why don't unlock_buffer call directly clear_and_wake_up_bit?

The unlock_buffer and clear_and_wake_up_bit call same the three function in the same sequence: 1.clear_bit_unlock 2.smp_mb__after_atomic 3.wake_up_bit So why don't unlock_buffer write the fllowing: void unlock_buffer(struct buffer_head *bh) { clear_and_wake_up_bit(BH_Lock,&bh->b_state); }