RFR: 8259862: MutableSpace's end should be atomic

[Kim Barrett]

On Sat, 30 Jan 2021 12:37:50 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Please review this change to MutableSpace, making its _end member volatile
>> and using Atomic operations to access the _top and _end members.  Some
>> unused accessor functions that would otherwise need updating are removed.
>> 
>> Testing:
>> mach5 tier1
>
> src/hotspot/share/gc/parallel/mutableSpace.hpp line 62:
> 
>> 60:   HeapWord* _bottom;
>> 61:   HeapWord* volatile _top;
>> 62:   HeapWord* volatile _end;
> 
> Maybe add some comments explaining how `_top` and `_end` are used in the concurrent setting.

I've added some comments describing `_bottom`, `_top`, and `_end`.

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PR: https://git.openjdk.java.net/jdk/pull/2323