Re: [Qemu-devel] [PATCH V2] Add -f option to qemu-nbd

[Paolo Bonzini]

On 11/18/2011 02:25 AM, Chun Yan Liu wrote:
> Yes. I have tested using same device twice as described in my previous
> mail. Without lock:
>
> If issuing "qemu-nbd -c /dev/nbd0 disk.img" and "qemu-nbd -c  /dev/nbd0
> disk1.img" almost at the same time, both can pass nbd_init() and get to
> nbd_client(), then the latter one will fail and exit, but the first one
> does not work well either (fail to show partitions.) That's why I think
> we should add a lock in an earlier time.

This is an initialization problem.  As Stefan wrote, functionality for 
atomic acquisition of NBD devices is provided by the kernel; the problem 
is simply that QEMU does not use it. :)

Paolo