[master] RFR: Load Klass* without causing monitor inflation

[Aleksey Shipilev]

On Tue, 19 Oct 2021 14:44:12 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> Until now, loading the Klass* in Lilliput may cause monitor inflation, because we needed a way to safely load the mark-word in the face of concurrent stack-locking or inflation happening. However, this caused troubles with concurrent GCs, because they may attempt to inflate monitors on from-space objects while traversing the heap for relocation/evacuation. Also, this may be a performance nuisance.
> It turns out that we don't have to fully inflate the monitor: we only need a partial inflation, up to where we install the transient INFLATING word to prevent concurrent threads from messing with the mark-word, and then read and return the mark-word safely, and swing back the original real mark word to let other threads continue.
> 
> Testing:
>  - [x] tier1
>  - [x] tier2
>  - [x] tier3
>  - [ ] tier4
>  - [ ] hotspot_gc

OK, I am approving this because `ObjectSynchronizer::stable_mark` is Lilliput-specific, and so changes there do not affect the upstream locking code (apart from Lilliput-specific paths).

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Marked as reviewed by shade (Committer).

PR: https://git.openjdk.java.net/lilliput/pull/25