Re: [Nbd] How to find a free NBD device?

[Andreas Klauer <Andreas.Klauer@...1264...>]

On Wed, Mar 05, 2014 at 03:20:55PM -0500, Paul Clements wrote:
> Although I wonder how bad the current situation really is.

The losetup --find --show being racy kind of stumped me, I'm not sure if 
this issue is known at all, Google doesn't give me anything except this:

http://www.spinics.net/lists/util-linux-ng/msg00221.html

It seems the --show was added in the first place to avoid race, yet it 
suffers from one; (for i in {1..100}; do losetup -f -s $i & done) does 
not give you a hundred loop devices... instead it keeps reusing already 
used ones.

If nobody notices such an issue in years it's probably not relevant 
in practice...

> Are people having difficulties with races and such in trying to 
> allocate nbds?

I guess the race is a problem of a very theoretical nature only... 
two processes trying to find a nbd device at the same millisecond
is just extremely unlikely.

Finding a free nbd device still seems a bit of a challenge to me. 
Although that may be because my buggy program is leaving behind nbd 
devices in a bad state, so they're still listed in /proc/partitions even 
though there is no process using them anymore, and even nbd-client -d
does not get rid of them.

The /sys/block/nbd*/pid seems to be the safest way to check, and never 
mind the race conditions...

Regards
Andreas Klauer