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Add comments and assertion to park/unpark

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regarding the synchronization.
This commit is contained in:
James Duley 2018-09-14 17:32:16 +01:00
parent 204d9608e3
commit f8a78bdfdf
1 changed files with 12 additions and 2 deletions
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14
src/libstd/thread/mod.rs
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@ -800,7 +800,11 @@ pub fn park() {
match thread.inner.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
Ok(_) => {}
Err(NOTIFIED) => {
thread.inner.state.swap(EMPTY, SeqCst);
// We must read again here, even though we know it will be NOTIFY,
// to synchronize with an write in `unpark` that occurred since we
// last read.
let old = thread.inner.state.swap(EMPTY, SeqCst);
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
} // should consume this notification, so prohibit spurious wakeups in next park.
Err(_) => panic!("inconsistent park state"),
@ -889,7 +893,11 @@ pub fn park_timeout(dur: Duration) {
match thread.inner.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
Ok(_) => {}
Err(NOTIFIED) => {
thread.inner.state.swap(EMPTY, SeqCst);
// We must read again here, even though we know it will be NOTIFY,
// to synchronize with an write in `unpark` that occurred since we
// last read.
let old = thread.inner.state.swap(EMPTY, SeqCst);
assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
return;
} // should consume this notification, so prohibit spurious wakeups in next park.
Err(_) => panic!("inconsistent park_timeout state"),
@ -1058,6 +1066,8 @@ impl Thread {
/// [park]: fn.park.html
#[stable(feature = "rust1", since = "1.0.0")]
pub fn unpark(&self) {
// We must unconditionally write NOTIFIED here to
// synchronize with a read in `park`.
match self.inner.state.swap(NOTIFIED, SeqCst) {
EMPTY => return, // no one was waiting
NOTIFIED => return, // already unparked