> -- Thread.new in finalizer raises SEGV
>
> The following code raises SEGV sometimes:
>
> $ ruby -e'ObjectSpace.define_finalizer(""){Thread.new{}}'
> $ ruby -e'ObjectSpace.define_finalizer(""){Thread.new{}}'
> SEGV received in SEGV handler

I think Thread.new in finalizer should raise argument error.
IOW, we shouldn't allow to create new thread in finalizer.
It's disaster. just my 2 cent.