RFR (M): JDK-8043239: G1: Missing post barrier in processing of j.l.ref.Reference objects

[Bengt Rutisson]

Hi all,

Can I have a couple of reviews for this change?

http://cr.openjdk.java.net/~brutisso/8043239/webrev.00/

https://bugs.openjdk.java.net/browse/JDK-8043239

As described in the bug report the reference processor was missing a 
write barrier call when manipulating the discovered list. This has 
always been the case but it was hidden because at the end of the 
reference processing we went through the complete discovered list and 
dirtied all the missed cards because we did an (unnecessary) write 
barrier when we set the next field to point to be a self pointer 
pointing back at the reference object itself.

The write barrier for setting the next field was removed since it was 
not needed, but that revealed the current bug. After some discussions 
and prototyping we came to the conclusion that there may be more 
barriers missing and that it is difficult to get the dirtying done the 
way our verification code assumes. A simpler solution seems to be to 
free the reference processing of all barriers and instead just make sure 
that we dirty all the right cards in the last pass.

The proposed fix thus re-introduces the post barrier when we iterate 
over the discovered list. This time it uses the discovered field for the 
barrier to be more explicit about what is going on.

Testing:
JPRT,
Kitchensink, 5 days
GC test suite
SPECjbb2013
Ad-hoc aurora run
Specific reproducer that illustrated the problem.

The specific reproducer was really good to pinpoint the problem but is 
hard to turn in to a JTreg test. Many thanks go to StefanK for helping 
out with creating the reproducer.

Thanks,
Bengt