Calling a Update task function module in a User-exit

I HOPE THIS IS WRITING DISCUSSION TO WRITE THIS IN!!! IF NOT, PLEASE LET ME KNOW THE RIGHT ONE.

I want to call a Update task function module(FM) in a User-exit ZXPADU02 for Infty 168. This FM is supposed to be called only on a commit work, and the commit work is reached but this FM is never executed. I tried writing "In background task" but its never executed. Please advise.

Thanks.

Kshitij.