How to avoid going back before the completion of the green card process due to expiration of visitor visa?

I am trying to apply for the green card for my parents as parents of a citizen. My parents are in US in visitor visa and it expires in four months. I believe it takes 5-6 months to process for the green card. I wanted to ask do they have to go back if their visa expires before the process is completed? Is there a way to avoid that? I have heard we can extend the visa but it is not reliable? Is there any other way? Please help!