RFR (S): 8182703: Correct G1 barrier queue lock orderings

[Thomas Schatzl]

Hi,

On Mon, 2017-06-26 at 15:34 +0200, Erik Österlund wrote:
> Hi,
> 
> Webrev: http://cr.openjdk.java.net/~eosterlund/8182703/webrev.02/
> Bug: https://bugs.openjdk.java.net/browse/JDK-8182703
> 

  looks good apart from the comment at Monitor::event_types. It now
contradicts itself from one sentence to the next ("special must be
lowest" and then "oh no, after all access must be lowest"). Please try
to find some better wording here :)

> The G1 barrier queues have very awkward lock orderings for the
> following reasons:
> 
[...]
> 
> I do recognize that long term, we *might* want a lock-free solution
> or something (not saying we do or do not). But until then, the ranks
> ought to be corrected so that they do not cause these problems
> causing everyone to bash their head against the awkward G1 lock ranks
> throughout the code and make hacks around it.
> 
> Testing: JPRT with hotspot all and lots of local testing.

Thanks,
  Thomas