RFR: 8187577: JVM crash during gc doing concurrent marking

[Poonam Parhar]

Hello,

Please review this fix for a G1GC crash occurring during concurrent marking.

Bug: JDK-8187577: <https://bugs.openjdk.java.net/browse/JDK-8187577> JVM 
crash during gc doing concurrent marking
Webrev: http://cr.openjdk.java.net/~poonam/8187577/webrev.00/

Problem:
A klass that has been considered unreachable by G1, can be looked up in 
the CLD/SD, and its _java_mirror or _class_loader can be stored in a 
root or any other reachable object making it alive again. Whenever a 
klass is resurrected in this manner, the SATB part of G1 needs to be 
notified about this, otherwise the concurrent marking remark phase will 
wrongly unload that klass.

In this particular crash, while a G1 is doing concurrent marking and has 
prepared its list of unreachable classes, JVMTI on a Java thread 
traverses classes in the CLD and stores thread-local JNIHandles for the 
java_mirror of the loaded classes. G1 does not have knowledge of these 
thread-local JNIHandles, and in the remark phase it unloads classes as 
per its prior knowledge of unreachable classes. But when these 
JNIHandles are later scanned, it leads to a crash.

Solution:
Inform G1's SATB that a klass has been resurrected and it should not be 
unloaded.

Testing:
- Customer verification testing
- mach5 jdk-tier1-3, hs-tier1-2

Thanks,
Poonam


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