[RESUMED] RFR: 8158946 - btree009 fails with assert(s > 0) failed: Bad size calculated

Kim Barrett kim.barrett at oracle.com
Wed Jun 22 02:07:19 UTC 2016

> On Jun 21, 2016, at 6:36 PM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
> Hi,
> On Tue, 2016-06-21 at 18:27 -0400, Derek White wrote:
>> Here's a more complete solution to the problem.
>> As Kim mentioned, the new version sets the object size field of a 
>> java.lang.Class object before it sets the object's header, so GC can 
>> reliably get the size of any object with it's header set.
>   just briefly looking over the code (maybe I am overlooking something
> obvious, I am only looking at the webrev) - wouldn't the code need some
> compiler and memory barriers that make sure that this required
> read/write orderings are observed?

I agree with Thomas.  I think there needs to be an
OrderAccess::storestore() between the setting of the size and the
setting of the klass.

And that makes me wonder why
CollectedHeap::post_allocation_setup_array doesn't appear to have
a similar store barrier.  It even has the corresponding comment about
how the array length must be set before setting the _klass field so
the object is parsable by concurrent GC.

Maybe this only causes problems when the object is allocated in the
old gen (perhaps because it is large).  Is there some other path for
large arrays, so we don't have a barrier for every array allocation?
I hope I'm missing something...

>> This fix works by adding a CollectedHeap::class_allocate() method and
>> associated helpers. These are implemented in CollectedHeap.inline.hpp
>> only because they share so much structure and code with 
>> CollectedHeap::obj_allocate() that I thought it best to keep them 
>> together (even though there is no performance reason to have the new 
>> code inlined).
>> Webrev: http://cr.openjdk.java.net/~drwhite/8158946/webrev.02
>> jprt in progress...
> Thanks,
>   Thomas

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