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Re: [libvirt] seg fault when running snapshot-create



On Thu, Apr 22, 2010 at 08:54:30AM -0400, Chris Lalancette wrote:
> On 04/22/2010 08:34 AM, Daniel P. Berrange wrote:
> > On Wed, Apr 21, 2010 at 05:16:21PM -0400, Chris Lalancette wrote:
> >> On 04/21/2010 04:34 PM, Stephen Shaw wrote:
> >>> I'm getting a seg fault when running virsh snapshot-create 1, but only
> >>> when virt-manager is open and connected.
> >>>
> >>> Here is some of the debug info I was able to come up with -
> >>> http://fpaste.org/9GO6/ (bt)
> >>> http://fpaste.org/7gkH/ ('thread apply all bt)
> >>>
> >>> * After the crash
> >>> (gdb) p mon->msg
> >>> $1 = (qemuMonitorMessagePtr) 0x0
> >>>
> >>>
> >>> nibbler:~ # libvirtd --version
> >>> libvirtd (libvirt) 0.8.0
> >>>
> >>>
> >>> Please let me know if there is any other information you need.
> >>> Stephen
> >>
> >> Thanks for the report.  To be perfectly honest, I can't see how what
> >> happened could happen :).  But I'll take a closer look at it and see
> >> if I can reproduce and see what is going on with it.
> > 
> > I see thread locking problems in the code
> > 
> >  - qemuDomainSnapshotCreateXML() is calling monitor commands, but has
> >    not run  qemuDomainObjBeginJobWithDriver() to ensure exclusive
> >    access to the monitor
> > 
> >  - qemuDomainSnapshotDiscard has same problem
> 
> Yep, just fixing those now.  I didn't quite understand the ObjBeginJob
> before, but I think I'm understanding it now.  This is probably the source of
> the problems.

There's some notes about the rules in src/qemu/THREADS.txt.

You must acquire locks on objects in the following order, not missing
any steps.

   qemud_driver             (qemudDriverLock)
   virDomainObjPtr          (implicit via virDomainObjFindByXXX)
   qemuMonitorPrivatePtr    (qemuDomainObjBeginJob)
   qemuMonitorPtr           (implicit via qemuDomainObjEnterMonitor)


Note that qemuDomainObjEnterMonitor() will release the locks on the
qemud_driver & virDomainObjPtr objects, once it has acquired the lock
on qemuMonitorPtr. The qemuMonitorPrivatePtr object has a condition 
variable that ensures continued mutual exclusion, even though the 
qemud_driver, virDomainObjPtr object are now unlocked.

So by missing qemuDomainObjBeginJob(), the condition variable was not
acquired, and mutual exclusion was not assured

Regards,
Daniel
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