Questions about the FIFO of simple protocol

[JK-Jiagn]

Hello, I have some questions about the FIFO of simple protocol.

1. When the sender finishes writing the data, what is the memory barrier function waiting for? Is it just waiting for the update of the tail?
2. Similarly, after the receiver finishes reading the data, will the memory barrier be called?
3. Is the tail updated by the receiver? Similarly, is the head updated by the sender? I have this question because I saw that the tail and head are local for recv and send respectively here.

Thank you very much.