JDK-8230459: Test failed to resume JVMCI CompilerThread

[David Holmes]

Hi Martin,

On 15/10/2019 12:50 am, Doerr, Martin wrote:
> Hi David,
> 
>>> As C1 and C2 compiler threads are hidden, I still don't see any problems reusing the Thread objects.
> 
>> They are not completely hidden. Those j.l.Thread objects are still part of the system thread group and can be found by other code.
> 
> I know that they are part of the system thread group, but I thought they 
> should not be visible to Java.
> 
> E.g.
> 
>          ThreadGroup tg = new ThreadGroup("A");
>          ThreadGroup stg = tg.getParent().getParent();
>          Thread[] list = new Thread[100];
>          stg.enumerate(list);
>          System.out.println("Listing elements of " + stg);
>          for (int i = 0; i < list.length; ++i) {
>              if (list[i] != null) {
>                  System.out.println(i + ": " + list[i]);
>              }
>          }
> 
> Finds:
> 
> Listing elements of java.lang.ThreadGroup[name=system,maxpri=10]
> 
> 0: Thread[Reference Handler,10,system]
> 1: Thread[Finalizer,8,system]
> 2: Thread[Signal Dispatcher,9,system]
> 3: Thread[main,5,main]
> 
> I can’t see any compiler threads.

Yes you are right. I had forgotten about this other quirk in this whole 
arrangement. Threads started directly by the VM, rather than via 
Thread.start, don't call ThreadGroup.add unless the VM explicitly makes 
that call - which it does for the Signal Dispatcher thread and a couple 
of others. The compiler threads aren't included so while they consider 
themselves to be part of the system ThreadGroup, from the group's 
perspective they are "unstarted threads" - something which is itself 
somewhat of an oddity. But that does mean they are not discoverable that 
way.

> Do you mean finding them by JVMTI agent?
> 
> As I understand the Spec, compiler threads should be excluded for 
> GetAllThreads: “The threads are Java programming language threads”

They are hidden from JVMTI (unlike the JVMCI compiler threads)

> Or what did you mean by “other code” which can see C1 and C2 threads?

I did mean by enumerating the system ThreadGroup.

So for C1/C2 compiler threads we can probably get away with reusing the 
j.l.Thread object.

Thanks,
David
-----

> Best regards,
> 
> Martin
>