Serial Port Configuration for offboard control

I’m looking at the C / C++ example interface code here: Looking in the serial_port files, it’s setup so that the port cannot be read from and written to simultaneously (the writing thread and reading thread have to acquire a lock before doing their business).

Why is it done like that? Is it an inherent limitation? Searching on google has mixed answers - some say it’s possible, others say it’s not. It should be possible since UARTs are full-duplex right?

I’m also curious how this affects performance, particularly given that only a single byte is read at a time (i.e. having to acquire the lock before each and every byte is read).

Thanks in advance.