RFR (S) 8247593: Shenandoah: should not block pacing reporters

[Roman Kennke]

Ok looks good.

Roman

On Mon, 2020-06-15 at 16:38 +0200, Aleksey Shipilev wrote:
> Bug:
>   https://bugs.openjdk.java.net/browse/JDK-8247593
> 
> After JDK-8247358, we are acquiring the _wait_monitor in
> ShenandoahPacer::report_internal. That runs
> into potential deadlocks with threads that are waiting on the same
> lock *and* safepointing at the
> same time, against the concurrent workers that want to report the
> progress before returning for
> subsequent safepoint.
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8247593/webrev.01/
> 
> Testing: hotspot_gc_shenandoah (some tests timed out before); ad-hoc
> benchmarks
>