RFR: 8273597: Rectify Thread::is_ConcurrentGC_thread() [v2]

[Stefan Karlsson]

On Mon, 13 Sep 2021 12:07:17 GMT, Per Liden <pliden at openjdk.org> wrote:

>> `Thread::is_ConcurrentGC_thread()` behaves differently to all other `Thread::is_xxx_thread()` functions, in the sense that it doesn't directly map to a distinct `Thread` sub-class. Instead, `is_ConcurrentGC_thread()` can today return true for both `ConcurrentGCThread` and `GangWorker`. These two classes have no super/sub-class relation. This is confusing and and potentially dangerous.
>> 
>> It would be reasonable to think that code like this would be correct:
>> 
>> 
>>   if (thread->is_ConcurrentGC_thread()) {
>>     conc_thread = static_cast<ConcurrentGCThread*>(thread);
>>     ...
>>   }
>> 
>> 
>> but it's not, since we might try to cast a `GangWorker` to a `ConcurrentGCThread`. And again, these two classes have no super/sub-class relation.
>> 
>> I propose that we clean this up, so that `is_ConcurrentGCThread()` only returns true for threads inheriting from `ConcurrentGCThread`. The main side-effect is that a handful of asserts need to be adjusted. In return, the code example above would become legal, and we can also remove some cruft from `WorkGang`/`GangWorker`.
>
> Per Liden has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Fix constructor call
>  - Add ConcurrentGCThread::cast()

Marked as reviewed by stefank (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/5463