CODEWITHSUNDEEP
javamultithreadingjvmjvm-hotspot
St.Antario
St.Antario

Reputation: 27435

Understanding notify method in HotSpot

I was trying to understand how notify wakes up a thread and faced some misunderstanding about implementation details in hotspot (jdk8).

We have wait/notify declared as a native methods inside Object and they are implemented here: wait and notify. Since static int Knob_MoveNotifyee = 2 ; I expected that the following code is responsible for performing wake up:

 if (Policy == 2) {      // prepend to cxq
     // prepend to cxq
     if (List == NULL) {
         iterator->_next = iterator->_prev = NULL ;
         _EntryList = iterator ;
     } else {
        iterator->TState = ObjectWaiter::TS_CXQ ;
        for (;;) {
            ObjectWaiter * Front = _cxq ;
            iterator->_next = Front ;
            if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
                break ;
            }
        }
     }
 }

But the thing is the method void ObjectWaiter::notify is wrapped into Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify"); /Thread::SpinRelease (&_WaitSetLock) ;.

  1. Why do we have CAS when prepending a dequeued thread from waiting queue to cxq? There seems no contention there since we acquired _WaitSetLock already.

  2. Who does modify the JavaThread state? We have iterator->wait_reenter_begin(this); at the end of void ObjectWaiter::notify, but this does not do set_thread_status(java_thread, java_lang_Thread::RUNNABLE); like in wait_reenter_end

Upvotes: 2

Views: 201

Answers (1)

apangin
apangin

Reputation: 98580

  1. _WaitSetLock protects _WaitSet only (the list of threads called wait on that object monitor). Meanwhile _cxq can be concurrently accessed not only from notify, but also from other functions, in particular, ObjectMonitor::enter and exit.

  2. notify does not modify the thread status. The status changes when the target thread leaves JVM_MonitorWait. It is done by JavaThreadInObjectWaitState destructor which implicitly calls ~JavaThreadStatusChanger().

Upvotes: 2

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