umount : fs busy

[PAUL SLOOTMAN]

When I want to umount a filesystem, I sometimes (entirely correctly)
get the message "umount failed: file system busy".
 
My question is: how can I find out who or what is keeping it busy?
Obviously it can be found, 'cos umount knows about it. The system
in question is SysV.
 
Thanks in advance for any answers.
 
 
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