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

Ok so there shouldn't be reentrancy if we use the `NmtVirtualMemory_lock` around `os::free`. But we also free in `ChunkPool::prune()` :

>> I'm not certain, but looking at it again, it seems that the ThreadCritical uses in ChunkPool::deallocate_chunk and ChunkPool::prune() are only needed for NMT and are independent of the other ThreadCritical uses in that code.

>At least for prune, that's needed for the chunk pool itself as it would otherwise be accessed concurrently.

So that means we'd need to have both `ThreadCritical` and `NmtVirtualMemory_lock` in that method (if we were to do the other replacements). One to protect the chunks and one to protect the malloc accounting.  It might also be good to rename `NmtVirtualMemory_lock` then too.

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

PR Comment: https://git.openjdk.org/jdk/pull/20852#issuecomment-2447631841