RFR: 8235669: G1: Stack walking API can expose AS_NO_KEEPALIVE oops

[erik.osterlund at oracle.com]

Hi,

When the stack is walked and e.g. locals are turned into StackValues, it 
might be that said local was made a constant oop by the JIT. In such 
cases, it is read from the nmethod using ON_STRONG_OOP_REF | 
AS_NO_KEEPALIVE. However, these oops need to be kept alive when 
concurrent marking is ongoing.
While I haven't seen crashes obviously linked to this yet, I don't think 
we should wait until we do, because it certainly will eventually.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8235669

Webrev:
http://cr.openjdk.java.net/~eosterlund/8235669/webrev.00/

Thanks,
/Erik