Re: RFR: 8319244: implement JVMTI handshakes support for virtual threads [v4]

[Serguei Spitsyn]

On Thu, 16 Nov 2023 16:27:23 GMT, Serguei Spitsyn <sspit...@openjdk.org> wrote:

>> So the problematic case I'm thinking is when the JvmtiVTMSTransitionDisabler 
>> starts after the vthread executed notifyJvmtiUnmount(), i.e the vthread is 
>> already outside the transition, but before changing the state to TERMINATED. 
>> JvmtiVTMSTransitionDisabler will proceed, and since the carrierThread field 
>> has already been cleared we will treat it as an unmounted vthread. Then we 
>> can see first that is alive in JvmtiHandshake::execute() but then we could 
>> hit the assert that is already TERMINATED in JvmtiEnvBase::get_vthread_jvf().
>
> Thanks! This is a valid concern. Will try to fix this.

I'm suggesting to fix it this way for the unmounted case only:

@@ -1976,6 +1976,13 @@ JvmtiHandshake::execute(JvmtiUnitedHandshakeClosure* 
hs_cl, ThreadsListHandle* t
       return;
     }
     if (target_jt == nullptr) {    // unmounted virtual thread
+      // JvmtiVTMSTransitionDisabler can start after the vthread executed 
notifyJvmtiUnmount(), i.e.
+      // the vthread is already outside the transition, but before changing 
the state to TERMINATED.
+      // Changing the state to TERMINATED is racy, so we check if the 
continuation is done in advance.
+      oop cont = java_lang_VirtualThread::continuation(target_h());
+      if (jdk_internal_vm_Continuation::done(cont)) {
+        return;
+      }
       hs_cl->do_vthread(target_h); // execute handshake closure callback on 
current thread directly
     }
   }

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

PR Review Comment: https://git.openjdk.org/jdk/pull/16460#discussion_r1396084391