JDK-8230459: Test failed to resume JVMCI CompilerThread

[Kim Barrett]

> On Oct 27, 2019, at 6:04 PM, David Holmes <david.holmes at oracle.com> wrote:
> 
> On 24/10/2019 12:47 am, Doerr, Martin wrote:
>> Hi Kim,
>> I didn't like using the OopStorage stuff directly, either. I just have not seen how to allocate a global handle and add the oop later.
>> Thanks for pointing me to JVMCI::make_global. I was not aware of that.
>> So I can imagine 3 ways to implement it:
>> 1. Use JNIHandles::destroy_global to release the handles. I just added that to
>> http://cr.openjdk.java.net/~mdoerr/8230459_JVMCI_thread_objects/webrev.01/
>> We may want to improve that further by setting the handle pointer to NULL and asserting that it is NULL before adding the new one.
>> I had been concerned about NULLs in the array, but looks like the existing code can deal with that.
> 
> I think it would be cleaner to both destroy the global handle and NULL it in the array at the same time.
> 
> This comment
> 
> 325         // Old j.l.Thread object can die here.
> 
> Isn't quite accurate. The j.l.Thread is still referenced via ct->threadObj() so can't "die" until that is also cleared during the actual termination process.

I think if there is such a thread here that it can't die, because the
death predicate (the can_remove stuff) won't see that old thread as
the last thread in _compiler2_objects.  That's what I meant by this:

> On Oct 25, 2019, at 4:05 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> I also think that here:
> 
> 947         jobject thread_handle = JNIHandles::make_global(thread_oop);
> 948         _compiler2_objects[i] = thread_handle;
> 
> should assert _compiler2_objects[i] == NULL.  Or if that isn't a valid
> assertion then I think there are other problems.

I think either that comment about an old thread is wrong (and the NULL
assertion I suggested is okay), or I think the whole mechanism here
has problems.  Or at least I was unable to figure out how it could work...