[NLM] fcntl(F_SETLKW) yields -ENOLCK when grace period expires.

[Frank van Maarseveen <frankvm@frankvm.com>]

Both client- and server run 2.6.39.3, NFSv3 over UDP (without the
relock_filesystem patch proposed earlier).

A second client has an exclusive lock on a file on the server. The
client under test calls fcntl(F_SETLKW) to wait for the same exclusive
lock. Wireshark sees NLM V4 LOCK calls resulting in NLM_BLOCKED.

Next the server is rebooted. The second client recovers the lock
correctly. The client under test now receives NLM_DENIED_GRACE_PERIOD for
every NLM V4 LOCK request resulting from the waiting fcntl(F_SETLKW). When
this changes to NLM_BLOCKED after grace period expiration the fcntl
returns -ENOLCK ("No locks available.") instead of continuing to wait.

server:/proc/locks shows two entries for the file after the -ENOLCK. When
the second client gives up its lock because the program running there
is killed one entry in server:/proc/locks remains indefinately: as a
result no NFS client can lock the file anymore.

-- 
Frank