RFR: 8212879: Make JVMTI TagMap table concurrent [v11]

[Kim Barrett]

On Wed, 18 Nov 2020 19:31:44 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

> test/hotspot/jtreg/vmTestbase/nsk/jvmti/AttachOnDemand/attach021/attach021Agent00.cpp:
> 
> The change below is not needed as the call to nsk_jvmti_aod_disableEventAndFinish() does exactly the same:
> 
> ```
> -    nsk_jvmti_aod_disableEventAndFinish(agentName, JVMTI_EVENT_OBJECT_FREE, success, jvmti, jni);
> +
> +    /* Flush any pending ObjectFree events, which may set success to 1 */
> +    if (jvmti->SetEventNotificationMode(JVMTI_DISABLE,
> +                                        JVMTI_EVENT_OBJECT_FREE,
> +                                        NULL) != JVMTI_ERROR_NONE) {
> +        success = 0;
> +    }
> +
> +    nsk_aod_agentFinished(jni, agentName, success);
>  }
> ```

This change really is needed.

The success variable in the test is a global, initially 0, set to 1 by the
ObjectFree handler.

In the old code, if the ObjectFree event hasn't been posted yet, we pass the
initial 0 value of success to nsk_jvmti_aod_disabledEventAndFinish, where
it's a local variable (so unaffected by any changes to the variable in the
test), so stays 0 through to the call to nsk_aod_agentFinished.  So the test
fails. 

The split in the change causes the updated post-ObjectFree event success
value of 1 to be passed to agentFinished.  So the test passes.

That required some head scratching to find at the time.  That's the point of
the comment about flushing pending events.  Maybe the comment should be
improved.

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

PR: https://git.openjdk.java.net/jdk/pull/967