RFR: 8215491: ICStubInterface::finalize finds zombie nmethod with ZGC concurrent class unloading

[Coleen Phillimore]

I think this looks good.  Then we don’t have to know who has already cleaned them or if they did it in the right order. 
 
Thanks
Coleen

> On Dec 18, 2018, at 4:30 AM, Erik Österlund <erik.osterlund at oracle.com> wrote:
> 
> Hi Coleen,
> 
>> On 2018-12-17 22:31, coleen.phillimore at oracle.com wrote:
>>> On 12/17/18 2:57 PM, Erik Österlund wrote:
>>> Hi Dean,
>>> 
>>>> On 2018-12-17 20:27, dean.long at oracle.com wrote:
>>>>> On 12/17/18 8:56 AM, Erik Österlund wrote:
>>>>> Hi,
>>>>> 
>>>>> Today, when converting a not_entrant nmethod to zombie, the sweeper first clears the IC stubs of that nmethod, and then make_zombie(). With concurrent class unloading, this ordering is a bit unfortunate. Between clearing the IC stubs and making it zombie, the concurrent GC threads may hit a timing window where they will create IC stubs for concurrently cleaning the ICs of this still is_alive() nmethod. 
>>>> 
>>>> Do we normally have to creates stubs to clean an IC?
>>> 
>>> Yes if you clean an IC in an nmethod that is_alive(), we currently have to do that using IC stubs. I have some ideas going forward for how we can stop doing that, but that is outside of scope for now.
>>> 
>>>> 
>>>>> The result is that during safepoint cleanup, when we finalize the IC stubs, we find one associated with a zombie.
>>>>> 
>>>>> The unregistering of the nmethod from the GC will block during concurrent nmethod unloading, so clearing the IC stubs after the nmethod has become zombie is a lot more sane as there can not be any such races then.
>>>>> 
>>>> 
>>>> Where is the unregistering happening?
>>> 
>>> Inside of make_zombie() we unregister_nmethod() on CollectedHeap. In there, ZGC makes sure we wait until concurrent nmethod unloading is over before continuing.
>> Can you move clean_ic_stubs() inside of make_not_entrant_or_zombie() to maintain the order dependence?
> 
> Sure. The implication of that is that the sweeper will redundantly check for IC stubs to clear on unloaded nmethods that can't have any. But I agree that the model is more clear if it is inside of make_not_entrant_or_zombie(), and think that is worth the cost.
> 
> New webrev:
> http://cr.openjdk.java.net/~eosterlund/8215491/webrev.01/
> 
> Thanks for reviewing.
> 
> /Erik
> 
>> thanks,
>> Coleen
>>> 
>>>> 
>>>> Doing the clearing after it's a zombie does sound safer.
>>> 
>>> Agreed!
>>> 
>>> Thanks for reviewing.
>>> 
>>> /Erik
>>> 
>>>> dl
>>>> 
>>>>> Bug:
>>>>> https://bugs.openjdk.java.net/browse/JDK-8215491
>>>>> 
>>>>> Webrev:
>>>>> http://cr.openjdk.java.net/~eosterlund/8215491/webrev.00/index.html
>>>>> 
>>>>> Thanks,
>>>>> /Erik
>>>>