RFR: 8280454: G1: ClassLoaderData verification keeps CLDs live that causes problems with VerifyDuringGC during Remark

[Stefan Karlsson]

On Mon, 30 May 2022 17:09:35 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   can I have reviews for this change that makes a few CLDG iterations no-keepalive during GC pauses?
> 
> This fixes a bug where the `VerifyDuringGC` verification otherwise kept alive all classes, effectively disabling class unloading during (g1) concurrent mark.
> 
> Afaict this verification code is only every called during GC pauses, so the change should be fine.
> 
> As for the change itself, I tried to avoid cluttering the code with `ClassLoaderDataGraphIterator<>` by using typedefs - unfortunately only C++ 17 allows omitting the `<>` if all template parameters are defaulted.
> 
> There is also a new test case for just this.
> 
> Testing: tier1-5
> 
> Thanks,
>   Thomas

src/hotspot/share/classfile/classLoaderDataGraph.cpp line 328:

> 326: public:
> 327:   ClassLoaderDataGraphIteratorBase() : _next(ClassLoaderDataGraph::_head), _thread(Thread::current()), _hm(_thread) {
> 328:     _thread = Thread::current();

Pre-existing: this code sets the _thread variable twice. Maybe clean that up?

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PR: https://git.openjdk.java.net/jdk/pull/8949