RFR: 8328866: Add raw monitor rank support to the debug agent. [v8]

[David Holmes]

On Fri, 17 May 2024 19:32:31 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> This PR adds ranked monitor support to the debug agent. The debug agent has a large number of monitors, and it's really hard to know which order to grab them in, and for that matter which monitors might already be held at any given moment. By imposing a rank on each monitor, we can check to make sure they are always grabbed in the order of their rank. Having this in place when I was working on [JDK-8324868](https://bugs.openjdk.org/browse/JDK-8324868) would have made it much easier to detect a deadlock that was occuring, and the reason for it. That's what motivated me to do this work
>> 
>> There were 2 or 3 minor rank issues discovered as a result of these changes. I also learned a lot about some of the more ugly details of the locking support in the process.
>> 
>> Tested with the following on all supported platforms and with virtual threads:
>> 
>> com/sun/jdi
>> vmTestbase/nsk/jdi
>> vmTestbase/nsk/jdb
>> vmTestbase/nsk/jdwp 
>> 
>> Still need to run tier2 and tier5.
>> 
>> Details of the changes follow in the first comment.
>
> Chris Plummer has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix a couple of minor typos in comments.

> Note we can't check if the current thread owns a lock without grabbing dbgRawMonitor. In fact the main purpose of dbgRawMonitor is to allow the current thread to check if it owns the monitor.

That smells fishy to me. A thread can always safely check if it owns something because it can't race with itself and get a wrong answer.

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PR Comment: https://git.openjdk.org/jdk/pull/19044#issuecomment-2119525715