RFR: Fix weakref precleaning

[Aleksey Shipilev]

On 05/31/2017 07:59 PM, Roman Kennke wrote:
> Weakref precleaning does infact need to do something in the
> keepalive-closure:
> 
> - It must update internal refs in j.l.r.Reference instances. I.e. make
> referent point to to-space
> - It must mark through the next field for references that are enqueued
> 
> And therefore the complete_gc closure also needs to process stuff on the
> marking stack.
> 
> The patch fixes those issues plus also drains SATB buffers, and adds a
> ResourceMark for -Xlog:gc+ref logging.
> 
> http://cr.openjdk.java.net/~rkennke/fixpreclean/webrev.00/

Thanks for doing this.

Comments:

 *) ShenandoahPrecleanKeepAliveUpdateClosure is not used. Seems like a bug! It
should be used on need_update_refs() path, right?

 *) Can use ReferenceProcessorIsAliveMutator, because RP is not null there

 *) Double ";"

  819     ReferenceProcessor* rp = sh->ref_processor();;

 *) Misleading comment:

  825                    true, // do not drain SATBs

 *) Leftovers:

  872     tty->print_cr("need update refs");
  ...
  880     tty->print_cr("don't need update refs");


Thanks,
-Aleksey