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Auto merge of #52349 - RalfJung:once, r=alexcrichton

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sync::Once use release-acquire access modes

Nothing here makes a case distinction like "this happened before OR after that". All we need is to get happens-before edges whenever we see that the state/signal has been changed. Release-acquire is good enough for that.
This commit is contained in:
bors 2018-07-20 02:52:19 +00:00
commit bc14d71622
1 changed files with 10 additions and 2 deletions
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12
src/libstd/sync/once.rs
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@ -220,7 +220,11 @@ impl Once {
#[stable(feature = "rust1", since = "1.0.0")]
pub fn call_once<F>(&self, f: F) where F: FnOnce() {
// Fast path, just see if we've completed initialization.
if self.state.load(Ordering::SeqCst) == COMPLETE {
// An `Acquire` load is enough because that makes all the initialization
// operations visible to us. The cold path uses SeqCst consistently
// because the performance difference really does not matter there,
// and SeqCst minimizes the chances of something going wrong.
if self.state.load(Ordering::Acquire) == COMPLETE {
return
}
@ -277,7 +281,11 @@ impl Once {
#[unstable(feature = "once_poison", issue = "33577")]
pub fn call_once_force<F>(&self, f: F) where F: FnOnce(&OnceState) {
// same as above, just with a different parameter to `call_inner`.
if self.state.load(Ordering::SeqCst) == COMPLETE {
// An `Acquire` load is enough because that makes all the initialization
// operations visible to us. The cold path uses SeqCst consistently
// because the performance difference really does not matter there,
// and SeqCst minimizes the chances of something going wrong.
if self.state.load(Ordering::Acquire) == COMPLETE {
return
}