RFR: 8366363: MemBaseline accesses VMT without using lock [v3]

[Johan Sjölen]

> Hi,
> 
> The `MemBaseline` used to access the VMT instance directly without a lock. We fix that, and we switch from using a `LinkedList` to a copied `RegionsTree` instead.

Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:

  Add misisng locks

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27003/files
  - new: https://git.openjdk.org/jdk/pull/27003/files/a125e582..800eca26

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27003&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27003&range=01-02

  Stats: 31 lines in 3 files changed: 16 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/27003.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27003/head:pull/27003

PR: https://git.openjdk.org/jdk/pull/27003