[libvirt] [PATCH] [PATCH] rpc: fix keep alive timer segfault

[Daniel P. Berrange]

On Mon, Apr 24, 2017 at 11:15:31AM +0200, Michal Privoznik wrote:
> On 04/24/2017 05:31 AM, wang.yi59 at zte.com.cn wrote:
> > Hi Michal,
> > 
> > 
> > 
> > Thanks for your review. The problem occured in a python applicatin using libvirt-python, which has no ref().  If we unref() first in virKeepAliveTimer, we may get a segfault in virObjectUnlock() when cleanup, so I suppose that my patch is safer, :-) Here is the backtrace:
> > 
> > 
> 
> Okay, now it makes more sense. However, the ref() I was referring to is our
> libvirt virObjectRef() not the one in python. The code where segfault occurs
> is too deep for python to see anyway. One thing that I am still worried
> about is doing virObjectRef() before virObjectLock(). I think these two
> steps should be swapped.

That ordering should not matter. At the time this code runs, we *must*
be guaranteed that we already hold a valid ref on the object. If we
did not have that existing ref, then both virObjectRef and virObjectLock
could segv. So the 'virObjectRef' must be acquiring a second reference,
and thus doing lock first would not give us any further protection

Regards,
Daniel
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