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

["Benno Lossin" <lossin@kernel.org>]

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),
> +            }
> +        }
> +    }
> +}