JDK-8230459: Test failed to resume JVMCI CompilerThread

[David Holmes]

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/webrev.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