Conflicting use of StackWatermark in StackWalker vs GC?

[Roman Kennke]

Hello all,

When running StackWalker tests with 'aggressive' Shenandoah mode (i.e. 
run GCs all the time, even if there is no work), then I observe crashes 
like this:

#  Internal Error 
(/home/rkennke/src/openjdk/jdk/src/hotspot/share/runtime/stackWatermark.cpp:178), 
pid=549168, tid=549230
#  assert(is_frame_safe(f)) failed: Frame must be safe

Full hs_err:
http://cr.openjdk.java.net/~rkennke/hs_err_pid549168.log

I strongly suspect that this is happening because of StackWalker's use 
of StackWatermark which conflicts with the GC's own use of StackWalker. 
IOW, it asserts that the frame has been processed, but the GC is still 
on it.

Are we missing some coordination between StackWalker and the GC here?

It can be reproduced using:
CONF=linux-x86_64-server-fastdebug make run-test 
TEST=java/lang/StackWalker 
TEST_VM_OPTS="-XX:+UnlockExperimentalVMOptions -XX:+UseShenandoahGC 
-XX:ShenandoahGCHeuristics=aggressive"

Thanks,
Roman