RFR: 8304824: NMT should not use ThreadCritical

[Gerard Ziemski]

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

Changes requested by gziemski (Committer).

src/hotspot/share/nmt/nmtCommon.hpp line 168:

> 166:       intx const current = os::current_thread_id();
> 167:       intx const owner = Atomic::load(&_owner);
> 168:       assert(current == owner, "NMT lock should be acquired in this section.");

How about we try to re-use `assert_locked()` in constructor and destructor, something like:


    NmtGuard() {
      assert(!assert_locked(), "Lock is not reentrant");
      _nmt_semaphore.wait();
      Atomic::store(&_owner, current);
    }

    ~NmtGuard() {
      assert(assert_locked(), "NMT lock should be acquired in this section.");
      Atomic::store(&_owner, (intx) -1);
      _nmt_semaphore.signal();
    }

    static bool assert_locked(){
      intx const current = os::current_thread_id();
      intx const owner = Atomic::load(&_owner);
      return (current == owner);
    }

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

PR Review: https://git.openjdk.org/jdk/pull/20852#pullrequestreview-2287095347
PR Review Comment: https://git.openjdk.org/jdk/pull/20852#discussion_r1747670483