[master] RFR: Implement Shenandoah support [v23]

[Roman Kennke]

On Wed, 20 Apr 2022 12:29:33 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Roman Kennke has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Zhengyu's suggestions
>
> src/hotspot/share/gc/shenandoah/shenandoahForwarding.inline.hpp line 91:
> 
>> 89:     } else if (prev_mark == markWord::INFLATING()) {
>> 90:       // This happens when we encounter a stack-locked object in from-space.
>> 91:       // Busy-wait for completion.
> 
> Can you explain how this can happen? I think we can only inflate to-space object, e.g. apply LRB first.

The trouble is that we need to fetch the size of an object during evacuation *before* the object is guaranteed to be in to-space. If we encounter an object that is stack-locked there, we may race with another thread which could, in the meantime, succeed to evacuate and unlock the object, in which case the stack-lock that we fetched from the from-space object would be invalid. We prevent that by CASing 0 into the header (e.g. the INFLATING marker) - this prevents both evacuation and unlocking, but it requires that our evacuation CAS is aware of it and retries when that happens.

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PR: https://git.openjdk.java.net/lilliput/pull/32