RHEL7: df and other commands hang on NFS client after NFS server reboot or loss of connection

Issue

NAS filer was rebooted after which the server cannot access the qtree and causing other simple commands to hang.
Like df, su , cd

On one server we were able to fix this situation by killing all processes accessing those nfs mounts and doing a umount -l , then stopping the database, rebooting the server. However, we would like to understand why the server cannot access the qtrees even after the filer is up. several other Linux and unix servers accessing the same qtrees/filer are able to access them after the filer came back up. Only having problems on these RHEL 7.2 servers.

NFS client connection recovery deadlocks with XPRT_CONNECTING set but no socket