RFR (XS/T) 8232176: Shenandoah: new assert in ShenandoahEvacuationTask is too strong

[Roman Kennke]

Hmm, ok.

Roman


> Recent regression:
>   https://bugs.openjdk.java.net/browse/JDK-8232176
> 
> JDK-8231947 added the assert in ShenandoahEvacuationTask that is too strong. There is a corner case
> when the region is collection-set-pinned (CSP), and the oom-evac-protocol waits for GC thread to
> complete the evacuation. There is a short window where GC thread can see the CSP region before
> seeing cancellation request.
> 
> It seems easier to remove the too strong assert for now. is_conc_move_allowed() == true is a lie
> right now. We can add cancelled_gc() check inside of it, but that would only be safe if we know that
> caller holds oom-evac-scope.
> 
> The assertion failure reliably reproduces with -XX:ShenandoahGCHeuristics=aggressive
> -XX:+ShenandoahOOMDuringEvacALot on SPECjvm2008.
> 
> Fix:
>   https://cr.openjdk.java.net/~shade/8232176/webrev.01/
> 
> Testing: broken tests
>