Comment by tedd4u

They don't quite spell it out fully but it seems like a good guess that they use a datatype (like a 64-bit word) for the version number that can be read and written atomically, so no synchronization is needed at all and it can be very fast. So it's only if you don't have the latest version that you do a slower fetch of the updated value (that you can cache) with a mutex to make sure you don't get a partial update.