Please review and comment...

This is the sixth patch to the NBD driver and it fixes a variety of 
outstanding locking issues with the ioctl user interface. This patch 
modifies both drivers/block/nbd.c and include/linux/nbd.h. It's intended 
to be applied incrementally (on top of the fifth patch).

Of major note is that since the linux 2.5 change in the block layer 
code, the distributions of the nbd-client user tool that I am familiar 
with (including the one in my anbd distribution up through release 0.4) 
will already be unable to correctly set the size of the served block 
device. This happens because nbd-client opens the nbd, calls the set 
size ioctl on its descriptor and eventually calls the do_it ioctl with 
that same still opened descriptor. The set size ioctl essentially just 
calls set_capacity() on the relevant gendisk structure. In the new block 
layer code however, the nbd descriptor has to be released after the set 
size ioctl call and then re-opened in order for the correct size to be 
registered by other openers. This could also be worked around in the 
driver but it has not been done (yet).

That said, this seems like an opportunistic time to further break 
compatibility with the existing nbd-client user tool and  do away with 
the problematic components of the ioctl user interface. This patch 
deprecates two ioctl's (NBD_SET_SOCK and NBD_CLEAR_SOCK) and changes the 
usage of the NBD_DO_IT ioctl to take the socket descriptor as its 
argument. As such, NBD_DO_IT rolls all the functionality of the three 
ioctls into just one. It simplifies the user interface and makes 
resource locking much easier. Consequently, several race conditions can 
be cleaned up in the driver itself (rather than via work arounds in 
nbd-client).

There are other ways of course... the new NBD_DO_IT style interface 
could be introduced instead as another ioctl completely and these 3 
ioctls could be maintained for backward compatibility for a while 
longer. That seems contradictory though to the idea of keeping the 
newest code the cleanest but at least it doesn't break existing 
nbd-client tools.

So look at the patch if you're interested and let me know what you 
think. Please don't make it part of the kernel distro though, at least 
not yet. I'll be "disconnected" from Thursday through Sunday so I won't 
be able to do any fixes should this introduce more problems than it 
fixes till next week.