RFR: 8273597: Rectify Thread::is_ConcurrentGC_thread()

[Stefan Karlsson]

On Fri, 10 Sep 2021 12:39:14 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`.

Thanks for cleaning this up.

src/hotspot/share/code/nmethod.cpp line 1563:

> 1561:   DEBUG_ONLY(bool called_by_gc = Universe::heap()->is_gc_active() ||
> 1562:                                  Thread::current()->is_ConcurrentGC_thread() ||
> 1563:                                  Thread::current()->is_Worker_thread();)

Three places use the same condition. Did you consider creating a helper function?

src/hotspot/share/gc/parallel/parallelScavengeHeap.hpp line 122:

> 120:     _old_pool(NULL),
> 121:     _workers("GC Thread",
> 122:              ParallelGCThreads) { }

Consider moving this up to the line above, like the G1 code.

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Marked as reviewed by stefank (Reviewer).

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