RFR: 8300915: G1: incomplete SATB because nmethod entry barriers don't get armed

[Erik Österlund]

On Wed, 25 Jan 2023 13:32:27 GMT, Richard Reingruber <rrich at openjdk.org> wrote:

> The change makes sure all nmethod entry barriers are armed when a G1 concurrent marking cycle starts by doing it unconditionally in `G1ConcurrentMark::pre_concurrent_start()`. The barrier is needed to add oop constants to the SATB.
> 
> This alone would be a conservative/minimal fix but I didn't like that `CodeCache::is_gc_marking_cycle_active()` can return true after a marking cycle start is undone. I.e. `G1ConcurrentMarkThread::_state == Idle && CodeCache::is_gc_marking_cycle_active()` is possible.
> Edit: It felt like an inconsistency but maybe its actually ok not to finish the CC marking cycle when the G1 marking is undone. Please comment...
> The changes in `G1ConcurrentMark::post_concurrent_undo_start()` were made to avoid it.
> 
> 
> Testing
> 
> * Rebased this PR onto [JDK-8297487](https://bugs.openjdk.org/browse/JDK-8297487), replaced related assertions with guarantees, and then executed test/langtools:tier1 in a loop for 12h without seeing the issues reported in [JDK-8299956](https://bugs.openjdk.org/browse/JDK-8299956).
> 
> * CI testing at SAP: This includes most JCK and JTREG tiers 1-4, also in Xcomp mode, on the standard platforms and also on ppc64le.
> 
> * GHA: build failures because download of apache ant failed. `serviceability/jvmti/vthread/SuspendResumeAll/SuspendResumeAll.java` had a timeout on windows-x64. Should be unrelated. It succeeded in our CI testing.

Does this change consider the case when we do an initiating mark YC, but there were no old objects, and hence the "finished" part is never executed as CM is skipped? It isn't obvious to me that said case is handled correctly.

-------------

PR: https://git.openjdk.org/jdk/pull/12194