RFR (XS): 7114095: G1: assert(obj == oopDesc::load_decode_heap_oop(p)) failed: p should still be pointing to obj

[John Cuthbertson]

Hi Everyone,

Can I have a couple of volunteers look over the fix for this CR? The 
webrev can be found at: 
http://http://cr.openjdk.java.net/~johnc/7114095/webrev.0/

Summary:
As a result of the changes for 4965777, we could apply the 
G1ParScanClosure twice to the discovered field of a reference object. 
Once because the closure itself has the apply_to_weak_discovered_field 
attribute (as I think it should) and the other because of the changes 
for 4965777. This causes the discovered field of an evacuated reference 
object to be pushed to the _refs_to_scan queue twice. Normally this is 
not a problem as the evacuation code can handle seeing the same object 
more than once. In this case however, if the result of the first push is 
stolen and the referenced object successfully evacuated by another 
worker thread, then the assert is too strong.  The other worker will 
update the discovered field to point to the new location while the 
current thread is pushing the discovered field for the second time. As a 
result when the current thread executes the assert, the discovered field 
could have been updated and trips the assert.

The solution is to weaken the assert slightly.

Testing: Both failing test cases running continuously overnight (they 
typically fail within a few iterations); the GC test suite with a delay 
in the offending code; jprt.

Thanks,

JohnC