RFR: 8286490: JvmtiEventControllerPrivate::set_event_callbacks CLEARING_MASK computation is incorrect

[Serguei Spitsyn]

On Tue, 24 May 2022 05:45:17 GMT, Serguei Spitsyn <sspitsyn at openjdk.org> wrote:

> It was a typo when the original CLEARING_MASK was initially introduced:
> 
> 
>  // Mask to clear normal event bits.
>   const jlong CLEARING_MASK = (1L >> (TOTAL_MIN_EVENT_TYPE_VAL - TOTAL_MIN_EVENT_TYPE_VAL)) - 1L;
>   // Avoid cleaning extension event bits.
>   jlong enabled_bits = CLEARING_MASK & env->env_event_enable()->_event_callback_enabled.get_bits();
> 
> ``` 
> 
> The first TOTAL_MIN_EVENT_TYPE_VAL has to be JVMTI_MIN_EVENT_TYPE_VAL as below:
> `  const jlong CLEARING_MASK = (1L >> (JVMTI_MIN_EVENT_TYPE_VAL - TOTAL_MIN_EVENT_TYPE_VAL)) - 1L;`
> 
> Let me provide some background to understand how this mask is used.
> 
> Normal JVMTI event types are numbered from 50 to 88. There are two constants:
> 
>   JVMTI_MIN_EVENT_TYPE_VAL = 50
>   JVMTI_MAX_EVENT_TYPE_VAL = 88
> 
> The extension event types are numbered from 47 to 49. There are also two constants:
> 
>   EXT_MIN_EVENT_TYPE_VAL = 47
>   EXT_MAX_EVENT_TYPE_VAL = 49
> 
> 
> There are also two additional constants:
> 
>   TOTAL_MIN_EVENT_TYPE_VAL = 47
>   TOTAL_MAX_EVENT_TYPE_VAL = 88
> 
> 
> The `enabled_bits` are shifted left by 47, so that the 0-bit in `enabled_bits` is for first extension event type. And the first normal JVMTI event type is numbered 3 in the `enabled_bits` bit mask. The `CLEARING_MASK` is used to clear only normal JVMTI event types but keep the extension event bits (0-2) unchanged.
> 
> Testing:
> I'll run all JVMTI tests including nsk.jvmti tests and serviceability/jvmti tests including those added in Loom for virtual thread coverage.

The extension events are not much used in the tests. There was only one extension event inn pre-jdk19 which is ClassUnload. This event was implemented to prove the event extension mechanism is working but it is is not really used anywhere except in our testing.
Two new extension events were introduced to support testing of virtual threads:
- VirtualThreadMount
- VirtualThreadUnmount

This particular issue depends on the order the extension events and normal JVMTI events are enabled.

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

PR: https://git.openjdk.java.net/jdk/pull/8860