RFR: 8057586: Explicit GC ignored if GCLocker is active [v6]

[Ivan Walulya]

> Hi All,
> 
> Please review this change to guarantee that at least a Full GC is executed between the invocation and return of `System.gc` or `WhiteBox.fullGC`, even if the call is concurrent with an active `GCLocker `. 
> 
> The change should also handle the issues reported in JDK-8299276.
> 
> Split into 3 commits, one commit for changes to each GC in [G1, Parallel, Serial]. 
> 
> Testing: Tier 1-5.

Ivan Walulya has updated the pull request incrementally with one additional commit since the last revision:

  Albert review

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13191/files
  - new: https://git.openjdk.org/jdk/pull/13191/files/005dc086..5fc0125f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13191&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13191&range=04-05

  Stats: 6 lines in 3 files changed: 0 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/13191.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13191/head:pull/13191

PR: https://git.openjdk.org/jdk/pull/13191