RFR: 8304824: NMT should not use ThreadCritical [v6]

[Thomas Stuefe]

On Fri, 20 Sep 2024 18:11:51 GMT, Robert Toyonaga <duke at openjdk.org> wrote:

>> ### Summary
>> This PR just replaces `ThreadCritical` with a lock specific to NMT.  `ThreadCritical` is a big lock and is unnecessary for the purposes of NMT. I've implemented the new lock with a semaphore so that it can be used early before VM init.  There is also the possibility of adding assertions in places we expect NMT to have synchronization. I haven't added assertions yet in many places because some code paths such as the (NMT tests)  don't lock yet. I think it makes sense to close any gaps in locking in another PR in which I can also add more assertions. 
>> 
>> Testing:
>> - hotspot_nmt
>> - gtest:VirtualSpace
>> - tier1
>
> Robert Toyonaga has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Inherit from ConditionalMutexLocker. Use current_or_null_safe instead of current_or_null.

src/hotspot/share/runtime/mutexLocker.hpp line 261:

> 259: // Same as MutexLocker but can be used during VM init.
> 260: // Performs no action if given a null mutex or with detached threads.
> 261: class NMTMutexLocker: public ConditionalMutexLocker {

Good, but either call this something generic (e.g. "SafeMutexLocker") or move this to the nmt subdir.

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PR Review Comment: https://git.openjdk.org/jdk/pull/20852#discussion_r1770421628