Probably, `std::memory_order_acquire` was intended [here](https://github.com/facebook/folly/blob/b028517fc58f3cec70905f05504e39e270a4457b/folly/synchronization/MicroSpinLock.h#L103), in `folly/folly/synchronization/MicroSpinLock.h`, as `xchg` is only used when obtaining the lock.
