I guess most locking is in place in order to allow multiplexing - different applications need access to different sockets, which however send and receive data f

I guess most locking is in place in order to allow multiplexing - different applications need access to different sockets, which however send and receive data from the same NIC. If you implement the whole stack in userspace and have only a single thread which processes all data you might get away with less locking. However as soon as there are multiple threads which want to send/receive on multiple endpoints there is the same need for synchronization and it would need to be implemented in userspace.