Comment 0 for bug 1689777

System:
Description: Ubuntu 14.04.5 LTS
Release: 14.04
Version signature: Ubuntu 4.4.0-75.96~14.04.1-generic 4.4.59

Using NFS-common and NFS-kernel-server version 1:1.2.8-6ubuntu1.2 before kernel update and after.

After an update from linux-image-generic-lts-xenial:amd64 4.4.0.64.50 to version 4.4.0.75.62, our NFS server refuses any exclusive file locks after every night. Symptoms are that NFS clients cannot execute programs that use exclusive file locks (e.g. cradle), if using a UI like Ubuntu Desktop, the whole UI is stuck. However files can still be accessed and created via shell on a client through nfs.
Following log is produced on a client that mounts nfs:

May 10 00:46:23 irminen kernel: [2972685.514114] lockd: server pranger.uni-trier.de OK
May 10 00:47:24 irminen kernel: [2972746.611297] lockd: server pranger.uni-trier.de not responding, still trying

The nfs server produces following log:

May 8 22:30:22 pranger kernel: [6267842.076551] Btrfs loaded
May 9 04:16:02 pranger kernel: [6288582.711415] lockd: cannot monitor client
May 9 04:17:02 pranger kernel: [6288642.871345] lockd: cannot monitor client
May 9 04:18:03 pranger kernel: [6288703.031317] lockd: cannot monitor client
May 9 04:19:03 pranger kernel: [6288763.191282] lockd: cannot monitor client
May 9 04:20:03 pranger kernel: [6288823.351247] lockd: cannot monitor client
May 9 04:21:03 pranger kernel: [6288883.511292] lockd: cannot monitor client
May 9 04:22:03 pranger kernel: [6288943.671215] lockd: cannot monitor client
May 9 04:23:03 pranger kernel: [6289003.831189] lockd: cannot monitor client

Restarting the NFS Server does not resolve the issue, I had to reboot the server completely. After one day running the same problem appears again.