RFR: 8308881: Strong CLD oop handle roots are demoted to non-roots concurrently [v2]

[Erik Österlund]

On Fri, 26 May 2023 12:47:04 GMT, Erik Österlund <eosterlund at openjdk.org> wrote:

>> It is illegal to remove strong roots concurrently without clearing them first. A SATB collector with concurrent root scanning assumes that when strong roots disappear from the object graph, they are cleared first, which makes SATB notice the root. All global strong roots do this. Except CLD strong roots, which are turned into non-roots by decrementing the keep_alive counter to 0, when bootstrapping weak hidden class CLDs. This is not valid behaviour. This patch tries to treat these oops like we do any other global strong handles that are unlinked from the system: clear them when they stop being strong roots.
>
> Erik Österlund has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Remove null store and improve comments

And whether the GC observes the old or new value of the keep alive counter doesn't matter either, as at least one thread ensures the oop location gets processed. If the GC sees the counter as 0 in this race, the mutator will process the oop location as I described. And then at least one thread has processed it.

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PR Comment: https://git.openjdk.org/jdk/pull/14154#issuecomment-1564436428