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

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.