This is an expensive operation because every live object might be moved, and every pointer to a moved object must be updated to the new location.
这是一项昂贵的操作，因为可能会移动每个活动对象，每个经过移动的对象的指针都必须更新为新位置。
2
Rather than wait for the other thread to finish, the current thread can "help" it out by finishing the operation for it by moving the tail pointer forward (b).
不必等候其他线程完成，当前线程就可以“帮助”它完成操作，把尾指针向前移动(B)。
3
You use the access_ok macro to validate a user-space pointer for a given operation.