Assert during stackwalk

[Roman Kennke]

Hello again,

I think I have now implemented the proper parts for Shenandoah GC in 
ShHeap::requires_barriers() now (will send PR shortly) and now I'm 
hitting another problem: the Fuzz.java test throws an assertion at me 
(when running in Shenandoah aggressive mode, which does a *lot* of GC work):

#  Internal Error 
(/home/rkennke/src/openjdk/loom/src/hotspot/share/runtime/stackWatermark.cpp:178), 
pid=54418, tid=54534
#  assert(is_frame_safe(f)) failed: Frame must be safe

The full hs_err:
http://cr.openjdk.java.net/~rkennke/hs_err_pid54418.log

It looks to me that the stackwalk uses the stack watermark mechanism, 
and this conflicts with Shenandoah's own use of the stack watermark to 
concurrently update thread roots during the evacuation phase. Am I 
missing some necessary coordination there?

(Also, is this all Loom tests, or should I also run other tests too?

make run-test TEST=java/lang/Continuation 
TEST_VM_OPTS="-XX:+UnlockDiagnosticVMOptions -XX:+UseShenandoahGC 
-XX:ShenandoahGCHeuristics=aggressive"

)

Thanks,
Roman