8340547: Starting many threads can delay safepoints

[snazarkin]

I'd like to backport this fix to fix the issue with starting a lot of threads in a burst.

Despite the low priority of this issue, some users have found this to be a barrier to migrating from JDK8.

The backport is not clean as it needs to adjust globals.hpp and replace ConditionalMutexLocker with classic MutexLocker. The rank nonleaf of UseThreadsLockThrottleLock has been chosen to be as close as possible to the rank of Threads_lock and to not cause a crash in debug mode:

#  Internal Error (/home/ubuntu/jdk17u-dev/src/hotspot/share/runtime/mutex.cpp:398), pid=1777, tid=1782
#  assert(false) failed: Attempting to acquire lock Threads_lock/22 out of order with lock ThreadsLockThrottle_lock/22 -- possible deadlock

Field _allow_vm_block is set to false to prevent

#  Internal Error (/home/ubuntu/jdk17u-dev/src/hotspot/share/runtime/thread.cpp:981), pid=56846, tid=56851
#  assert(false) failed: Possible safepoint reached by thread that does not allow it

Similar PR is on review at jdk21
Original fix and discussion are openjdk/jdk#21111

Tested with tier1 (release), tier2(fastdebug), and reproducers from JDK-8340547, JDK-8307970

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
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• Commit message must refer to an issue
• JDK-8340547 needs maintainer approval

Issue

• JDK-8340547: Starting many threads can delay safepoints (Enhancement - P4)

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[bridgekeeper]

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[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[openjdk]

This backport pull request has now been updated with issue from the original commit.

[mlbridge]

Webrevs

• 00: Full (23719c2a)

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