Re: [Qemu-devel] [PATCH 1/5] block: added lock image option and callback

[Fam Zheng]

On Tue, 01/12 13:24, Daniel P. Berrange wrote:
> On Tue, Jan 12, 2016 at 09:17:51PM +0800, Fam Zheng wrote:
> > On Tue, 01/12 13:28, Kevin Wolf wrote:
> > > Am 12.01.2016 um 12:33 hat Fam Zheng geschrieben:
> > > > On Tue, 01/12 11:10, Kevin Wolf wrote:
> > > > > 
> > > > > The problem is that libvirt already takes a lock, as Dan mentioned in
> > > > > another reply in this thread, so we can't enable locking in qemu by
> > > > > default. It would always fail when run under libvirt.
> > > > > 
> > > > > Unless I'm seriously mistaken, this means that flock() inside qemu is
> > > > > dead.
> > > > 
> > > > Yes, I see the problem with libvirt, but can we instead do these?
> > > > 
> > > >   1) Do a soft flock() in QEMU invocation. If it fails, sliently ignore.
> > > >   2) Do a hard flock() in qemu-img invocation. If it fails, report and 
> > > > exit.
> > > > 
> > > > This way, if libvirt is holding flock, we can assume libvirt is actually
> > > > "using" the image: 1) just works as before, but 2) will not break the 
> > > > qcow2.
> > > > That is still a slight improvement, and does solve the reckless 
> > > > "qemu-img
> > > > snapshot create" user's problem.
> > > 
> > > This makes two assumptions:
> > > 
> > > 1. qemu is only ever invoked by libvirt
> > > 2. qemu-img is only ever invoked by human users
> > > 
> > > Both of them are wrong. 1. just means that manually started QEMUs are
> > > unprotected (which is already bad), but 2. means that qemu-img called by
> > > libvirt fails (which is obviously not acceptable).
> > 
> > No, my assumptions are:
> > 
> > a. libvirt calls flock() when invoking qemu;
> > b. libvirt doesn't call flock() when invoking qemu-img; (if I read the 
> > libvirt
> >    code correctly, input from libvirt folks needed);
> 
> b. is /currently/ true, but I wouldn't guarantee that will always be
> true, because we've (vague) plans to extend our locking infrastructure
> to cover our storage pools APIs too, at which point we'd likely be
> have locking around qemu-img based API calls too. There's also likelihood
> we'll make our locking API public, in which case it is possible that
> an app using libvirt could have acquired locks on the file.
> 

This is not a problem. When you extend that in libvirt, you can at meanwhile
modify it to always specify "nolock=on" when invoking the new qemu-img so that
it doesn't check flock().

Fam