RFR: Region sampling should lock while gathering region data

[Christine Flood]

This looks fine to me.

On Fri, Dec 1, 2017 at 5:56 AM, Aleksey Shipilev <shade at redhat.com> wrote:
> http://cr.openjdk.java.net/~shade/shenandoah/sampling-lock/webrev.01/
>
> Another corner case discovered when testing humongous moves: when Full GC is in full swing, heap
> region states might be inconsistent. It fails the asserts in get_live() that searches for humongous
> start, and cannot find it when Full GC moves humongous. Full GC does lock the heap when changing
> heap region states, and thus we can lock the heap sampler too, to always get consistent view of the
> heap.
>
> Testing: hotspot_gc_shenandoah, Visualizer runs
>
> Thanks,
> -Aleksey
>