RFR: 8278602: CDS dynamic dump may access unloaded classes [v4]

[Calvin Cheung]

On Fri, 17 Dec 2021 18:23:01 GMT, Ioi Lam <iklam at openjdk.org> wrote:

>> Cause of crash:
>> 
>> When dumping a CDS archive,  while iterating over entries of the `SystemDictionaryShared::_dumptime_table`, we do not check whether the classes are already unloaded. In the crash, we are trying to call `InstanceKlass::signer()` but the class has already been unloaded.
>> 
>> Fix:
>> 
>> Override the  template function `DumpTimeSharedClassTable::iterate` to ensure iteration safety.  Do not iterate over a class if its `class_loader_data` is no longer alive.
>> 
>> The assert in `DumpTimeSharedClassTable::IterationHelper` found another existing bug -- we were calling `SystemDictionaryShared::is_dumptime_table_empty()` without holding the `DumpTimeTable_lock`. I delayed the call until we have grabbed the lock.
>> 
>> Testing:
>> 
>> I have attached a test case into the bug report. Without the fix, it would reproduce the same crash in less than a minute. With the fix, the crash is no longer reproducible.
>> 
>> Unfortunately, the test case requires a ZGC patch (thanks to @stefank) that adds delays to  increase the likelihood of seeing unloaded classes inside the `_dumptime_table`. Therefore, I cannot integrate the test as a jtreg test. I'll mark the bug as **noreg-hard**
>
> Ioi Lam has updated the pull request incrementally with one additional commit since the last revision:
> 
>   @calvinccheung comments -- removed unused code

Marked as reviewed by ccheung (Reviewer).

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PR: https://git.openjdk.java.net/jdk/pull/6859