Comment 31 for bug 683938

What lies behind the comment on the umount call is this: the NFS protocol requests that clients tell servers when they unmount partitions, so that servers can stop reporting it in tools like showmount. The client doesn't care at all if the server ever gets it, and even if you do an unmount RPC, the server is not allowed to stop handling the filehandles, which are (officially) permanent.

So since the client doesn't care beans about the umount call, or even if it works or gets blackholed, the code in mount_clnt.c does minimal retransmits and short timeouts. When it says that it doesn't expect data back, that doesn't mean that the RPC has no return (the server does follow the usual sunrpc semantics and does send back a reply) but the client doesn't care what the return is in any way.