RFR: 8319896: Remove monitor deflation from final audit [v3]

[Coleen Phillimore]

On Thu, 16 Nov 2023 20:32:48 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> In [JDK-8318757](https://bugs.openjdk.org/browse/JDK-8318757) we can see that running a monitor deflation pass during a safepointed operation can interleave with the async monitor deflation, which causes various problems. The fix for that is to stop deflating monitors in the thread dump operation and instead only collect relevant monitors.
>> 
>> There is yet another place where we call monitor deflation from outside of the monitor deflation thread. That place is the "final audit" part, which walks over monitors and performs verification and logging just before we exit the JVM. Before the walk over the list of monitors we perform a monitor deflation pass to prune the system from "uninteresting" monitors.
>> 
>> I propose that we remove the monitor deflation from the final audit, and that we instead only visit "interesting" monitors (those that have an owner or "is busy"). After this change it's only the monitor deflation thread that performs monitor deflation. It is unclear to me if the final audit can actually interleave with the async monitor deflation, but this removal makes it easier to reason around monitor deflation since it is only one thread that is performing it.
>> 
>> This PR is dependent on #16519. I intend to run extended testing on both of these PRs over the weekend.
>
> Stefan Karlsson has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Update comments

src/hotspot/share/runtime/synchronizer.cpp line 1624:

> 1622:   size_t unlinked_count = 0;
> 1623:   size_t deleted_count = 0;
> 1624:   if (deflated_count > 0) {

There's a conditional at line 1631 is_monitor_deflation_thread, and an else at 1657 that deflating idle monitors is at a safepoint.  Should there not be this conditional, ie am I reading the description right?

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PR Review Comment: https://git.openjdk.org/jdk/pull/16605#discussion_r1396505808