RFR: 8210330: Make CLD claiming allow multiple claim bits

[Erik Österlund]

Hi Per,

Thanks for the review!

/Erik

On 2018-09-04 16:50, Per Liden wrote:
> Hi Erik,
>
> On 09/04/2018 04:45 PM, Erik Österlund wrote:
>> Hi Coleen,
>>
>> Thank you for the review.
>>
>> Per wanted the following naming change: claim_value -> claim, claim() 
>> -> try_claim(). And the strong_claim to be 3 instead of 1, so that 
>> one does not accidentally set the wrong value when using it from ZGC 
>> (strong claimed always implies final claimed too). Hope you are also 
>> okay with that.
>
> Thanks for adjusting that.
>
>>
>> Incremental:
>> http://cr.openjdk.java.net/~eosterlund/8210330/webrev.00_01/
>>
>> Full:
>> http://cr.openjdk.java.net/~eosterlund/8210330/webrev.01/
>
> Looks good to me!
>
> /Per
>
>>
>> Thanks,
>> /Erik
>>
>> On 2018-09-04 14:29, coleen.phillimore at oracle.com wrote:
>>>
>>> This change looks good to me.
>>> Coleen
>>>
>>> On 9/4/18 4:42 AM, Erik Österlund wrote:
>>>> Hi,
>>>>
>>>> Today, the claim value of a CLD is logically a boolean, stored as 
>>>> an int. Either a CLD is claimed, or not claimed. This is used by 
>>>> the GC to see if oops_do can be skipped on the CLD, because its 
>>>> handles have already been marked.
>>>> With ZGC supporting concurrent class unloading, the CLD holder can 
>>>> be marked finalizably reachable first, and afterwards strongly 
>>>> reachable. This requires 2 claim bits in the CLD: one for claiming 
>>>> it for strong marking of the oop handles, and one for finalizable 
>>>> marking.
>>>>
>>>> Rather than making a separate mechanism for ZGC only, I would like 
>>>> to use these shared claim bits. I think this is a fascility that 
>>>> anyone who has concurrent reference processing and concurrent class 
>>>> unloading will need, it just happens to be that ZGC is the first GC 
>>>> to do that in hotspot. I also have the hypothesis that this can be 
>>>> beneficial for other uses such as the JFR leak profiler, which can 
>>>> have its own claim bits, instead of clearing the claim bit, doing 
>>>> its thing, and then restoring the claim bit to what it was before.
>>>>
>>>> As part of this change, I changed the "must_claim" boolean that is 
>>>> passed around to instead take an int for the desired claim value, 
>>>> and made it explicit and not implicit.
>>>>
>>>> Webrev:
>>>> http://cr.openjdk.java.net/~eosterlund/8210330/webrev.00/
>>>>
>>>> Bug:
>>>> https://bugs.openjdk.java.net/browse/JDK-8210330
>>>>
>>>> Thanks,
>>>> /Erik
>>>
>>