RFR: 8253180: ZGC: Implementation of JEP 376: ZGC: Concurrent Thread-Stack Processing [v3]

[Albert Mingkun Yang]

On Wed, 23 Sep 2020 14:27:55 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Erik Österlund has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Review: SteafanK CR 2
>
> Marked as reviewed by stefank (Reviewer).

Given names like `StackWatermarkSet::lowest_watermark`, I wonder if some diagrams could be provided in the code
comments for `class StackWatermarks` so that readers will have the correct mental pictures of the layout of stack
frames and watermarks.

Some docs on `volatile uint32_t _state;` in `StackWatermark` would be nice, such as what info is encoded in this state,
typical state transitions, who can access this state (tying it to volatile) etc.

I wonder if it's possible to remove `bool _is_done;` in `StackWatermarkFramesIterator`; just calling `is_done()` on the
underlying frame stream seems cleaner.

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PR: https://git.openjdk.java.net/jdk/pull/296