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Auto merge of #114591 - joboet:thread_parking_ordering_fix, r=thomcc

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Synchronize with all calls to `unpark` in id-based thread parker

[The documentation for `thread::park`](https://doc.rust-lang.org/nightly/std/thread/fn.park.html#memory-ordering) guarantees that "park synchronizes-with all prior unpark operations". In the id-based thread parking implementation, this is not implemented correctly, as the state variable is reset with a simple store, so there will not be a *synchronizes-with* edge if an `unpark` happens just before the reset. This PR corrects this, replacing the load-check-reset sequence with a single `compare_exchange`.
This commit is contained in:
bors 2023-08-18 19:03:38 +00:00
commit d06ca0ffaf
1 changed files with 6 additions and 11 deletions
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17
library/std/src/sys_common/thread_parking/id.rs
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@ -56,18 +56,14 @@ impl Parker {
self.init_tid(); self.init_tid();
// Changes NOTIFIED to EMPTY and EMPTY to PARKED. // Changes NOTIFIED to EMPTY and EMPTY to PARKED.
let mut state = self.state.fetch_sub(1, Acquire).wrapping_sub(1); let state = self.state.fetch_sub(1, Acquire);
if state == PARKED { if state == EMPTY {
// Loop to guard against spurious wakeups. // Loop to guard against spurious wakeups.
while state == PARKED { // The state must be reset with acquire ordering to ensure that all
// calls to `unpark` synchronize with this thread.
while self.state.compare_exchange(NOTIFIED, EMPTY, Acquire, Relaxed).is_err() {
park(self.state.as_ptr().addr()); park(self.state.as_ptr().addr());
state = self.state.load(Acquire);
} }
// Since the state change has already been observed with acquire
// ordering, the state can be reset with a relaxed store instead
// of a swap.
self.state.store(EMPTY, Relaxed);
} }
} }
@ -78,8 +74,7 @@ impl Parker {
if state == PARKED { if state == PARKED {
park_timeout(dur, self.state.as_ptr().addr()); park_timeout(dur, self.state.as_ptr().addr());
// Swap to ensure that we observe all state changes with acquire // Swap to ensure that we observe all state changes with acquire
// ordering, even if the state has been changed after the timeout // ordering.
// occurred.
self.state.swap(EMPTY, Acquire); self.state.swap(EMPTY, Acquire);
} }
} }