Re: locking option doesn't work as expected

["Daniel P. Berrangé" <berrange@redhat.com>]

On Wed, Nov 04, 2020 at 11:35:56AM -0500, Masayoshi Mizuma wrote:
> Hello,
> 
> It seems that locking option doesn't work as expected.
> When I run qemu as following options, then I got an error and failed to
> boot the guest:
> 
>   qemu-system-x86_64 \
>     -machine pc \
>     -enable-kvm \
>     -cpu host \
>     -smp 1 -m 4G \
>     -nographic \
>     -serial telnet::1235,server,nowait \
>     -blockdev driver=qcow2,node-name=disk,file.driver=file,file.filename=/mnt/guest.qcow2,file.locking=auto \
>     -device virtio-blk-pci,scsi=off,drive=disk,id=virtio-disk0,bootindex=1
> 
>   qemu-system-x86_64: -blockdev driver=qcow2,node-name=disk,file.driver=file,file.filename=/mnt/guest.qcow2,file.locking=auto: Failed to lock byte 100
> 
> The error happens when the filesystem doesn't support OFD lock.
> qemu_probe_lock_ops() judges whether qemu can use OFD lock or not with doing
> fcntl(F_OFD_GETLK) to /dev/null, so the error happens if /dev/null supports OFD lock,
> but the filesystem doesn't support the lock.
> 
> I'm thinking how to fix the error. My idea is to add locking=posix option to use posix
> lock to the file, but I'm not sure the idea is good way to fix the error...
> 
> I would appreciate it if you could give me some advises to fix the error.

Ideally we would not attempt to probe it on /dev/null at all. Instead just
attempt to lock the actual file using F_OFD_SETLK and if that fails, then
falback to F_SETLK.  We can do similar when checking whether the lock
currently exists. AFAIK, the locking code isn't so performance critical
that the extra syscall would hurt us in the case where OFD doesn't exist
or isn't supported by the FS.

> 
> BTW, locking=off may be useful for the workaround so far, however, locking=off
> doesn't work on the splitting blockdev configs as followings...
> I split the blockdev option as libvirt doing.
> 
>    -blockdev driver=file,filename=/mnt/guest.qcow2,node-name=storage,auto-read-only=on,locking=off \
>    -blockdev node-name=format,read-only=off,driver=qcow2,file=storage \
> 
> Thanks,
> Masa
> 

Regards,
Daniel
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