Integrated: 8345750: Shenandoah: Test TestJcmdHeapDump.java#aggressive intermittent assert(gc_cause() == GCCause::_no_gc) failed: Over-writing cause

[William Kemper]

On Fri, 17 Jan 2025 23:12:14 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This occurs when an attempt to produce a heap dump conflicts with a concurrent cycle. The heap dump vm operation attempts to run a cycle directly on the VM thread with no regard for the state of the concurrent collection. Although Shenandoah _is_ technically capable of running an entire _full_ GCs on the vm thread, I would rather not add another dependency on full GCs, nor would I like to cancel an in-progress concurrent GC. Instead, Shenandoah will follow the pattern established ZGC in which the calling thread waits for a concurrent cycle to complete before taking the heap dump.
> 
> ## Testing
> 
> Shenandoh's jtreg test: `gc/shenandoah/TestJcmdHeapDump.java` simply takes a heap dump of itself. Before this change I observed a failure rate of approximately 1/6000 executions. The assertion is violated when the VM thread attempts to run a collection which is not coordinated by Shenandoah's control thread. This specific assertion happens because the default implementation of `collect_as_vm_thread` modifies the `_gc_cause` field in ways that Shenandoah does not expect. Without this assertion, one can only imagine the chaos that would ensue.

This pull request has now been integrated.

Changeset: 6a29a811
Author:    William Kemper <wkemper at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/6a29a8110ec38b4adc8163ba8651cbc935353f1d
Stats:     23 lines in 4 files changed: 15 ins; 1 del; 7 mod

8345750: Shenandoah: Test TestJcmdHeapDump.java#aggressive intermittent assert(gc_cause() == GCCause::_no_gc) failed: Over-writing cause

Reviewed-by: kdnilsen, ysr

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PR: https://git.openjdk.org/jdk/pull/23186