RFR: 8304824: NMT should not use ThreadCritical

[Thomas Stuefe]

On Wed, 4 Sep 2024 14:17:08 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

I plan to look at this start of next week.

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

PR Comment: https://git.openjdk.org/jdk/pull/20852#issuecomment-2333532290