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 :|