From: Paul Clements <Paul.Clements@SteelEye.com>

Here's the updated patch to fix several race conditions in nbd. It
requires reverting the already included (but incomplete)
nbd-race-fix.patch that's in -mm5.

This patch fixes the following race conditions:

1) adds an increment of req->ref_count to eliminate races between
do_nbd_request and nbd_end_request, which resulted in the freeing of
in-use requests -- there were races between send/receive, send/shutdown
(killall -9 nbd-client), and send/disconnect (nbd-client -d), which are
now all fixed

2) adds locking and properly orders the code in NBD_CLEAR_SOCK to
eliminate races with other code

3) adds an lo->sock check to nbd_clear_que to eliminate races between
do_nbd_request and nbd_clear_que, which resulted in the dequeuing of
active requests

4) adds an lo->sock check to NBD_DO_IT to eliminate races with
NBD_CLEAR_SOCK, which caused an Oops when "nbd-client -d" was called