[libvirt] seg fault when running snapshot-create
Daniel P. Berrange
berrange at redhat.com
Thu Apr 22 13:09:53 UTC 2010
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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