Judge whether the current process is running when SMP is turned on

I found that there is a function called task_running in the kernel, and its judgment logic is as follows

static inline int task_current(struct rq *rq, struct task_struct *p){    return rq->curr == p;}static inline int task_running(struct rq *rq, struct task_struct *p){#ifdef CONFIG_SMP    return p->on_cpu;#else    return task_current(rq, p);#endif}

Is there any difference between rq->curr and p->on_cpu? I think they both mean that the process is being scheduled by the current cpu. Why separate judgments are required under SMP