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

[Erik Österlund]

> 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

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Changes:
  - all: https://git.openjdk.org/jdk/pull/14154/files
  - new: https://git.openjdk.org/jdk/pull/14154/files/b75fce32..7aa800be

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=14154&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=14154&range=00-01

  Stats: 29 lines in 1 file changed: 19 ins; 3 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/14154.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14154/head:pull/14154

PR: https://git.openjdk.org/jdk/pull/14154