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.
Daniel
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