RFR: 8304824: NMT should not use ThreadCritical [v9]

[Robert Toyonaga]

On Mon, 28 Oct 2024 16:12:39 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 two additional commits since the last revision:
> 
>  - add a comment explaining lock rank
>  - remove unnecessary dropping of tracking level

Also, those uses of `ThreadCritical` are not protecting NMT's model of virtual memory. They are only protecting the reporting of malloc info specifically.  Although, it may still make sense to replace those instances with `NmtVirtualMemory_lock` since it's is still related to NMT.    

What do you think @tstuefe?

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PR Comment: https://git.openjdk.org/jdk/pull/20852#issuecomment-2447247092