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Re: [libvirt] [PATCH 03/14] Fix crash when deleting monitor while a command is in progress



On Thu, Dec 03, 2009 at 12:47:02AM +0100, Matthias Bolte wrote:
> 2009/11/26 Daniel P. Berrange <berrange redhat com>:
> > If QEMU shuts down while we're in the middle of processing a
> > monitor command, the monitor will be freed, and upon cleaning
> > up we attempt to do  qemuMonitorUnlock(priv->mon) when priv->mon
> > is NULL.
> >
> > To address this we introduce proper reference counting into
> > the qemuMonitorPtr object, and hold an extra reference whenever
> > executing a command.
> >
> > * src/qemu/qemu_driver.c: Hold a reference on the monitor while
> >  executing commands, and only NULL-ify the priv->mon field when
> >  the last reference is released
> > * src/qemu/qemu_monitor.h, src/qemu/qemu_monitor.c: Add reference
> >  counting to handle safe deletion of monitor objects
> 
> The locking pattern below results in destroying a locked mutex. It
> this intended?

No, that's a bug, the qemuMonitorUnref() call itself should have
called unlock if the ref count hit 0, before destroying it.

> qemuMonitorLock(mon);
> [...]
> if (qemuMonitorUnref(mon) > 0)
>     qemuMonitorUnlock(mon);
> 
> Well, this patch makes the TCK deadlock for me, seems to be a lock
> ordering issue combined with a race condition; it doesn't happen every
> run. I don't understand all details of the locking and refcounting
> scheme of the QEMU monitor yet, it's quite complex and gets even more
> complex.

Yes, that problem shown by valgrind will indeed deadlock. I'll fix
this. The domain object lock must never be acquired if the thread
holds the monitor lock already. We must have strict ordering from
top to bottom (driver -> domain object -> qemu monitor)


Daniel
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