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Use Ordering::Acquire in Arc::try_unwrap #150709

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Use Ordering::Acquire in Arc::try_unwrap #150709

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6 changes: 5 additions & 1 deletion library/alloc/src/sync.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -1104,7 +1104,11 @@ impl<T, A: Allocator> Arc<T, A> {
#[inline]
#[stable(feature = "arc_unique", since = "1.4.0")]
pub fn try_unwrap(this: Self) -> Result<T, Self> {
if this.inner().strong.compare_exchange(1, 0, Relaxed, Relaxed).is_err() {
// This `Acquire` synchronizes with the `Release` in the `Arc::drop`
// of the penultimate `Arc`, allowing the resulting value to contain
// all modifications that may have happened due to that copy of the
// `Arc`.
if this.inner().strong.compare_exchange(1, 0, Acquire, Relaxed).is_err() {

@tmiasko tmiasko Jan 5, 2026 •
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Did you perhaps missed the fact that the success path already contains acquire fence?

@cramertj cramertj Jan 5, 2026

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I did, yes! Thank you.

return Err(this);
}

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