What is the correct steps if tar process is killed during pg_start_backup() and pg_stop_backup()

I have goolged this issued for some time, but I do not find much valuable information.

Here is my questions, I would like to know when I invoke pg_start_backup() and execute tar for backup my cluster data, but due to some reasons, tar is stopped. Q1) Should I invoke pg_stop_backup() to complete the procedure if I do not want to re-tar again? Q2) What if I decide to re-tar again my data, should I need to invoke pg_stop_backup() first and perform pg_start_backup() -> tar -> pg_stop_backup()? 

Yours sincerely,

Z.H Shin