Re: [libvirt] Making the Thread-safety issues with vbox driver ?

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On Thu, Apr 19, 2012 at 12:51:10PM +0200, Jean-Baptiste Rouault wrote:
> On Wednesday 04 April 2012 17:56:12 Jean-Baptiste Rouault wrote:
> > On Monday 16 January 2012 11:34:53 Matthias Bolte wrote:
> > > Okay, without looking deeper into this here are some ideas:
> > > 
> > > The XPCOM API libvirt uses might not be threadsafe, or needs to be
> > > initialized for every thread that wants to use it. Currently its only
> > > initialized for the thread that opens the driver. I know that this is
> > > the case on Windows were VirtualBox uses MSCOM for its API and you
> > > need to call CoInitialize on every thread. This is currently not done
> > > for the MSCOM glue in libvirt, so I know that on Windows the
> > > VirtualBox driver is not threadsafe currently. Also I didn't look into
> > > a solution for this yet. Maybe we need a thread local variable that
> > > holds whether (MS/XP)COM was already initialized for this thread and
> > > add a check to every driver function to initialize it when needed.
> > > 
> > > Did you try to open a connection for each thread instead of trying to
> > > share one? If that works reliable it might indicate that there is an
> > > VirtualBox API initialization problem.
> > 
> > I tried today with one connection for each thread and it works.
> > 
> > I changed the vbox driver so that the pfnComInitialize function is called
> > only when the first connection is opened : it breaks the test, even with
> > one connection per thread.
> > 
> > I guess we'll have to use a thread local variable as you suggested, unless
> > someone has a better idea to handle this problem.
> 
> Hi,
> 
> I looked deeper into these thread-safety issues, once a new connection is 
> opened for each thread, everything works well.
> However, opening and closing connections isn't thread-safe at all for two 
> reasons :
> 
> - VirtualBox C bindings initialization and uninitialization functions aren't 
> thread-safe. I talked about it with upstream on IRC and they are probably not 
> going to fix it, but would accept a patch fixing the issue. I'm going to contact 
> upstream again to get some advices so I can write a patch.
> 
> - In the libvirt vbox driver, for each new connection, modification of the 
> global variable g_pVBoxGlobalData isn't protected (see line 1040 of 
> vbox_tmpl.c). 
> 
> First of all, is it really necessary to replace it on each new connection, or 
> would it be ok to initialize it only when the first connection is opened ?
> 
> A global mutex is needed in the vbox driver to protect access to 
> g_pVBoxGlobalData, the vboxRegister() function seems to be the best place to 
> initialize such a mutex unless there is another entry point to do this ?

I can't comment on where the best place to put hte lock is, but I agree
that we should make sure we do all the locking in libvirt. Not only because
upstream isnt likely to make it threadsafe soon, but also because it will
ensure we have compatibilty & safety with all existing/previous releases
or VirtualBox

Daniel
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