Unknown symbol find_task_by_pid_ns

You will come across this error

Unknown symbol find_task_by_pid_ns

if you are trying to write a module using the function find_task_by_pid_ns and your kernel version is above 2.6.30.
This is because in kernels after 2.6.30 this symbol is no longer exported and instead of this the function pid_task needs to be used.

Ref: http://kerneltrap.org/mailarchive/git-commits-head/2009/6/18/6028583

Here is an example module for using the pid_task to find out a process from its pid. "Module to find a task from its pid"