RFR(S): 8047812: Ensure ClassLoaderDataGraph::classes_unloading_do only delivers klasses from CLDs with non-reclaimed class loader oops

[Markus Grönlund]

Greetings,

 

Kindly asking for reviews for the following change:

 

Bug: https://bugs.openjdk.java.net/browse/JDK-8047812

Webrev: http://cr.openjdk.java.net/~mgronlun/8047812/webrev01 

 

Description:

The "8038212: Method::is_valid_method() check has performance regression 
  impact for stackwalking" - changeset introduced a change in how the ClassLoaderDataGraph::_unloading list of ClassLoaderData's is purged. 

This change to the purging of the CLD's work the same as before for most GC's, but when using CMS GC, SystemDictionary::do_unloading() is called twice with no explicit purge call in between. On the second call (post-sweep), we can now get stale class loader oops delivered as part of the Klass closure callbacks from the _unloading list. Again, this is because there is no explicit purge call in between these two entries to SystemDictionary::do_unloading() - and being CMS and concurrent, it is very hard to accommodate a timely and proper purge call here.

The first do_unloading call comes after CMS concurrent marking, and the second comes from a Full GC triggered while sweeping the CMS heap. 

This fix ensures the unloading purge mechanism to work correctly also for the CMS collector, in that only CLDs with non-reclaimed class loader oops will deliver klasses from the _unloading list. In addition, this will ensure a single "logical" pass is achieved when iterating the unloading list in-between purges (avoiding the processing of the same data twice). 

This fix is precipitated by nightly testing failures with CMS after the introduction of 8038212: Method::is_valid_method() check has performance regression 
  impact for stackwalking" - for example "nsk/sysdict/vm/stress/jck12a//sysdictj12a008" which is crashing because of following up stale klass loader oop's from the ClassLoaderDataGraph::_unloading list.

 

Thanks

Markus
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