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

[serguei.spitsyn at oracle.com]

Markus,

It looks correct in general and the comments are good.

However, it makes it a little bit more complex.
I'd suggest to rename the "_saved_unloading" to something more meaningful.
What about "_iterated_unloading", "_processed_unloading" or 
"_posted_unloading" ?
No pressure, just some thinking. :)

Thanks,
Serguei

On 6/23/14 8:02 AM, Markus Grönlund wrote:
> 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.
>
>   
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> Thanks
>
> Markus