Ubuntu :: Several Scripts Simultaneously Accessing One File

I made a script for batch processing brain scans (with FreeSurfer). I have a batch file stored on an NFS server that looks something like this:

Code:

subject1
subject2
subject3
subject4

now my script is performed on several computers at a time, first it marks an item to be in use (with sed -i 's/.../...'), then processes it (it gets the actual subject files from elsewhere on the server, then puts them back), then marks it to be finished in the list. so during action the batch file should look something like this:

Code:

subject1 finished
subject2 inprogress on host1
subject3 inprogress on host2
subject4

so i tried a run and watched the batch file, now i observed something like this:

Code:

subject1 finished
subject2 inprogress on host1
subject3 inprogress on host2

[code]....

there was only one script running on host1. the subjects never got marked as finished, although i know for a fact that they were finished. also, the script sometimes seemed to hang for moments.