RFR: 8340826: Should not send unload notification for scratch classes [v2]

[Coleen Phillimore]

On Thu, 26 Sep 2024 18:23:48 GMT, Leonid Mesnik <lmesnik at openjdk.org> wrote:

>> The jvmti class redefinition creates temporary scratch classes for it's own purposes. These classes are added to corresponding classloaders and might be unloaded.
>> In this case the jvmti/jfr and log events are generated twice: for original class and for it's scratch.
>> 
>> The bug could be reproduced by jfr test
>> jdk/jfr/api/metadata/eventtype/TestUnloadingEventClass.java
>> with '-Xcomp -XX:TieredStopAtLevel=1' or with '-Xcomp'
>> 
>> The test log (modified slightly) shown
>> 
>> 
>> [167.294s][info ][class,unload] unloading class jdk.jfr.api.metadata.eventtype.TestUnloadingEventClass$ToBeUnloaded 0x00000000af1006d8 allocated
>> [167.294s][info ][class,unload] unloading class jdk.jfr.api.metadata.eventtype.TestUnloadingEventClass$ToBeUnloaded 0x00000000af100248 fully_initialized
>> [167.345s][trace][class,unload] unlinking class (subclass): jdk.jfr.api.metadata.eventtype.TestUnloadingEventClass$ToBeUnloaded
>> [167.872s][trace][gc          ] GC(0) Restored 3597 marks, occupying 57552 B
>> [167.924s][info ][gc          ] GC(0) Pause Full (System.gc()) 34M->2M(136M) 691.041ms
>> Unloaded count: 2
>> 
>> 
>> instead of expected
>> 
>> 
>> 
>> [159.737s][info ][class,unload] unloading class jdk.jfr.api.metadata.eventtype.TestUnloadingEventClass$ToBeUnloaded 0x0000000041100248 state: fully_initialized
>> [159.800s][trace][class,unload] unlinking class (subclass): jdk.jfr.api.metadata.eventtype.TestUnloadingEventClass$ToBeUnloaded
>> [160.341s][trace][gc          ] GC(0) Restored 3597 marks, occupying 57552 B
>> [160.384s][info ][gc          ] GC(0) Pause Full (System.gc()) 34M->2M(136M) 710.422ms
>> 
>> 
>> 
>> The test hang because got 2 events while waiting for one.
>> The "allocated" version is the scratch class generated by JVMTI JFR agent that redefine classes.
>> 
>> The fix is to don't send notification for scratch classes. The scratch classes shouldn't have dependency so added assertion. Also, we don't expect any other not loaded classes during unloaded.
>> 
>> Thanks Coleen for details about scratch classed. 
>> 
>> Tested with tier1-5 and with :jdk_jfr with Xcomp and c1.
>
> Leonid Mesnik has updated the pull request incrementally with one additional commit since the last revision:
> 
>   space added

Thanks!

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Marked as reviewed by coleenp (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21166#pullrequestreview-2332068915