RFR(S): 8173743: Failures during class definition can lead to memory leaks in metaspace

Volker Simonis volker.simonis at gmail.com
Mon Feb 6 18:40:09 UTC 2017


can somebody please review the following change:


It fixes some problems during class definitions where instance klasses
can leak into the metaspace in cases where the class definition fails
after the class was successfully loaded from the bytecode stream.

There are actually two cases to consider:

1. If we load the same class several times into the same class loader,
we will get LinkageErrors after the first successful load. But
nevertheless we will first construct a new instanceKlass for each new
load attempt and this instanceKlass will never be deleted.

2. If we have a parallel capable class loader, set
-XX:-UnsyncloadClass and/or -XX:+AllowParallelDefineClass and load a
class from several threads at the same time in parallel, it can happen
that we create several instance klasses for the same class. At the end
only one of them will be added to the system dictionary, but all the
other ones will never be deleted. Notice that if we run this scenario
without setting either of -XX:-UnsyncloadClass or
-XX:+AllowParallelDefineClass, this scenario will degrade into the
case above and we will get LinkageErrors for all but the first
successful load.

The change comes with a regression test which checks for the two cases
just describe and also for the failing class redefinition case, which
currently doesn't produce a memory leak.

I've already committed this to the hs-demo-submit/hotspot/ forest and
it went through without a problem. So in theory it should have passed
the internal JPRT tests although I'm not sure if the test set of the
"demo-submit" forest and the real hotspot repo are exactly the same
(CC'ed Tim and Brian).

Thank you and best regards,

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