关于异常和中断及调度的讨论(1), 感谢wheelz-wangxingchao2010-ChinaUnix博客

同事推荐了篇帖子，写得不错,基本上回答了“中断里不能睡眠”的问题。但”异常里可以睡眠 “，这个问题我还是不太懂。
相信再研究一段时间，中断异常的不同机制明白了，就懂了。

帖子地址

原帖子问题及回答：
中文是我的注释

On Tue, 2003-07-01 at 09:22, Martin Maletinsky wrote:

> I know, that you are not allowed to sleep in an interrupt handler.
> How ever I am interested to understand the reason(s) for that. Was
> this a design decision (e.g. to simplify things) for the Linux
> kernel, or is there a fundamental reason that makes sleeping
> interrupt handlers simply impossible? (是kernel design还是其它“根本”的原因)
What about the page fault
> handler - it (probably) sleeps when it has to swap in a page,
> why is it possible to sleep for the page fault handler and not for
> an interrupt handler?

You cannot sleep in an interrupt handler because interrupts do not have
a backing process context(我对这个backing process context的理解是，在由进程切换到中断环境时,已经保存的进程上下文环境与当前的中断环境相比,前者是可以满足进程切换的条件的，而后者不可以。中断的环境，姑且称之为中断上下文吧，相对进程上下文比较light,仅仅是些寄存器或其它环境变量，所以调度器无法在离开此种环境后再切换回来), and thus there is nothing to reschedule back
into. In other words, interrupt handlers are not associated with a task,
so there is nothing to "put to sleep" and (more importantly) "nothing to
wake up". They must run atomically.(上面这两句，就是我在之前帖子里提到的“睡不下去”和“醒不来”的问题了。当前的环境（中断环境--A），是由进程上下文--B--里被打断而形成的. A 类似于运行在B之上,而B又不是一个task,调度器无法识别之,要进行sleep进程切换时,nothing to put sleep。另外就是wake_up了。去唤醒谁呢？就算是从当前的A里切换出去了，调度器重新切换回来时,已经找不到A这种环境了。我在上面帖子里同样提到这个问题，找不到A可以找到B啊，为什么不稍做这样的处理，在A要sleep时,把A的环境同样也保存了，以进程B为代表去切换。实际上这样也有问题，原帖子举例说明了，比如spin lock,wait-queue等.总之，这是一个"费力不讨好"的事情。从kernel design的角度，这样做意义不大,麻烦不少)

This is not unlike other operating systems. In most operating systems,
interrupts are not threaded. Bottom halves often are, however.

The reason the page fault handler can sleep is that it is invoked only
by code that is running in process context. Because the kernel's own
memory is not pagable, only user-space memory accesses can result in a
page fault. Thus, only a few certain places (such as calls to
copy_{to,from}_user()) can cause a page fault within the kernel. Those
places must all be made by code that can sleep (i.e., process context,
no locks, et cetera).

这一段回复，需要再进一步研究。

Hope this helped.

Robert Love
大家可以查看原贴，查看其它回复

另外，最近kernel新增了中断线程化(interurpt thread),不知道跟这个有没有关系呢？

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