libvirt users - Apr 2012 - Is this Race Condition?

Hi, everyone

    I'm  writing application using libvirt and I need call
virDomainGetCPUStats to get domain cpu stats . I learn from API
reference that this function has to be called when domain is running.
So my question is if there is an atomic way to test the domain's state
and get its stats?

   Or, it's ok to call this function when domain is not active, but
caller just gets error returned.

   In my code, I just call it directly and return error. I wonder if
this programming practice has subtle problem like race condition.

On Thu, Apr 05, 2012 at 06:06:43PM +0800, Zhihua Che
wrote:
> Hi, everyone
> 
>     I'm  writing application using libvirt and I need call
> virDomainGetCPUStats to get domain cpu stats . I learn from API
> reference that this function has to be called when domain is running.
> So my question is if there is an atomic way to test the domain's state
> and get its stats?
> 
>    Or, it's ok to call this function when domain is not active, but
> caller just gets error returned.
> 
>    In my code, I just call it directly and return error. I wonder if
> this programming practice has subtle problem like race condition.
AFAIK there is no way to do this without a race, not least because a
VM can power itself off / crash at literally any time.  In virt-top,
we just catch the error and ignore it.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming blog: http://rwmj.wordpress.com
Fedora now supports 80 OCaml packages (the OPEN alternative to F#)
http://cocan.org/getting_started_with_ocaml_on_red_hat_and_fedora

On Thu, Apr 05, 2012 at 06:06:43PM +0800, Zhihua Che
wrote:
> Hi, everyone
> 
>     I'm  writing application using libvirt and I need call
> virDomainGetCPUStats to get domain cpu stats . I learn from API
> reference that this function has to be called when domain is running.
> So my question is if there is an atomic way to test the domain's state
> and get its stats?
> 
>    Or, it's ok to call this function when domain is not active, but
> caller just gets error returned.
> 
>    In my code, I just call it directly and return error. I wonder if
> this programming practice has subtle problem like race condition.
For a function which is only callable when the domain is running, libvirt
will return a special error code  VIR_ERR_OPERATION_INVALID  if you invoke
it when the domain is shutoff.

Thus the race-free way to deal with this scenario is to just call the
API, and check for VIR_ERR_OPERATION_INVALID. Any other error code is
an indication of a "real" error situaton.

Regards,
Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|