CRR (S): 7119027: G1: use atomics to update RS length / predict time of inc CSet

[Tony Printezis]

Hi all,

I'd like a couple of code reviews for this small change that resolves 
the race that we identified a few weeks ago (and which we already guard 
against in the code):

http://cr.openjdk.java.net/~tonyp/7119027/webrev.0/

Instead of using atomics I decided to take an alternative approach to 
eliminate the race which as described in the CR:

"The final fix is somewhat different to what I had originally planned 
and does not use atomics. The observation is that only two threads can 
update the fields in question: a mutator thread that retires a region 
before allocating a new one (and it has to take the Heap_lock in order 
to do so - so those updates are serialized anyway) and a concurrent 
refinement thread that samples the RSets in the inc CSet. What we'll do 
is to accumulate the updates from each thread on separate fields and 
we'll just combine them at the start of a GC."

Thanks,

Tony