RFR: 8274196: Crashes in VM_HeapDumper::work after JDK-8252842

[Coleen Phillimore]

On Fri, 24 Sep 2021 13:13:39 GMT, Lin Zang <lzang at openjdk.org> wrote:

> The root cause for crash in ZGC is that the JNIHandles are processed before object iteration. And ZGC would update the JNIHandles at object iteration with read barrier. So the crash is cause by accessing the invalid address which can be dummy info after zgc, and hence crash.
> 
> The lock rank issue can be fixed because the related mutexes are acquired in safepoint. so the safepoint_check_required could be safepoint_check_always.
> 
> The Epsilon issue is caused by wrong _num_dumper_thread calculated when the gang==NULL.

src/hotspot/share/services/heapDumper.cpp line 751:

> 749:   static void before_work() {
> 750:     assert(_lock == NULL, "ParDumpWriter lock must be initialized only once");
> 751:     _lock = new (std::nothrow) PaddedMonitor(Mutex::leaf, "ParallelHProfWriter_lock", Mutex::_safepoint_check_always);

If you change these locks to _safepoint_check_always, you have to acquire them without the Mutex::_no_safepoint_check flags so I don't know why you don't get that assert.

-------------

PR: https://git.openjdk.java.net/jdk/pull/5681