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

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

On Fri Jun 27, 2025 at 3:53 PM CEST, Boqun Feng wrote:
> On Fri, Jun 27, 2025 at 10:58:43AM +0200, Benno Lossin wrote:
>> 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`?
>> 
>
> FYI, in Rust std, this operation is called `swap()`, what's the reason
> of using a name that is neither the Rust convention nor Linux kernel
> convention?

Ah, well then my suggestion would be `swap()` instead :)

> As for naming, the reason I choose xchg() and cmpxchg() is because they
> are the name LKMM uses for a long time, to use another name, we have to
> have a very good reason to do so and I don't see a good reason
> that the other names are better, especially, in our memory model, we use
> xchg() and cmpxchg() a lot, and they are different than Rust version
> where you can specify orderings separately. Naming LKMM xchg()/cmpxchg()
> would cause more confusion I believe.

I'm just not used to the name shortening from the kernel... I think it's
fine to use them especially since the ordering parameters differ from
std's atomics.

Can you add aliases for the Rust names?

> Same answer for compare_exchange() vs cmpxchg().
>
>> > +        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?
>> 
>
> Usually in kernel that means in a failure case you need to use a barrier
> afterwards, for example:
>
> 	if (old != cmpxchg(v, old, new)) {
> 		smp_mb();
> 		// ^ following memory operations are ordered against.
> 	}

Do we already have abstractions for those?

>> 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() {
>
> You should use `Full` if you want AcqRel-like behavior when succeed.

I think it would be pretty valuable to document this. Also any other
"direct" translations from the Rust memory model are useful. For example
is `SeqCst` "equivalent" to `Full`?

---
Cheers,
Benno