JDK-8230459: Test failed to resume JVMCI CompilerThread

[Doerr, Martin]

Hi David,

> I don't think factoring out CompileBroker::clear_compiler2_object when
> it is only used once was warranted, but that's call for compiler team to
> make.
I did that because _compiler2_objects is private and there's currently no setter available.
But let's see what the compiler folks think.

> Otherwise changes seem fine and I have noted the use of the
> MutexUnlocker as per your direct email.
Thanks a lot for reviewing. It was not a trivial one ��

You had noticed an incorrect usage of the CHECK macro. I've created a new bug for that:
https://bugs.openjdk.java.net/browse/JDK-8233193
Would be great if you could take a look if that's what you meant and made adaptions if needed.

Best regards,
Martin


> -----Original Message-----
> From: David Holmes <david.holmes at oracle.com>
> Sent: Mittwoch, 30. Oktober 2019 05:47
> To: Doerr, Martin <martin.doerr at sap.com>; Kim Barrett
> <kim.barrett at oracle.com>
> Cc: Vladimir Kozlov (vladimir.kozlov at oracle.com)
> <vladimir.kozlov at oracle.com>; hotspot-compiler-dev at openjdk.java.net;
> David Holmes <David.Holmes at oracle.com>
> Subject: Re: JDK-8230459: Test failed to resume JVMCI CompilerThread
> 
> Hi Martin,
> 
> On 29/10/2019 12:06 am, Doerr, Martin wrote:
> > Hi David and Kim,
> >
> > I think it's easier to talk about code. So here's a new webrev:
> >
> http://cr.openjdk.java.net/~mdoerr/8230459_JVMCI_thread_objects/webr
> ev.03/
> 
> I don't think factoring out CompileBroker::clear_compiler2_object when
> it is only used once was warranted, but that's call for compiler team to
> make. Otherwise changes seem fine and I have noted the use of the
> MutexUnlocker as per your direct email.
> 
> Thanks,
> David
> -----
> 
> > @Kim:
> > Thanks for looking at the handle related parts. It's ok if you don't want to
> be a reviewer of the whole change.
> >
> >> I think it's weird that can_remove() is a predicate with optional side
> >> effects.  I think it would be simpler to have it be a pure predicate,
> >> and have the one caller with do_it = true perform the updates.  That
> >> should include NULLing out the handle pointer (perhaps debug-only, but
> >> it doesn't cost much to cleanly maintain the data structure).
> > Nevertheless, it has the advantage that it enforces the update to be
> consistent.
> > A caller could use it without holding the lock or mess it up otherwise.
> > In addition, I don't what to change that as part of this fix.
> >
> >> So far as I can tell, THREAD == NULL here.
> > This is a very tricky part (not my invention):
> > EXCEPTION_MARK contains an ExceptionMark constructor call which sets
> __the_thread__ to Thread::current().
> > I don't want to publish my opinion about this ��
> >
> > @David:
> > Seems like this option is preferred over option 3
> (possibly_add_compiler_threads part of webrev.02 and leave the
> initialization as is).
> > So when you're ok with it, I'll request a 2nd review from the compiler folks
> (I should change the subject to contain RFR).
> >
> > Thanks,
> > Martin
> >
> >
> >> -----Original Message-----
> >> From: David Holmes <david.holmes at oracle.com>
> >> Sent: Montag, 28. Oktober 2019 05:04
> >> To: Kim Barrett <kim.barrett at oracle.com>
> >> Cc: Doerr, Martin <martin.doerr at sap.com>; Vladimir Kozlov
> >> (vladimir.kozlov at oracle.com) <vladimir.kozlov at oracle.com>; hotspot-
> >> compiler-dev at openjdk.java.net
> >> Subject: Re: JDK-8230459: Test failed to resume JVMCI CompilerThread
> >>
> >> On 28/10/2019 1:42 pm, Kim Barrett wrote:
> >>>> 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/webr
> >> ev.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...
> >>>
> >>
> >> I'm not following sorry. You can't assert NULL unless it's actually set
> >> to NULL which it presently isn't. But it could be set NULL as Martin
> >> suggested:
> >>
> >> "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."
> >>
> >> and which I also supported. But that aside once the delete_global has
> >> been called that JNIHandle no longer references the j.l.Thread that it
> >> did, at which point it is only reachable via the threadObj() of the
> >> CompilerThread.
> >>
> >> David