dropIdentsOlderThan() acquires _dropMutex and then a GlobalLock, while every caller to immediatelyCompletePendingDrop() acquires them in the opposite order. This typically works because GlobalLock(MODE_IX) permits multiple writers at a time and we normally can't have enough callers to immediatelyCompletePendingDrop() to exhaust all write tickets, but we've observed it happen.