RFR: 8247972: incorrect implementation of JVM TI GetObjectMonitorUsage [v9]

[David Holmes]

On Thu, 15 Feb 2024 19:55:08 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: JDWP monitor_info spec clarification; removed debugging code from objmonusage001

Thanks for the test updates.

A couple of other queries.

src/hotspot/share/prims/jvmtiEnvBase.cpp line 1496:

> 1494:   nWant = wantList->length();
> 1495: 
> 1496:   if (mon != nullptr) {

Shouldn't the call to `get_pending_threads` only happen if `mon != nullptr`? Otherwise the `wantList` has to be empty.

src/hotspot/share/prims/jvmtiEnvBase.cpp line 1500:

> 1498:     for (int i = 0; i < nWait; i++) {
> 1499:       if (waiter == nullptr || (i != 0 && waiter == mon->first_waiter())) {
> 1500:         // robustness: the waiting list has gotten smaller

We are at a safepoint so I don't see how the wait list can shrink. I initially thought perhaps a waiter could timeout, but the code that does the timed park is wrapped in ` ThreadBlockInVMPreprocess` which will block at a safepoint if one is active.

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

PR Review: https://git.openjdk.org/jdk/pull/17680#pullrequestreview-1884365884
PR Review Comment: https://git.openjdk.org/jdk/pull/17680#discussion_r1491992136
PR Review Comment: https://git.openjdk.org/jdk/pull/17680#discussion_r1491997307