering::TYPE {
> + OrderingType::Full => T::Repr::atomic_xchg(a, v),
> + OrderingType::Acquire => T::Repr::atomic_xchg_acquire(a, v),
> + OrderingType::Release => T::Repr::atomic_xchg_release(a, v),
> + OrderingType::Relaxed => T::Repr::atomic_xchg_relaxed(a, v),
> + }
> + };
> +
> + T::from_repr(ret)
> + }
> +
> + /// Atomic compare and exchange.
> + ///
> + /// Compare: The comparison is done via the byte level comparison between the atomic variables
> + /// with the `old` value.
> + ///
> + /// Ordering: When succeeds, provides the corresponding ordering as the `Ordering` type
> + /// parameter indicates, and a failed one doesn't provide any ordering, the read part of a
> + /// failed cmpxchg should be treated as a relaxed read.

This is a bit confusing to me. The operation has a store and a load
operation and both can have different orderings (at least in Rust
userland) depending on the success/failure of the operation. In
userland, I can supply `AcqRel` and `Acquire` to ensure that I always
have Acquire semantics on any read and `Release` semantics on any write
(which I would think is a common case). How do I do this using your API?

Don't I need `Acquire` semantics on the read in order for
`compare_exchange` to give me the correct behavior in this example:

pub struct Foo {
data: Atomic<u64>,
new: Atomic<bool>,
ready: Atomic<bool>,
}

impl Foo {
pub fn new() -> Self {
Self {
data: Atomic::new(0),
new: Atomic::new(false),
ready: Atomic::new(false),
}
}

pub fn get(&self) -> Option<u64> {
if self.new.compare_exchange(true, false, Release).is_ok() {
let val = self.data.load(Acquire);
self.ready.store(false, Release);
Some(val)
} else {
None
}
}

pub fn set(&self, val: u64) -> Result<(), u64> {
if self.ready.compare_exchange(false, true, Release).is_ok() {
self.data.store(val, Release);
self.new.store(true, Release);
} else {
Err(val)
}
}
}

IIUC, you need `Acquire` ordering on both `compare_exchange` operations'
reads for this to work, right? Because if they are relaxed, this could
happen:

Thread 0 | Thread 1
------------------------------------------------|------------------------------------------------
get() { | set(42) {
| if ready.cmpxchg(false, true, Rel).is_ok() {
| data.store(42, Rel)
| new.store(true, Rel)
if new.cmpxchg(true, false, Rel).is_ok() { |
let val = self.data.load(Acq); // reads 0 |
ready.store(false, Rel); |
Some(val) |
} | }
} | }

So essentially, the `data.store` operation is not synchronized, because
the read on `new` is not `Acquire`.

> + ///
> + /// Returns `Ok(value)` if cmpxchg succeeds, and `value` is guaranteed to be equal to `old`,
> + /// otherwise returns `Err(value)`, and `value` is the value of the atomic variable when
> + /// cmpxchg was happening.
> + ///
> + /// # Examples
> + ///
> + /// ```rust
> + /// 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));
> + /// ```
> + #[doc(alias(
> + "atomic_cmpxchg",
> + "atomic64_cmpxchg",
> + "atomic_try_cmpxchg",
> + "atomic64_try_cmpxchg"
> + ))]
> + #[inline(always)]
> + pub fn cmpxchg<Ordering: All>(&self, mut old: T, new: T, o: Ordering) -> Result<T, T> {

`compare_exchange`?

> + /// Atomic compare and exchange and returns whether the operation succeeds.
> + ///
> + /// "Compare" and "Ordering" part are the same as [`Atomic::cmpxchg()`].
> + ///
> + /// Returns `true` means the cmpxchg succeeds otherwise returns `false` with `old` updated to
> + /// the value of the atomic variable when cmpxchg was happening.
> + #[inline(always)]
> + fn try_cmpxchg<Ordering: All>(&self, old: &mut T, new: T, _: Ordering) -> bool {

`try_compare_exchange`?

---
Cheers,
Benno

> + let old = (old as *mut T).cast::<T::Repr>();
> + let new = T::into_repr(new);
> + let a = self.0.get().cast::<T::Repr>();
> +
> + // SAFETY:
> + // - For calling the atomic_try_cmpchg*() function:
> + // - `self.as_ptr()` is a valid pointer, and per the safety requirement of `AllowAtomic`,
> + // a `*mut T` is a valid `*mut T::Repr`. Therefore `a` is a valid pointer,
> + // - per the type invariants, the following atomic operation won't cause data races.
> + // - `old` is a valid pointer to write because it comes from a mutable reference.
> + // - For extra safety requirement of usage on pointers returned by `self.as_ptr():
> + // - atomic operations are used here.
> + unsafe {
> + match Ordering::TYPE {
> + OrderingType::Full => T::Repr::atomic_try_cmpxchg(a, old, new),
> + OrderingType::Acquire => T::Repr::atomic_try_cmpxchg_acquire(a, old, new),
> + OrderingType::Release => T::Repr::atomic_try_cmpxchg_release(a, old, new),
> + OrderingType::Relaxed => T::Repr::atomic_try_cmpxchg_relaxed(a, old, new),
> + }
> + }
> + }
> +}