RFR: 8307365: JvmtiStressModule hit SIGSEGV in JvmtiEventControllerPrivate::recompute_thread_enabled

[Patricio Chilano Mateo]

The following patch fixes a bug introduced while refactoring the VirtualThreadStart/End events. Specifically, the code to delete the JvmtiThreadState of a terminating vthread was moved before we start the VTMS transition. That allowed said code to run concurrently with recompute_enabled() leading to different crashing modes. I wrote the detailed sequence of events leading to the crash in the bug comments. 

To fix it I moved the cleanup code back after the call to VTMS_unmount_begin(). Now, since the rebinding of the JvmtiThreadState to that of the carrier has to be done after this cleanup code is executed, otherwise we would delete the wrong JvmtiThreadState state, I had to add a boolean argument to VTMS_unmount_begin() to differentiate the last unmount call from the other ones. This is unfortunate since ideally VTMS_unmount_begin() would be oblivious to these two cases as with VTMS_mount_end() where we don't need to check if this is the first mount.
I looked for other ways to solve it instead of the extra boolean argument but wasn't convinced. One way would be to have another JvmtiExport::cleanup_thread() that would handle this case. Another way which is very simple is to move the rebind_to_jvmti_thread_state_of() call to VTMS_unmount_end() instead. But that means during the transition the _jvmti_thread_state field of the carrier would be either null or that of the vthread, unlike today which is always that of the carrier during the transitions. I didn't want to change that behavior in this fix but I can also explore that route.

I tested the patch with the reproducer I attached to the bug, plus I also run tiers1-3 in mach5.

Thanks,
Patricio

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Commit messages:
 - v1

Changes: https://git.openjdk.org/jdk/pull/13949/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13949&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8307365
  Stats: 12 lines in 2 files changed: 4 ins; 1 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/13949.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13949/head:pull/13949

PR: https://git.openjdk.org/jdk/pull/13949