samba - Nov 2013 - keepalive not working?

I am running sernet samba 4.0.10-7 on a network with XP and W7 PCs.

As I saw that many smb connections remain open, especially from W7 clients,
I added this to smb.conf:

         socket options = TCP_NODELAY SO_KEEPALIVE

However, it appears to be not working.
Over 2 hours after clients have logged off, smbstatus still shows the
connections
with locked files and all.

When I kill the associated smbd pid, of course the connection goes away.
However, I expected that SO_KEEPALIVE would fix this.
Why not?

Rob