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

[Erik Helin]

Looks good, Reviewd.

Thanks,
Erik

On Monday 23 June 2014 08:26:51 AM Markus Grönlund wrote:
> Sending this to the Hotspot-GC-dev group as well.
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> /Markus
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> 
> From: Markus Grönlund
> Sent: den 23 juni 2014 17:03
> To: hotspot-runtime-dev; serviceability-dev
> Subject: RFR(S): 8047812: Ensure ClassLoaderDataGraph::classes_unloading_do
> only delivers klasses from CLDs with non-reclaimed class loader oops
> 
>  
> 
> Greetings,
> 
>  
> 
> Kindly asking for reviews for the following change:
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> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8047812
> 
> Webrev: http://cr.openjdk.java.net/~mgronlun/8047812/webrev01
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> 
> 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.
> 
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> 
> Thanks
> 
> Markus