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Re: [libvirt] [PATCH 08/15] Locking of the qemuMonitorPtr object



On Wed, Nov 04, 2009 at 05:51:36PM +0100, Daniel Veillard wrote:
> On Tue, Nov 03, 2009 at 02:50:02PM -0500, Daniel P. Berrange wrote:
> > In preparation of the monitor I/O process becoming fully asynchronous,
> > it is neccessary to ensure all access to internals of the qemuMonitorPtr
> > object is protected by a mutex lock.
> > 
> > * src/qemu/qemu_monitor.c, src/qemu/qemu_monitor.h: Add mutex for locking
> >   monitor.
> > * src/qemu/qemu_driver.c: Add locking around all monitor commands
> > ---
> >  src/qemu/qemu_driver.c  |  299 +++++++++++++++++++++++++++++++++++------------
> >  src/qemu/qemu_monitor.c |   19 +++
> >  src/qemu/qemu_monitor.h |    3 +
> >  3 files changed, 248 insertions(+), 73 deletions(-)
> 
> [..]
> > +++ b/src/qemu/qemu_driver.c
> > @@ -141,6 +141,22 @@ static void qemuDomainObjPrivateFree(void *data)
> >  }
> >  
> >  
> > +static void qemuDomainObjEnterMonitor(virDomainObjPtr obj)
> > +{
> > +    qemuDomainObjPrivatePtr priv = obj->privateData;
> > +
> > +    qemuMonitorLock(priv->mon);
> > +}
> > +
> > +
> > +static void qemuDomainObjExitMonitor(virDomainObjPtr obj)
> > +{
> > +    qemuDomainObjPrivatePtr priv = obj->privateData;
> > +
> > +    qemuMonitorUnlock(priv->mon);
> > +}
> > +
> 
>   so we're using pthread mutex here and basically there is no way this
> could go wrong and there is no need to handle errors, okay
> 
> >  static int qemuCgroupControllerActive(struct qemud_driver *driver,
> >                                        int controller)
> >  {
> 
>   the patch is quite regular
> 
> > +        qemuDomainObjEnterMonitor(vm);
> > +        if (qemuMonitorStopCPUs(priv->mon) < 0) {
> > +            qemuDomainObjExitMonitor(vm);
> >              goto cleanup;
> > +        }
> > +        qemuDomainObjExitMonitor(vm);
> 
>  to some extend I wonder if we shouldn't kill all those blocks and
> always use the more regular form
> 
>            qemuDomainObjEnterMonitor(vm);
>            ret = qemuMonitorStopCPUs(priv->mon);
>            qemuDomainObjExitMonitor(vm);
>            if (ret < 0)
>                goto cleanup;
> 
> it's simpler, easier to read, we don't have to think about matching
> of Enter and Exit, and the generated code probably will be identical
> In a number of places the refactoring introduced the later way but
> the former is still present in a number of place, closer to original
> code but doesn't make the patch safer I'm afraid. The only inconvenience
> is the declaration of the temporary variable ...

Yes, I think I'll do a further add-on patch to make that change.

Daniel
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