Re: [Xen-devel] Re: [RFC] "xs_read(): uuid get error" of qemu-dm

[Masami Watanabe <masami.watanabe@xxxxxxxxxxxxxx>]

Hi Ewan,

> I've just put a patch in that ought to help.  We can't reproduce this
> race here, but perhaps you could give it a try for me.

Special thanks for your patch.
Your patch splendidly solved "xs_read(): uuid get error" that occurred
on c/s 12307 and before that.

When is this patch committed ?

Masami



On Wed, 8 Nov 2006 18:35:37 +0000, Ewan Mellor wrote:
> On Wed, Nov 08, 2006 at 03:13:43PM +0900, Masami Watanabe wrote:
> 
> > Hi Keir,
> > 
> > My explanation was insufficient.
> > 
> > "xs_read(): uuid get error" happens when uuid can't read from xenstore
> > in xenstore_read_vncpasswd@tools/ioemu/xenstore.c.
> > 
> > c/s 12187 evaded this problem when the guest rebooted in a lot of
> > environments. As for my environment, the problem was corrected by this
> > correction.
> > 
> > However, Afterwards, following problem keeps happening.
> > I think that it is a problem.
> > 
> > [Xen-devel] VMX status report 12254:f8ffeb540ec1
> >   http://lists.xensource.com/archives/html/xen-devel/2006-11/msg00288.html
> > [Xen-devel] VMX status report 12217:20204db0891b
> >   http://lists.xensource.com/archives/html/xen-devel/2006-11/msg00183.html
> > >  IA32/PAE/IA32E: Windows and Linux VMX domains may fail to be
> > > created, the qemu-dm process is <defunct>, and the qemu log says
> > > "xs_read(): uuid get error."
> > 
> > 
> > I examined it.
> > As a result, In the environment that allocated two or more CPU in
> > Dom0, this problem was able to be caused.
> > The result of the confirmation is as follows.
> > - uuid cannot be read with xenstore_read_vncpasswd() in qemu-dm.
> > - uuid can often be read by changing the order of vm.initDomain()
> >   and vm.storeVmDetails() in create()@XendDomainInfo.py.
> > - And, when the read timing is delayed in qemu-dm, It was possible
> >   to always read.
> > 
> > >From the above, I thought that this problem was a problem of the timing
> > of writing and reading to xenstore from another process.
> > 
> > 
> > > Is the passwd getting written after qemu
> > > is started and hence racing the xs_read() in xenstored?
> > 
> > Yes, maybe. I understand the order of processing xend as follows.
> > Is it my misunderstanding ?
> > 
> >   create()@XendDomainInfo.py+135
> >       start()
> >           _initDomain()
> >               _createDevices()
> >                   createDeviceModel(self)@image.py
> >                       os.spawnve()  ==============>  start qemu-dm process
> >           _storeVmDetails()
> >               _writeVm()            ==============>  write to xenstore
> >               _setVmPermissions()
> 
> I've just put a patch in that ought to help.  We can't reproduce this race
> here, but perhaps you could give it a try for me.
> 
> diff -r 9a43cc89ae0a tools/python/xen/xend/XendDomainInfo.py
> --- a/tools/python/xen/xend/XendDomainInfo.py   Wed Nov 08 18:27:31 2006 +0000
> +++ b/tools/python/xen/xend/XendDomainInfo.py   Wed Nov 08 18:08:28 2006 +0000
> @@ -678,6 +678,7 @@ class XendDomainInfo:
>          t.remove()
>          t.mkdir()
>          t.set_permissions({ 'dom' : self.domid })
> +        t.write('vm', self.vmpath)
> 
>      def _storeDomDetails(self):
>          to_store = {
> 
> 
> The /vm/<uuid>/vncpasswd node is written before the call to createDeviceModel,
> in configVNC, but you need the /local/domain/<domid>/vm node to be present
> too, and it's this one that isn't written until after qemu-dm is started.
> 
> HTH,
> 
> Ewan.


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