Understanding Linux Network Internals 1.2.3

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1.2.3. Reference Counts
When a piece of code tries to access a data structure that has already been freed, the kernel is not very happy, and the user is rarely happy with the kernel's reaction. To avoid those nasty problems, and to make garbage collection mechanisms easier and more effective (see the section "Garbage Collection" later in this chapter), most data structures keep a reference count. Good kernel citizens increment and decrement the reference count of every data structure every time they save and release a reference, respectively, to the structure. For any data structure type that requires a reference count, the kernel component that owns the structure usually exports two functions that can be used to increment and decrement the reference count. Such functions are usually called xxx_hold and xxx_release, respectively. Sometimes the release function is called xxx_put instead (e.g., dev_put for net_device structures).
1.2.3. 叁考计数
当一块代码试着存取被释放的数据结构的时候，内核不是很快乐，而且用户很少地对内核的反应感到满意。避免那些脏问题,去构造垃圾收集机制是比较容易而且是更有效的 (请见 " 垃圾收集 " 本章节后面的部分) 大多数的数据结构保存一个叁考计数。优秀的内核成员逐渐增加或者渐减每个数据结构的叁考计数 ,每次他们保存或释放叁考，对结构。对于任何的数据结构类型所需要叁考计数,通常的内核成员拥有结构输出二种功能去递增和渐减叁考计数。 分别地,这样功能通常被称为 xxx_hold 和 xxx_release 。 有时释放功能也可以用 xxx_put 代替 。 (例如, dev_put 对于 net_device 结构)
While we like to assume there are no bad citizens in the kernel, developers are human, and as such they do not always write bug-free code. The use of the reference count is a simple but effective mechanism to avoid freeing still-referenced data structures. However, it does not always solve the problem completely. This is the consequence of forgetting to balance increments and decrements:
我们喜欢设想在内核中没有不健康的部分，但是开发者是人的，同样地他们不总是写出没有bug的代码。 叁考计数的使用作为一个简单而有效的机制-避免释放静止的叁考数据结构。 然而，它不是总能完美地解决问题。 这是忽略了平衡递增和渐减的结果:
If you release a reference to a data structure but forget to call the xxx_release function, the kernel will never allow the data structure to be freed (unless another buggy piece of code happens to call the release function an extra time by mistake!). This leads to gradual memory exhaustion.
如果你发布关于数据结构的参考但是忘记了声明 xxx_release 函数, 内核将不会释放这个数据结构 (除非发生其他更多的 bug 代码区块这个发布的函数才会被声明在特别的时间通过错误的形式!)。 这将导致存储空间逐渐的耗尽。
If you take a reference to a data structure but forget to call xxx_hold, and at some later point you happen to be the only reference holder, the structure will be prematurely freed because you are not accounted for. This case definitely can be more catastrophic than the previous one; your next attempt to access the structure can corrupt other data or cause a kernel panic that brings down the whole system instantly.
如果你提取关于数据结构的参考但是忘记声明 xxx_hold, 而且在稍后指出 , 你恰好是唯一的叁考持有人, 因为你没有说明，所以结构将会过早的被释放。 这种情形一定是更灾难的超过前面的; 你接下来尝试接近结构可能会破坏其他的数据或者引起内核错误是整个系统崩溃。
When a data structure is to be removed for some reason, the reference holders can be explicitly notified about its going away so that they can politely release their references. This is done through notification chains. See the section "Reference Counts" in Chapter 8 for an interesting example.
当数据结构因为一些原因被移动, 叁考持有人能明确地被通知到有关它离开的情况以便他们能很斯文的（politely）释放他们的叁考。 通过通知链来完成。一个有趣的例子在第 8 章中的 " 叁考计数 " 。
The reference count on a data structure typically can be incremented when:
在数据结构上的叁考计数代表性被增加:
There is a close relationship between two data structure types. In this case, one of the two often maintains a pointer initialized to the address of the second one.
二种数据结构类型之间有紧密的关系。在这情况,其中一个经常会维持一个指针到第二个类型的住址去设定初值。
A timer is started whose handler is going to access the data structure. When the timer is fired, the reference count on the structure is incremented, because the last thing you want is for the data structure to be freed before the timer expires.
管理者要存取数据结构会开启一个定时器。当定时器被开启的时候，叁考计数在结构上被增加, 最后一件事，在定时器期满之前，你需要释放这个数据结构。
A successful lookup on a list or a hash table returns a pointer to the matching element. In most cases, the returned result is used by the caller to carry out some task. Because of that, it is common for a lookup routine to increase the reference count on the matching element, and let the caller release it when necessary.
一次成功的查询在一个列表或者 Hash表中返回一个指针到匹配的要素。在大多数情况下，送回的结果被访客去实行一些工作。所以，一个查询常式在相配要素上增加叁考计数, 而且让访客释放它通常是必需的。
When the last reference to a data structure is released, it may be freed because it is not needed anymore, but not necessarily.
当一个数据结构的最后参考被发布, 它可能被释放, 因为它再也不被需要 ，但是也不是必然的。
The introduction of the new sysfs filesystem has helped to make a good portion of the kernel code more aware of reference counts and consistent in its use of them.
新 sysfs 文件系统介绍到，它已经帮助创建部分好的内核代码，使得更多的一致的来意识到参考计数被它所应用。


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