NFS is hung but hung_task timer is not firing

Issue

• I am using NFS and I know NFS is hung because processes trying to do operations on NFS files are blocked indefinitely. I thought this would cause the hung_task timer to fire, but it does not. Why not?
• I did a 'dd' write to an NFS file, and it is blocked indefinitely. I've configured the hung_task timer per https://access.redhat.com/site/solutions/60572 but it is not firing.