RFR: 8224966: ZGC: Don't skip oop verification in OopMapSet::all_do()

[Per Liden]

Thanks for reviewing, Stefan. I agree, and I'll wait until Nils' patch 
has been pushed.

/Per

On 6/5/19 10:22 AM, Stefan Karlsson wrote:
> On 2019-05-29 11:27, Per Liden wrote:
>> When ZGC is enabled, we skip oop verification in OopMapSet::all_do(). 
>> There's no good reason to skip that. The comment suggesting this is 
>> skipped because a safepoint can happen between a load and the barrier 
>> is incorrect. We can still do the verification as is_in_or_null() 
>> doesn't require the address to have a "good" color.
> 
> I think the comment is correct. The following situation can cause 
> failure in the verification:
> 
> register = Object.field
> safepoint
> load barrier for register
> 
> If Object.field points into the old collection set, and we perform a 
> mark start or mark end pause, we'll perform a stack walk and hit this 
> assert because Heap::is_in_or_null(Object.field) will return false.
> 
> I applied the patch and ran stress testing with large collection sets, 
> and eventually hit this assert during the mark start phase.
> 
>>
>> Also, once JDK-8224675 is integrated, this comment will make even less 
>> sense.
> 
> Right. After Nils' changes this shouldn't be a problem anymore.
> 
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8224966
>> Webrev: http://cr.openjdk.java.net/~pliden/8224966/webrev.0
> 
> Looks good, after Nils' changes have been pushed.
> 
> StefanK
> 
>>
>> /Per