RFR: 8247972: incorrect implementation of JVM TI GetObjectMonitorUsage [v22]

[David Holmes]

On Tue, 5 Mar 2024 09:04:08 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> The loop is endless without this extra condition, so we are getting a test execution timeout.
>> The `waiters` seems to be `circular doubly linked list` as we can see below:
>> 
>> inline void ObjectMonitor::AddWaiter(ObjectWaiter* node) {
>>   assert(node != nullptr, "should not add null node");
>>   assert(node->_prev == nullptr, "node already in list");
>>   assert(node->_next == nullptr, "node already in list");
>>   // put node at end of queue (circular doubly linked list)
>>   if (_WaitSet == nullptr) {
>>     _WaitSet = node;
>>     node->_prev = node;
>>     node->_next = node;
>>   } else {
>>     ObjectWaiter* head = _WaitSet;
>>     ObjectWaiter* tail = head->_prev;
>>     assert(tail->_next == head, "invariant check");
>>     tail->_next = node;
>>     head->_prev = node;
>>     node->_next = head;
>>     node->_prev = tail;
>>   }
>> }
>> 
>> I'll make sure nothing is missed and check the old code again.
>
> Okay. Now I see why the old code did not have endless loops.

Apologies I should have checked the code - I'd forgotten the list is circular to avoid needing an explicit tail pointer.

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PR Review Comment: https://git.openjdk.org/jdk/pull/17680#discussion_r1519058800