RFR: 8269268: JDWP: Properly fix thread lookup assert in findThread() [v2]

[David Holmes]

On Thu, 24 Jun 2021 16:51:59 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> Re-enable the assert that was disabled (with some overhead) by [JDK-8265683](https://bugs.openjdk.java.net/browse/JDK-8265683). Explanation is in the CR and also in comments included with the changes.
>> 
>> I tested by running `vmTestbase/nsk/jdb/suspend/suspend001/suspend001.java` and `vmTestbase/nsk/jdb/wherei/wherei001/wherei001.java` 100's of times, and did not see any failures. I also verified the original issue was still reproducible by temporarily not setting `gdata->handlingVMDeath = JNI_TRUE`, which did trigger the assert as expected.
>
> Chris Plummer has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Rename flag and make some minor adjustments.

Just to follow up:

> As far as any other synchronizing or memory barriers being needed, I'm a bit unclear on that part. The only thing that matters is that gdata->jvmtiCallBacksCleared being set true is visible to other threads before the callbacks get cleared. It seems the call to SetEventCallbacks() should ensure this.

In practice this should be fine as `SetEventCallbacks` acquires a monitor/mutex; and `findThread` is also called while holding a monitor, so although different monitors, in practice the acquire/release semantics should make this work. Additionally the complexity of the interaction between the two threads involved will introduce other synchronization actions.

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PR: https://git.openjdk.java.net/jdk/pull/4580