JDK-8280454 : G1: ClassLoaderData verification keeps CLDs live that causes problems with VerifyDuringGC during Remark
  • Type: Bug
  • Component: hotspot
  • Sub-Component: gc
  • Affected Version: 19
  • Priority: P4
  • Status: Resolved
  • Resolution: Fixed
  • Submitted: 2022-01-21
  • Updated: 2024-06-27
  • Resolved: 2022-06-10
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JDK 20
20 b02Fixed
Related Reports
Relates :  
JDK-8288203 - runtime/ClassUnload/UnloadTestWithVerifyDuringGC.java fails with release VMs
Relates :  
JDK-8335059 - Consider renaming ClassLoaderData::keep_alive
Relates :  
JDK-8326820 - Metadata artificially kept alive
Relates :  
JDK-8297427 - Avoid keeping class loaders alive when executing ClassLoaderStatsVMOperation
Description
When running with -XX:+VerifyDuringGC G1 will do verification during the Remark and Cleanup pause. The "before" verification in Remark is causing class unloading to not work as expected because iterating over the CLDGs keeps all CLDs live. The call to ClassLoaderData::holder() in ClassLoaderDataGraphIterator::get_next() pushes it into the SATB queue, which is processed during Remark just a bit later.

At least for the TestHumongousClassLoader.java this causes a test failure as the CLDs are not unloaded. This can be reproduce using -XX:VerifySubSet=classloader_data_graph (i.e. only do that specific verification) and if you instead run with -XX:VerifySubSet=heap the tests passes.

One workaround could be to only verify the heap in the "before" verification for remark, exclude the CLDG verification in this verification or add a special iteration for this particular verification.
Comments
Changeset: 94b473e4 Author: Thomas Schatzl <tschatzl@openjdk.org> Date: 2022-06-10 07:57:32 +0000 URL: https://git.openjdk.org/jdk/commit/94b473e4642a5a4626faeb73341b4aea128ccb31
10-06-2022
A pull request was submitted for review. URL: https://git.openjdk.java.net/jdk/pull/8949 Date: 2022-05-30 17:09:35 +0000
31-05-2022
System dictionary verification has the same issue, the called method ClassLoaderDataGraph::verify_dictionary() keeps alive objects in error.
30-05-2022
My guess is that the ClassLoaderDataGraphIterator puts ClassLoaderData instances it finds into handles, which may keep them alive until they are cleaned out. Not sure exactly why they are marked as live throughout the remark pause as on the outside handling seems okay (i.e. some handlemark is used)
24-01-2022