C++ 为什么在4.9.37版的内核中，kfio还需要smp_wmb呢

内核lib/kfifo.c中有两个段码，如下:

#define kfifo_in_spinlocked(fifo, buf, n, lock) \
({ \
    unsigned long __flags; \
    unsigned int __ret; \
    spin_lock_irqsave(lock, __flags); \
    __ret = kfifo_in(fifo, buf, n); \
    spin_unlock_irqrestore(lock, __flags); \
    __ret; \
})

static void kfifo_copy_in(struct __kfifo *fifo, const void *src,
    unsigned int len, unsigned int off) {
    unsigned int size = fifo->mask + 1;
    unsigned int esize = fifo->esize;
    unsigned int l;

    off &= fifo->mask;
    if (esize != 1) {
        off *= esize;
        size *= esize;
        len *= esize;
    }
    l = min(len, size - off);

    memcpy(fifo->data + off, src, l);
    memcpy(fifo->data, src + l, len - l);
    /*
     * make sure that the data in the fifo is up to date before
     * incrementing the fifo->in index counter
     */
    smp_wmb();
}

kfifo_in_spinlocked中的函数kfifo_in最终会调用kfifo_copy_in函数，我的问题是，为什么还要使用spin_lock_irqsave,smp_wmb？CPU获得自旋锁使其他CPU停顿，因此它可以独占地修改变量和数据地址，当它解锁时，其他CPU可以获得最新的数据和数据地址？为什么需要smp_wmb？