From: Ingo Molnar <mingo@elte.hu>

I'd also suggest the following patch below, to clarify the use of
unsynchronized list_empty().  list_empty_careful() can only be safe in the
very specific case of "one-shot" list entries which might be removed by
another CPU.  (but nothing else can happen to them and this is their only
final state.) list_empty_careful() is otherwise completely unsynchronized
on both the compiler and CPU level and is not 'SMP safe' in any way.