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

[Lin Zang]

On Fri, 24 Sep 2021 13:46:13 GMT, Lin Zang <lzang at openjdk.org> wrote:

>> 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.
>
> I think it may be because this is actually not a JavaThread. So the assert in `Mutex::check_no_safepoint_state` would pass. 
> Moreover, I have tried to use `PaddedMonitor(Mutex::nosafepoint, "ParallelHProfWriter_lock", Mutex::_safepoint_check_never);` here, but the slowdebug would report errors as you mentioned in JDK-8274245.

I agree the flag here and at the place of lock acquiring  seems problematic. I will try to see whether I can use `Mutex::_safepoint_check_never` here and get rid of the assert.

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