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style: Use an acquire load for is_unique in servo_arc.

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Fixes #21186

Bug: 1476445
Reviewed-by: Manishearth
Differential Revision: https://phabricator.services.mozilla.com/D2205
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Bobby Holley 2018-07-17 22:23:26 +00:00 • committed by Emilio Cobos Álvarez
parent 498592ff61
commit 89880727ec
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11
components/servo_arc/lib.rs
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@ -349,15 +349,10 @@ impl<T: ?Sized> Arc<T> {
#[inline]
pub fn is_unique(&self) -> bool {
// We can use Relaxed here, but the justification is a bit subtle.
// See the extensive discussion in [1] for why this needs to be Acquire.
//
// The reason to use Acquire would be to synchronize with other threads
// that are modifying the refcount with Release, i.e. to ensure that
// their writes to memory guarded by this refcount are flushed. However,
// we know that threads only modify the contents of the Arc when they
// observe the refcount to be 1, and no other thread could observe that
// because we're holding one strong reference here.
self.inner().count.load(Relaxed) == 1
// [1] https://github.com/servo/servo/issues/21186
self.inner().count.load(Acquire) == 1
}
}