When is to preempt safe?

Hi, Preemption has two cases: user preemption and kernel preemption. I have tow questions about them. 1. According to Love, "If the kernel is returning to user-space, it knows it is in a safe quiescent state. In other words, if it is safe to continue executing the current task, it is also safe to pick a new task to execute." What's the meaning of user preemption's safety? How can we deduce safety of schedule from the current task going on? 2. Another statement from Love is that "the kernel can preempt a task running in the kernel so long as it does not hold a lock". Why is it not safe while kernel code holding lock?