RFR (XS): 8033922: G1: Back out 8033601 and go back to use the to-obj for chunked arrays.

Stefan Karlsson stefan.karlsson at oracle.com
Fri Feb 7 12:38:15 UTC 2014

On 2014-02-07 13:29, Bengt Rutisson wrote:
> Hi all,
> Can I have a couple of reviews for backing out the changes for 
> 8033601. Those changes cause verification failures in our nightly 
> testing.
> http://cr.openjdk.java.net/~brutisso/8033922/webrev.00/



> https://bugs.openjdk.java.net/browse/JDK-8033922
> Background from the CR:
> The fix for 8033601 changed the code for chunked arrays to use the 
> from obj length as index rather than the to object.
> This was nice since it made G1 look more like the other collectors.
> However, another difference in G1 is that it writes the forward 
> pointer before it writes the to-object. That means that there are 
> times when an object is forwarded, but there is no to-object image 
> that can be trusted.
> One such place is
> void G1ParCopyClosure<barrier, do_mark_object>
> ::do_oop_work(T* p)
> We can find that the object is forwarded, but it may not have been 
> marked yet. So, we end up calling mark_forwarded_object() which needs 
> the size to call _cm->grayRoot(to_obj, (size_t) from_obj->size(), 
> _worker_id);
> mark_forwarded_object() has a comment that tries to describe this 
> situation:
>   // The object might be in the process of being copied by another
>   // worker so we cannot trust that its to-space image is
>   // well-formed. So we have to read its size from its from-space
>   // image which we know should not be changing.
>   _cm->grayRoot(to_obj, (size_t) from_obj->size(), _worker_id);
> So, just as the comment say, we can not trust the to-space image. 
> However, after the change for 8033601 we can not trust the from-space 
> image either.
> This seems like a show-stopper for the suggested change for 8033601 so 
> we need to back it out.
> Thanks,
> Bengt

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