RFR: 8304824: NMT should not use ThreadCritical [v5]

[David Holmes]

On Tue, 17 Sep 2024 22:07:40 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 with a new target base due to a merge or a rebase. The pull request now contains five commits:
> 
>  - Merge master
>  - replace tty in pd_release_memory while holding NMT lock.
>  - Switch to using a Hotspot Mutex.
>  - Comments. Hide assertion behind DEBUG. Replace MemoryFileTracker locker. make reentrant.
>  - Replace ThreadCritical with a specific NMT lock

Changes requested by dholmes (Reviewer).

src/hotspot/os/windows/os_windows.cpp line 3785:

> 3783:       log_warning(os)("bad release: [" PTR_FORMAT "-" PTR_FORMAT "): %s", p2i(start), p2i(end), err);
> 3784: #ifdef ASSERT
> 3785:       fileStream fs(stderr);

tty generally prints to stdout, but now this prints to stderr. I guess it doesn't really matter as we are about to crash anyway.

src/hotspot/share/runtime/mutexLocker.hpp line 197:

> 195:     _mutex(mutex) {
> 196:     bool no_safepoint_check = flag == Mutex::_no_safepoint_check_flag;
> 197:     if (_mutex != nullptr && Thread::current_or_null() != nullptr) {

I do not like this addition to the mutex locker code. First, it probably needs to be `Thread::current_or_null_safe` in case we ever lock in a signal-handling context. But second this slows down all mutex uses. Instead `NMTLocker` should extend ConditionalMutexLocker and make the current thread check the condition.

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

PR Review: https://git.openjdk.org/jdk/pull/20852#pullrequestreview-2311303401
PR Review Comment: https://git.openjdk.org/jdk/pull/20852#discussion_r1764175522
PR Review Comment: https://git.openjdk.org/jdk/pull/20852#discussion_r1764239104