RFR: 8166607: G1 needs klass_or_null_acquire

Kim Barrett kim.barrett at oracle.com
Fri Nov 18 14:03:50 UTC 2016

> On Nov 15, 2016, at 6:58 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
>> On Nov 15, 2016, at 5:21 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
>> Hi Kim,
>> On Mon, 2016-11-07 at 14:38 -0500, Kim Barrett wrote:
>>>> On Nov 7, 2016, at 5:53 AM, Thomas Schatzl <thomas.schatzl at oracle.c
>>>> om> wrote:
>>>> Maybe it would be cleaner to call a method in the barrier set
>>>> instead of inlining the dirtying + enqueuing in lines 685 to 691?
>>>> Maybe as an additional RFE.
>>> We could use _ct_bs->invalidate(dirtyRegion).  That's rather
>>> overgeneralized and inefficient for this situation, but this
>>> situation should occur *very* rarely; it requires a stale card get
>>> processed just as a humongous object is in the midst of being
>>> allocated in the same region.
>> I kind of think for these reasons we should use _ct_bs->invalidate() as
>> it seems clearer to me. There is the mentioned drawback of having no
>> other more efficient way, so I will let you decide about this.
> I've made the change to call invalidate, and also updated some comments.
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8166607
> Webrevs:
> full: http://cr.openjdk.java.net/~kbarrett/8166607/webrev.03/
> incr: http://cr.openjdk.java.net/~kbarrett/8166607/webrev.03.inc/
> Also, see RFR: 8166811, where I've included a webrev combining the
> latest changes for 8166607 and 8166811, since they are rather
> intertwined.  I think I'll do as Erik suggested and push the two
> together.

Sorry folks, but I want to revert this part and go back to the old
code where it locked the shared queue and enqueued there.

If the executing invocation of refine_card is from a Java thread,
e.g. this is the "mutator helps with refinement" case, calling
invalidate would enqueue to the current thread's buffer.  But that is
effectively a reentrant call to enqueue, and the Java thread case of
enqueue is not reentrant-safe.  Only enqueue to the shared queue is

I think that scenario presently can't happen, since the mutator helps
case is dealt with by the mutator processing it's own buffer.  In that
situation, all the cards in the buffer came from writes by this thread
to an object this thread either allocated or has access to, so the
klass must be there.  But that's getting uncomfortably subtle in what
is already difficult to analyze code.

Also, we've talked about changing the mutator helps case to not
immediately process it's own buffer but instead add its buffer to the
pending buffer list and process the next (FIFO ordered) buffer, in
order to let its buffer age.  (I have a change for that in my post-JDK
9 collection of pending changes.  The mutator-invoked enqueue might be
reentrant-safe in that change, but I don't think I want to make that

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