// Why do we CAS a 0 into the mark-word instead of just CASing the // mark-word from the stack-locked value directly to the new inflated state? // Consider what happens when a thread unlocks a stack-locked object. // It attempts to use CAS to swing the displaced header value from the // on-stack basiclock back into the object header. Recall also that the // header value (hashcode, etc) can reside in (a) the object header, or // (b) a displaced header associated with the stack-lock, or (c) a displaced // header in an objectMonitor. The inflate() routine must copy the header // value from the basiclock on the owner's stack to the objectMonitor, all // the while preserving the hashCode stability invariants. If the owner // decides to release the lock while the value is 0, the unlock will fail // and control will eventually pass from slow_exit() to inflate. The owner // will then spin, waiting for the 0 value to disappear. Put another way, // the 0 causes the owner to stall if the owner happens to try to // drop the lock (restoring the header from the basiclock to the object) // while inflation is in-progress. This protocol avoids races that might // would otherwise permit hashCode values to change or "flicker" for an object. // Critically, while object->mark is 0 mark->displaced_mark_helper() is stable. // 0 serves as a "BUSY" inflate-in-progress indicator.
for (;;) { assert (THREAD == _owner, "invariant") ;
// Knob_ExitPolicy默认为0 if (Knob_ExitPolicy == 0) { // code 1：先释放锁，这时如果有其他线程进入同步块则能获得锁 OrderAccess::release_store_ptr (&_owner, NULL) ; // drop the lock OrderAccess::storeload() ; // See if we need to wake a successor // code 2：如果没有等待的线程或已经有假定继承人 if ((intptr_t(_EntryList)|intptr_t(_cxq)) == 0 || _succ != NULL) { TEVENT (Inflated exit - simple egress) ; return ; } TEVENT (Inflated exit - complex egress) ;
if (QMode == 3 && _cxq != NULL) { // 将cxq中的元素插入到EntryList的末尾 w = _cxq ; for (;;) { assert (w != NULL, "Invariant") ; ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; if (u == w) break ; w = u ; } assert (w != NULL , "invariant") ;
ObjectWaiter * q = NULL ; ObjectWaiter * p ; for (p = w ; p != NULL ; p = p->_next) { guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; p->TState = ObjectWaiter::TS_ENTER ; p->_prev = q ; q = p ; }
// Append the RATs to the EntryList // TODO: organize EntryList as a CDLL so we can locate the tail in constant-time. ObjectWaiter * Tail ; for (Tail = _EntryList ; Tail != NULL && Tail->_next != NULL ; Tail = Tail->_next) ; if (Tail == NULL) { _EntryList = w ; } else { Tail->_next = w ; w->_prev = Tail ; }
// Fall thru into code that tries to wake a successor from EntryList }
if (QMode == 4 && _cxq != NULL) { // 将cxq插入到EntryList的队首 w = _cxq ; for (;;) { assert (w != NULL, "Invariant") ; ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; if (u == w) break ; w = u ; } assert (w != NULL , "invariant") ;
ObjectWaiter * q = NULL ; ObjectWaiter * p ; for (p = w ; p != NULL ; p = p->_next) { guarantee (p->TState == ObjectWaiter::TS_CXQ, "Invariant") ; p->TState = ObjectWaiter::TS_ENTER ; p->_prev = q ; q = p ; }
// Prepend the RATs to the EntryList if (_EntryList != NULL) { q->_next = _EntryList ; _EntryList->_prev = q ; } _EntryList = w ;
// Fall thru into code that tries to wake a successor from EntryList }
w = _EntryList ; if (w != NULL) { // 如果EntryList不为空，则直接唤醒EntryList的队首元素 assert (w->TState == ObjectWaiter::TS_ENTER, "invariant") ; ExitEpilog (Self, w) ; return ; }
// EntryList为null，则处理cxq中的元素 w = _cxq ; if (w == NULL) continue ;
// 因为之后要将cxq的元素移动到EntryList，所以这里将cxq字段设置为null for (;;) { assert (w != NULL, "Invariant") ; ObjectWaiter * u = (ObjectWaiter *) Atomic::cmpxchg_ptr (NULL, &_cxq, w) ; if (u == w) break ; w = u ; } TEVENT (Inflated exit - drain cxq into EntryList) ;
