RFR: 8288139: JavaThread touches oop after GC barrier is detached

[David Holmes]

On Wed, 15 Jun 2022 16:06:08 GMT, Daniel D. Daugherty <dcubed at openjdk.org> wrote:

> Update SharedRuntime::get_java_tid() to verify that the calling thread is safely
> accessing its own threadObj(). This check uses the new is_gc_barrier_detached()
> function added by [JDK-8288497](https://bugs.openjdk.org/browse/JDK-8288497) add support for JavaThread::is_gc_barrier_detached().
> 
> The above check was used to reproduce the failure mode without Shenandoah
> and the remainder of the fix relocates the offending code from
> ThreadsSMRSupport::remove_thread() to Threads::remove(). The work of
> removed the 'tid' entry from the ThreadIdTable is still done under the
> protection of the Threads_lock.
> 
> This fix along with the fix for JDK-8288497 has been tested in Mach5 Tier[1-8].
> There are no related failures in Mach5 Tier[1-7]; Mach5 Tier8 is still running.

src/hotspot/share/runtime/sharedRuntime.cpp line 998:

> 996: JRT_END
> 997: 
> 998: jlong SharedRuntime::get_java_tid(Thread* thread) {

Additional RFE: can we not declare this to take JavaThread and so avoid the checks and casts below?

src/hotspot/share/runtime/thread.cpp line 3603:

> 3601:       ThreadIdTable::remove_thread(tid);
> 3602:     }
> 3603: 

I don't like exposing `ThreadSMR` internal details like this. Can we at least make this a little `ThreadSMRSupport` method that we call from here.

Or ... can we just move `ThreadsSMRSupport::remove_thread(p);` to before `on_thread_detach`?

Or can we grab the pid before `on_thread_detach` and pass it to `ThreadsSMRSupport::remove_thread`?

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PR: https://git.openjdk.org/jdk19/pull/21