[libvirt] [PATCH 1/2] virObjectUnref: Set pointer to NULL on dispose

[Daniel P. Berrange]

On Fri, Nov 08, 2013 at 08:54:39AM +0100, Michal Privoznik wrote:
> On 08.11.2013 06:27, Daniel P. Berrange wrote:
> > On Thu, Nov 07, 2013 at 11:39:27AM +0100, Michal Privoznik wrote:
> >> Similarly to VIR_FREE() we can set the pointer passed to virObjectUnref
> >> to NULL in case of disposing the object. However, to avoid overwriting
> >> nearly thousands line of code, the virObjectUnref is turned into a macro
> >> which passes the address of pointer and calls virObjectUnrefInternal
> >> (the modified version of original virObjectUnref).
> > 
> > I have to say I'm not really liking this, and your impl is not race
> > free since you're not atomically updating the point.
> > 
> > Daniel
> > 
> 
> 
> I don't think I follow you there. AFAIU, the whole 'if' body is executed
> exactly once iff obj->refs is zero after decrement. And I don't see how
> can I possibly race with others.
> 
> If two threads calls virObjectUnref on the very same object with
> refcount = 1, do you expect them both to have the *ptr = NULL?

The first thread decrements refcount, so it hits zero. Now a short time
later it sets *obj = NULL, but at the same time another thread is running
virObjectUnref(). It will check *obj != NULL, which races with setting
*obj = NULL.

Daniel
-- 
|: http://berrange.com      -o-    http://www.flickr.com/photos/dberrange/ :|
|: http://libvirt.org              -o-             http://virt-manager.org :|
|: http://autobuild.org       -o-         http://search.cpan.org/~danberr/ :|
|: http://entangle-photo.org       -o-       http://live.gnome.org/gtk-vnc :|