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

[Erik Osterlund]

Hi Dean,

Thanks for the review.

/Erik

> On 20 Dec 2018, at 02:18, dean.long at oracle.com wrote:
> 
> +1
> 
> dl
> 
>> On 12/19/18 11:48 AM, Coleen Phillimore wrote:
>> 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
>