From: Eric Dumazet <redacted>
Date: Wed, 27 Feb 2013 09:05:03 -0800

Processing pure ACK on behalf of the thread blocked in tcp_recvmsg()
is a waste of resources, as thread has to immediately sleep again
because it got no payload.

More than one thread can be operating on the socket, the other one
could be waiting for the window to open up in order to do a send.  Are
you absolutely sure that we won't have a problem in that situation?
In fact I wonder if that does the right thing right now.