ldd tips

关于机制和策略(mechanism and policy)

The distinction between mechanism and policy is one of the best ideas behind the Unix design. Most programming problems can indeed be split into two parts: “what capabilities are to be provided” (the mechanism) and “how those capabilities can be used” (the policy). If the two issues are addressed by different parts of the program, or even by different programs altogether, the software package is much easier to develop and to adapt to particular needs.

长久以来，我都把这个概念与接口和实现(interface and implementation)混淆在一起，傻傻分不清楚。谁谁谁(乔治~康师傅？)说过，如果能把一个概念向你隔壁的花朵或者大妈解释清楚，那么你就是真的理解了。好吧，我试试吧：

拿锤子举个例吧，你捡到一个锤子，这就是你的”命运之锤“，你的”机制“，拿来干嘛呢？你可以用锤子砸核桃，砸冰箱(老罗？)，或者表演杂技(档部碎大石？)。这就是策略。换个角度，锤子提供锤东西的能力可以说是个接口，而具体锤子是用铁做的，铜做的，还是用钛合金做的，就是实现。

关于抢占式(preemptible)和可重入(reentrant)

Finally, in 2.6, kernel code has been made preemptible; this change causes even uniprocessor systems to have many of the same concurrency issues as multiprocessor systems.

As a result, Linux kernel code, including driver code, must be reentrant – it must be capable of running in more than one context at the same time. Data structures must be carefully designed to keep multiple threads of execution separate, and the code must take care to access shared data in ways that prevent corruption of the data.

关于内核模块编译

内核使用一套自己的Makefile系统(Kbuild)来生成内核映像和模块二进制文件，可能是目前最复杂的makefile系统了吧。makefile 的写法与应用程序的写法也大不相同。但是文本配置起来都非常简单(更不用提各种可视化配置界面了)，就几条规则:

持久编译进内核映像
```
 obj-y += obj1.o obj2.o
```
编译成模块二进制文件(*.ko)
```
 obj-m += obj-m1.o obj-m2.o
```
不进行编译
```
 obj-n += obj-n1.o obj-n2.o
```
多个目标文件链接成一个
```
 obj-y += abc.o
 abc-y := a.o b.o c.o
```
利用Kconfig定制功能
```
 obj-$(CONFIG_FOO) += foo.o
```
当运行 make menuconfig或类似的可视化配置界面时，其实就是在配置一个一个的CONFIG_XXX，正如你所想的，一般来说三种值，y|m|n 。配置好后将在顶层目录生成 .config文件供后面编译参考。
自动切换目录
```
 obj-y += power/
```
这里加入的不是文件，而是目录，意思是切换到 power/ 目录，并处理其中的 Makefile。

掌握以上六点差不多你就掌握了内核源码中大部分的 Makefile 了，就能随心所欲得定制内核功能了。是不是很简单:)

对于源码树外的自己编写的内核模块的话，makefile 会稍许复杂。典型的(节选自 kernel/Documentation/kbuild/modules.txt):

...
ifneq ($(KERNELRELEASE),)
# kbuild part of makefile
obj-m  := hello.o

else
# normal makefile
KDIR ?= /lib/modules/$(shell uname -r)/build
PWD := $(shell pwd)

default:
	$(MAKE) -C $(KDIR) M=$(PWD) modules

endif

当我们运行make时，这个makefile会被读取两次。第一次，变量 KERNELRELEASE 还没有设置，所以会执行 else 内的代码块。 KDIR指定内核源码路径，PWD指定当前路径，即模块源码路径。我们来简单分析下命令 $(MAKE) -C $(KDIR) M=$(PWD) modules:

$(MAKE)是makefile的系统变量，即make命令。
-C $(KDIR)是make的一个选项，意思是在读取makefile之前，把当前路径切换到 KDIR下，那么此时载入的makefile就是内核源码顶层目录的Makefile，而不再是模块目录下的了。
M=$(PWD)其实是内核源码主 Makefile 里的一个对外部模块的支持选项 (External module support).make M=dir modules的功能就是联编 dir 目录下的所有模块。

所以，合起来看，这条命令就是切换至内核源码顶层目录，载入主 Makefile，联编模块目录下的所有指定模块。这个联编过程即，第二次载入模块 Makefile，注意，主 Makefile 内定义了 KERNELRELEASE 变量，这次就执行obj-m += hello.o命令了。你看，不就是源码树内的模块编译规则吗？

今天在 Hacker News 上看到一篇写 twitter，facebook，youtube 的 humble Beginnings [h]，挺有意思的(youtube 最初居然是个约炮网站。。现在微信拍马赶上。。呃扯远了)。最后一节提到：

Simple in terms of product mind you, not necessarily technically.

……

Technical difficulties though, were in service of product simplicity.

Kbuild 系统这么复杂，但是其目的不正是为了给我们提供简便而又强大的功能吗？

最后两段话讲了创业的事，我文笔拙劣就不翻译了：

The key is to find a market with a small and easily definable problem to solve, but also a market large enough to accomodate further growth and disruption. Solving a specific problem will get your foot in the door, giving you leverage to throw it right open later down the line.

Being flexible is much more important than knowing the end game. Just come in, build something small, and grow it one step at a time. Everything starts from humble beginnings.

Published at 17 December 2012

Tags ldd 1 kernel 2

ldd tips tips, quotes, sentences

关于机制和策略(mechanism and policy)

关于抢占式(preemptible)和可重入(reentrant)

关于内核模块编译