On Thu, Nov 17, 2011 at 11:34 AM, Chun Yan Liu <cy...@suse.com> wrote: > Thanks for your suggestions. > > For the usage "qemu-nbd -f disk.img", adding some code could implement it. I > think it could be like "losetup -f" usage. > > #qemu-nbd -f > > show the first free nbd device at this moment. > > user can choose to issue "qemu-nbd -c THAT_DEVICE disk.img" or not. > > #qemu-nbd -f disk.img > > find a free nbd device and connect disk.img to that device. > > How do you think? > > For the race conditions caused by executing multiple qemu-nbd -f at the same > time, I've tried both ways (1. lock; 2. if one device not work, trying other > devices until one works). > > In my testing, the 2nd way has problem. When issuing "qemu-nbd -c /dev/nbd0 > disk.img -v" and "qemu-nbd -c /dev/nbd0 disk1.img -v" at the same time, the > latter one will eventually exit with EXIT_FAILURE, but the first one cannot > work normally as well, it cannot show disk partitions. Executing multiple > "qemu-nbd -f" has same problem.
The problem is that qemu/nbd.c is doing it wrong. The linux/drivers/block/nbd.c driver returns -EBUSY from ioctl(NBD_SET_SOCK) if the device already has a socket configured. However, qemu/nbd.c issues NBD_SET_BLKSIZE and other settings ioctls *before* NBD_SET_SOCK. This clobbers the existing nbd connection settings. Then we take over the device using NBD_CLEAR_SOCK and NBD_SET_SOCK instead of noticing there is already a socket configured. I think qemu/nbd.c:nbd_init() should be fixed to: 1. NBD_SET_SOCK. This fails with -EBUSY if an existing socket is configured. This is the atomic acquire/test operation. 2. NBD_SET_BLKSIZE and other settings ioctls. The only user-visible change is that qemu-nbd -c /dev/nbd0 no longer hijacks the nbd device. Instead it properly detects -EBUSY. This seems like a reasonable change to make although we could restrict it to only qemu-nbd -f disk.img in order to preserve the current behavior for qemu-nbd -c /dev/nbd0. I don't think using file locks is necessary since it can be done properly through the nbd ioctl interface. Stefan
