RFR: 8299414: JVMTI FollowReferences should support references from VirtualThread stack [v2]

[Alex Menkov]

On Thu, 6 Apr 2023 18:53:59 GMT, Alan Bateman <alanb at openjdk.org> wrote:

> FollowReferences is a graph walk so it will visit the reachable virtual Threads. If I'm not mistake, VThreadClosure will iterate over unreachable Virtual Thread objects. That might be okay as they should be terminated and thus not have any frames, but maybe the other approach needs to be explored too.

The fix is for the case when FollowReferences is called with null initial_object.
Per spec in the case "references are followed from the heap roots".
And: "The heap root are the set of system classes, JNI globals, references from thread stacks, and other objects used as roots for the purposes of garbage collection."

FollowReferences visits all reachable virtual threads only if agent callback always returns JVMTI_VISIT_OBJECTS.
If agent callback doesn't return JVMTI_VISIT_OBJECTS for some object, references from the object are not traversed, so some unmounted vthreads may be missed.

VThreadClosure iterates over all VirtualThread objects, but skips mounted and terminated threads.
After reporting the object is marked "visited" and won't be reported again when some objects have reference to it.

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PR Comment: https://git.openjdk.org/jdk/pull/13254#issuecomment-1499733642