RFR: 8346123: [REDO] NMT should not use ThreadCritical [v13]

[David Holmes]

On Wed, 8 Jan 2025 16:38:50 GMT, Robert Toyonaga <duke at openjdk.org> wrote:

>> This is a redo of [JDK-8304824](https://bugs.openjdk.org/browse/JDK-8304824) which was backed out by [JDK-8343726](https://bugs.openjdk.org/browse/JDK-8343726) due to problems documented in [JDK-8343244](https://bugs.openjdk.org/browse/JDK-8343244).
>> 
>> The problem was that `NmtVirtualMemoryLocker` was not locking when the current thread is not attached by checking `Thread::current_or_null_safe() != nullptr`. This is necessary during VM init, but should not be allowed afterward. NMT may be used in `attach_current_thread` before the current thread is set. The lock was not being acquired in that case, which intermittently caused NMT accounting to become corrupted, triggering various assertions when future NMT operations are done.  To fix this, I've adopted [Thomas' suggestion](https://github.com/openjdk/jdk/pull/21928#issuecomment-2460238057) to reverse the order of 
>> 
>> 
>> thread->register_thread_stack_with_NMT();
>> thread->initialize_thread_current();
>> 
>> 
>> in `attach_current_thread`.  This allows `NmtVirtualMemoryLocker` to be locked after current thread is set. 
>> 
>> To allow for `NmtVirtualMemoryLocker` to be used during VM init, I've replaced the `ConditionalMutexLocker` check `Thread::current_or_null_safe() != nullptr` with a new flag: `_done_bootstrap`. This flag prevents the lock from being used during VM init, at which point we are single threaded anyway. This avoids errors due to Hotspot mutexes and current thread not yet being ready. 
>> 
>> I also added new asserts in `virtualMemoryTracker.cpp` to catch future bugs like this where the lock is not held when it should be. I updated the appropriate VMT tests to also lock (there were a few cases where locking was being bypassed) so they can pass the new asserts.
>> 
>> I also removed the unused `_query_lock` variable in `MemTracker`.
>> 
>> Testing: 
>> 
>> - On Linux amd64, I was able to consistently reproduce the errors described in [JDK-8343244](https://bugs.openjdk.org/browse/JDK-8343244) by increasing the number of test threads in `java/lang/Thread/jni/AttachCurrentThread/AttachTest.java`. The test consistently passes with the new changes in this PR.
>> - hotspot_nmt , gtest:VirtualSpace, tier1
>
> Robert Toyonaga has updated the pull request incrementally with one additional commit since the last revision:
> 
>   use Threads::is_single_threaded()

Thanks for the update but the `is_single_threaded()` check can be done more efficiently.

src/hotspot/share/runtime/threads.hpp line 159:

> 157:     }
> 158:     return count <= 1;
> 159:   }

Suggestion:

  // VM initialization is single-threaded until the first NJT (WatcherThread) is created
  // by the main thread, but that thread disappears during VM exit so we can't just
  // check for it. To reduce the overhead we check for > 1 JavaThreads first.
  static bool is_single_threaded() {
    if (number_of_threads() > 1) {
        return false;
    }
    NonJavaThread::Iterator njti;
    return njti->current() == nullptr;
  }

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

Changes requested by dholmes (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/22745#pullrequestreview-2538579113
PR Review Comment: https://git.openjdk.org/jdk/pull/22745#discussion_r1908054366