Convert warning: "Thread holding lock at safepoint that vm can block on: ..." into assertion?

[Vladimir Ivanov]

Hi,

I don't see much value in having the following warning in 
Thread::check_for_valid_safepoint_state:
src/share/vm/runtime/thread.cpp:
    933           warning("Thread holding lock at safepoint that vm can 
block on: %s", cur->name());

It's very easy to miss such warnings during testing, but they may 
manifest real problems which are hard to trigger/diagnose in the field.

What do people think about converting corresponding check into 
assertion? It is already present only in non-product builds.

Best regards,
Vladimir Ivanov