RFR: 8247972: incorrect implementation of JVM TI GetObjectMonitorUsage [v4]

[David Holmes]

On Sat, 10 Feb 2024 04:06:37 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

>> The implementation of the JVM TI `GetObjectMonitorUsage` does not match the spec.
>> The function returns the following structure:
>> 
>> 
>> typedef struct {
>>     jthread owner;
>>     jint entry_count;
>>     jint waiter_count;
>>     jthread* waiters;
>>     jint notify_waiter_count;
>>     jthread* notify_waiters;
>> } jvmtiMonitorUsage;
>> 
>> 
>> The following four fields are defined this way:
>> 
>> waiter_count  [jint] The number of threads waiting to own this monitor
>> waiters  [jthread*] The waiter_count waiting threads
>> notify_waiter_count  [jint]  The number of threads waiting to be notified by this monitor
>> notify_waiters  [jthread*] The notify_waiter_count threads waiting to be notified
>> 
>> The `waiters` has to include all threads waiting to enter the monitor or to re-enter it in `Object.wait()`.
>> The implementation also includes the threads waiting to be notified in `Object.wait()` which is wrong.
>> The `notify_waiters` has to include all threads waiting to be notified in `Object.wait()`.
>> The implementation also includes the threads waiting to re-enter the monitor in `Object.wait()` which is wrong.
>> This update makes it right.
>> 
>> The implementation of the JDWP command `ObjectReference.MonitorInfo (5)` is based on the JVM TI `GetObjectMonitorInfo()`. This update has a tweak to keep the existing behavior of this command.
>> 
>> The follwoing JVMTI vmTestbase tests are fixed to adopt to the `GetObjectMonitorUsage()` correct behavior:
>> 
>>   jvmti/GetObjectMonitorUsage/objmonusage001
>>   jvmti/GetObjectMonitorUsage/objmonusage003
>> 
>> 
>> The following JVMTI JCK tests have to be fixed to adopt to correct behavior:
>> 
>> vm/jvmti/GetObjectMonitorUsage/gomu001/gomu00101/gomu00101.html
>> vm/jvmti/GetObjectMonitorUsage/gomu001/gomu00101/gomu00101a.html
>> vm/jvmti/GetObjectMonitorUsage/gomu001/gomu00102/gomu00102.html
>> vm/jvmti/GetObjectMonitorUsage/gomu001/gomu00102/gomu00102a.html
>> 
>> 
>> 
>> A JCK bug will be filed and the tests have to be added into the JCK problem list located in the closed repository.
>> 
>> Also, please see and review the related CSR:
>>  [8324677](https://bugs.openjdk.org/browse/JDK-8324677): incorrect implementation of JVM TI GetObjectMonitorUsage
>> 
>> The Release-Note is:
>> [8325314](https://bugs.openjdk.org/browse/JDK-8325314): Release Note: incorrect implementation of JVM TI GetObjectMonitorUsage
>>  
>> Testing:
>>  - tested with mach5 tiers 1-6
>
> Serguei Spitsyn has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review: fixed issues in get_object_monitor_usage; extended test coverage in objmonusage003

Sorry really struggling to understand this now. We have gone from a simple miscalculation to apparently doing everything wrong.

IIUC this API does not currently handle virtual threads correctly -i s that the case? If so I would like to see that fix factored out and done separately before this fix is done. Thanks.

src/hotspot/share/runtime/javaThread.cpp line 196:

> 194: oop JavaThread::vthread_or_thread() const {
> 195:     oop result = vthread();
> 196:     if (result == nullptr) {

Is that really sufficient? If so why do we have `is_vthread_mounted` which checks the continuation?

test/hotspot/jtreg/vmTestbase/nsk/jvmti/GetObjectMonitorUsage/objmonusage003.java line 31:

> 29: 
> 30:     final static int JCK_STATUS_BASE = 95;
> 31:     final static int NUMBER_OF_ENTERER_THREADS = 4;

Nit: ENTERING not ENTERER

test/hotspot/jtreg/vmTestbase/nsk/jvmti/GetObjectMonitorUsage/objmonusage003.java line 33:

> 31:     final static int NUMBER_OF_ENTERER_THREADS = 4;
> 32:     final static int NUMBER_OF_WAITER_THREADS  = 4;
> 33:     final static int NUMBER_OF_THREADS = NUMBER_OF_ENTERER_THREADS + NUMBER_OF_WAITER_THREADS;

This testing is not sufficient. We have three states of interest:
- entering the monitor directly
- waiting in the monitor via Object.wait
- re-rentering the monitor after being notified (or timed-out or interrupted)
- we need to check all of these conditions in permutations covering zero and non-zero for each one, and then see that we get the correct counts back from JVM TI.

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

PR Review: https://git.openjdk.org/jdk/pull/17680#pullrequestreview-1876986745
PR Review Comment: https://git.openjdk.org/jdk/pull/17680#discussion_r1487299437
PR Review Comment: https://git.openjdk.org/jdk/pull/17680#discussion_r1487304912
PR Review Comment: https://git.openjdk.org/jdk/pull/17680#discussion_r1487311675