RFR: 8328866: Add raw monitor rank support to the debug agent. [v6]

[Alex Menkov]

On Thu, 9 May 2024 06:10:04 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:
> 
>   Flip rank order. Some cleanup and better comments for verifyMonitorRank().

src/jdk.jdwp.agent/share/native/libjdwp/util.c line 1145:

> 1143:  */
> 1144: 
> 1145: static jrawMonitorID dbgRawMonitor;

As the monitor is used to synchronize access to dbg_monitors array, maybe rename it to something like dbg_monitors_lock?

src/jdk.jdwp.agent/share/native/libjdwp/util.c line 1276:

> 1274: 
> 1275: static void
> 1276: assertIsCurrentThread(JNIEnv *env, jthread thread, jthread current_thread)

The function gets both threads as arguments, I think `assertIsSameThread` would be more correct name

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PR Review Comment: https://git.openjdk.org/jdk/pull/19044#discussion_r1605494862
PR Review Comment: https://git.openjdk.org/jdk/pull/19044#discussion_r1605498977