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:
quoted
+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?
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.
Same answer for compare_exchange() vs cmpxchg().
quoted
+ 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.
}
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.
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() {
Same.
Regards,
Boqun
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`.
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