RFR: 8328866: Add raw monitor rank support to the debug agent. [v7]

[Kevin Walls]

On Thu, 16 May 2024 15:31:15 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:
> 
>   Simplify by getting rid of special logic around leaf monitors.

src/jdk.jdwp.agent/share/native/libjdwp/threadControl.c line 1781:

> 1779:  * popFrameEventLock. Meanwhile when the target thread gets the SINGLE_STEP event,
> 1780:  * it always enters popFrameProceedLock first (which is always available), then
> 1781:  * popFrameProceedLock second. It will always succeed because the Reader thread

Is that "then popFrameEventLock second"

Drawing these out in two columns I can't see a deadlock either 8-)

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PR Review Comment: https://git.openjdk.org/jdk/pull/19044#discussion_r1603777908