Re: [Qemu-devel] Disable image locking for snapshot drive?

[Andrew Baumann]

> From: Fam Zheng [mailto:address@hidden
> Sent: Wednesday, 19 July 2017 23:53
> 
> On Tue, 07/18 16:19, Andrew Baumann wrote:
> > > From: Eric Blake [mailto:address@hidden
> > > Sent: Tuesday, 18 July 2017 8:07
> > > On 07/17/2017 07:33 PM, John Snow wrote:
> > > > On 07/17/2017 07:30 PM, Andrew Baumann via Qemu-devel wrote:
> > > >> I'm running a recent Linux build of qemu on Windows Subsystem for
> Linux
> > > (WSL) which doesn't appear to implement file locking:
> > > >>
> > > >> $ qemu-system-aarch64 ... -drive file=test.vhdx,if=none,id=hd0 -
> device
> > > virtio-blk-pci,drive=hd0
> > > >> qemu-system-aarch64: -drive file=test.vhdx,if=none,id=hd0: Failed to
> > > unlock byte 100
> > >
> > > Does WSL implement fcntl(F_SETLK) but not fcntl(F_OFD_SETLK)?
> >
> > Yes, this appears to be the case (there's also one report that it's broken):
> > https://github.com/Microsoft/BashOnWindows/issues/1927
> 
> What does fcntl(F_OFD_SETLK) return? If it is -ENOTSUP, we can probably
> detect
> that and disable locking. Can you try the patch pasted in the end?

It returns -EINVAL. Your patch works, if I change the error code. Maybe testing 
for either ENOTSUP or EINVAL, and only allowing the fallback if the user didn't 
force locking on, would be a reasonable compromise?

Cheers,
Andrew

> ---
> 
> diff --git a/block/file-posix.c b/block/file-posix.c
> index cfbb236f6f..0be5bbbd53 100644
> --- a/block/file-posix.c
> +++ b/block/file-posix.c
> @@ -493,6 +493,12 @@ static int raw_open_common(BlockDriverState *bs,
> QDict *options,
>      }
>      s->fd = fd;
> 
> +    if (s->use_lock) {
> +        int ret0 = qemu_unlock_fd(fd, 0, 0);
> +        if (ret0 == -ENOTSUP) {
> +            s->use_lock = false;
> +        }
> +    }
>      s->lock_fd = -1;
>      if (s->use_lock) {
>          fd = qemu_open(filename, s->open_flags);