RFR: 8364314: java_lang_Thread::get_thread_status fails assert(base != nullptr) failed: Invalid base [v2]

[Daniel D. Daugherty]

On Wed, 30 Jul 2025 06:24:39 GMT, David Holmes <dholmes at openjdk.org> wrote:

>> After the changes in  JDK-8361912 we could "return " the carrier thread from `cv_internal_thread_to_JavaThread`, but before we hit the transition disabler the virtual thread could unmount. As a result when we execute this code:
>> 
>>   if (is_virtual) {
>>     // 1st need to disable mount/unmount transitions
>>     transition_disabler.init(jthread);
>> 
>>     carrier_thread = Handle(THREAD, java_lang_VirtualThread::carrier_thread(thread_h()));
>>     if (carrier_thread != nullptr) {
>>       java_thread = java_lang_Thread::thread(carrier_thread());
>>     }
>>   }
>> 
>> we hit the implicit else where "`carrier_thread == nullptr`" and we do nothing, but `java_thread` still holds the old carrier, which we then perform the handshake operation with:
>> 
>>   void do_thread(Thread* th) override {
>>     Thread* current = Thread::current();
>> 
>>     bool is_virtual = java_lang_VirtualThread::is_instance(_thread_h());
>>     if (_java_thread != nullptr) {
>>       if (is_virtual) {
>>         // mounted vthread, use carrier thread state
>>         oop carrier_thread = java_lang_VirtualThread::carrier_thread(_thread_h());
>>         _thread_status = java_lang_Thread::get_thread_status(carrier_thread);
>>       } else {
>> 
>> But the `_java_thread` no longer has a carrier, so `get_thread_status` is passed null and we crash.
>> 
>> Simple fix is to clear `java_thread` when we find a null carrier oop. Also added an assert to guard against a null carrier oop in the handshake code, and added some additional commentary.
>> 
>> Testing:
>>  - com/sun/management/HotSpotDiagnosticMXBean/DumpThreads.java
>>  -  tier 5 and 6
>> 
>> Thanks
>
> David Holmes has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:
> 
>  - Remove from ProblemList
>  - Merge branch 'master' into 8364314-threadSMR
>  - 8364314: java_lang_Thread::get_thread_status fails assert(base != nullptr) failed: Invalid base

For the calls to `tlh.cv_internal_thread_to_JavaThread` in src/hotspot/share/prims/jvm.cpp,
the new logic added by:
[JDK-8361912](https://bugs.openjdk.org/browse/JDK-8361912) ThreadsListHandle::cv_internal_thread_to_JavaThread does not deal with a virtual thread's carrier thread

will allow the carrier JavaThread to be returned to the caller.
However, NONE of the callers in jvm.cpp use a JvmtiVTMSTransitionDisabler
to prevent the virtual thread from being unmounted from the
carrier thread. So at the time of the remainder of the logic in
the JVM calls, the `JavaThread* receiver` may be a stale
carrier thread that no longer has a virtual thread mounted.

I don't know if this is an issue or not.

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

PR Comment: https://git.openjdk.org/jdk/pull/26544#issuecomment-3137198268