Integrated: 8278602: CDS dynamic dump may access unloaded classes

[Ioi Lam]

On Thu, 16 Dec 2021 03:46:10 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**

This pull request has now been integrated.

Changeset: 09cf5f19
Author:    Ioi Lam <iklam at openjdk.org>
URL:       https://git.openjdk.java.net/jdk/commit/09cf5f19d76b17790ffb899aad247f821a27d46b
Stats:     269 lines in 8 files changed: 261 ins; 4 del; 4 mod

8278602: CDS dynamic dump may access unloaded classes

Reviewed-by: coleenp, ccheung

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