[jdk17u-dev] RFR: 8319048: Monitor deflation unlink phase prolongs time to safepoint [v2]

[Aleksey Shipilev]

> Unclean backport to eliminate another safepoint hiccup caused by monitor deflation. The uncleanliness comes mostly from `NULL` vs `nullptr` differences in affected hunks. I chose to accept the new version that uses `nullptr` consistently, even though some `NULL`-s might have been left alone.
> 
> I was concerned that we might still have problems with interleaving with end-of-VM monitor deflation. This was one of the minor reasons to do [JDK-8319896](https://bugs.openjdk.org/browse/JDK-8319896), but I see that we only do that in `VMThread` when exiting, so we are not actually at risk here.
> 
> Additional testing:
>   - [x] Linux x86_64 server fastdebug, `all` tests pass

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge branch 'master' into JDK-8319048-monitor-deflation-unlink
 - Backport efc392259c64986bbbe880259e95b09058b9076a

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

Changes:
  - all: https://git.openjdk.org/jdk17u-dev/pull/2309/files
  - new: https://git.openjdk.org/jdk17u-dev/pull/2309/files/c15e247e..59b388ad

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2309&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2309&range=00-01

  Stats: 26782 lines in 832 files changed: 8402 ins; 15877 del; 2503 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2309.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2309/head:pull/2309

PR: https://git.openjdk.org/jdk17u-dev/pull/2309