[jdk21u-dev] RFR: 8340547: Starting many threads can delay safepoints [v4]

[Sergey Nazarkin]

> I'd like to backport this fix to fix the issue with starting a lot of threads in a burst. Some users found this to be a barrier to migrating from JDK8. 
> 
> The backport is not clean as required to adjust `globals.hpp` and replace `ConditionalMutexLocker` with classic `MutexLocker`. Original fix and discussion are [here](https://github.com/openjdk/jdk/pull/21111)
> 
> Tested with `tier1` and reproducers from [JDK-8340547](https://bugs.openjdk.org/browse/JDK-8340547), [JDK-8307970](https://bugs.openjdk.org/browse/JDK-8307970)

Sergey Nazarkin has updated the pull request incrementally with one additional commit since the last revision:

  Add extra space to outline new option

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Changes:
  - all: https://git.openjdk.org/jdk21u-dev/pull/1365/files
  - new: https://git.openjdk.org/jdk21u-dev/pull/1365/files/7794dc7d..2a9dda33

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1365&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1365&range=02-03

  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1365.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1365/head:pull/1365

PR: https://git.openjdk.org/jdk21u-dev/pull/1365