RFR: JDK-8275704: Metaspace::contains() should be threadsafe

[Thomas Stuefe]

Metaspace::contains() is used in many places. It is not threadsafe since it walks the list of metaspace mappings, which can be altered concurrently. This is suspected to be the cause of JDK-8271124.

Currently, it does not lock, and adding a lock is not realistic either. It should work lockless.

This patch builds atop of https://bugs.openjdk.java.net/browse/JDK-8275582, which removed the old (pre JEP 387) technique of uncommitting metaspace memory. As a side effect, that patch changed the mapping list to an add-only structure. The only remaining place where it gets modified is in VirtualSpaceList::create_node(). Modifications are synchronized via lock. The only place where we walk the list locklessly is in Metaspace::contains(). This patch adds the appropriate memory barriers to those two places.

Tests:
- GHAs
- SAP nightlies (queued)

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Commit messages:
 - make metaspace contains threadsafe

Changes: https://git.openjdk.java.net/jdk/pull/6060/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6060&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8275704
  Stats: 23 lines in 2 files changed: 15 ins; 1 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6060.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6060/head:pull/6060

PR: https://git.openjdk.java.net/jdk/pull/6060