如何配置nginx + Unicorn以避免超时错误？

I have a Rails (v3.2.13, Ruby 2.0.0) application running on nginx + Unicorn (Ubuntu 12.04). All is working well, except when an admin user is uploading users (thousands) via a CVS file. The problem is that I have set timeout to 30 seconds and the import process takes much more time. So, after 30 seconds I get an nginx 502 Bad Gateway page (Unicorn worker is killed).I have a Rails (v3.2.13, Ruby 2.0.0) applicatio