diff options
| -rw-r--r-- | src/liballoc/sync.rs | 13 |
1 files changed, 7 insertions, 6 deletions
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs index 2abd9c85c57..4244b09b18f 100644 --- a/src/liballoc/sync.rs +++ b/src/liballoc/sync.rs @@ -886,13 +886,14 @@ impl<T: ?Sized> Arc<T> { // holder. // // The acquire label here ensures a happens-before relationship with any - // writes to `strong` prior to decrements of the `weak` count (via drop, - // which uses Release). + // writes to `strong` (in particular in `Weak::upgrade`) prior to decrements + // of the `weak` count (via `Weak::drop`, which uses release). If the upgraded + // weak ref was never dropped, the CAS here will fail so we do not care to synchronize. if self.inner().weak.compare_exchange(1, usize::MAX, Acquire, Relaxed).is_ok() { - // Due to the previous acquire read, this will observe any writes to - // `strong` that were due to upgrading weak pointers; only strong - // clones remain, which require that the strong count is > 1 anyway. - let unique = self.inner().strong.load(Relaxed) == 1; + // This needs to be an `Acquire` to synchronize with the decrement of the `strong` + // counter in `drop` -- the only access that happens when any but the last reference + // is being dropped. + let unique = self.inner().strong.load(Acquire) == 1; // The release write here synchronizes with a read in `downgrade`, // effectively preventing the above read of `strong` from happening |