Hello, Trond!
Could you point to idea how userspace process can know about
lost lock on NFSv4 server (due to server reboot) ?
See "case -NFS4ERR_DENIED" at nfs4_reclaim_locks():
http://lxr.free-electrons.com/source/fs/nfs/nfs4state.c#L1184
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Thanks,
Vitaliy