kernel 源码中的 ACCESS_ONCE()

ACCESS_ONCE()也是一个宏，宏定义如下：

#define ACCESS_ONCE(x) (*(volatile typeof(x) *)&(x))

它还有一堆注释，不仔细看的话会很令人困惑，先放在这里，下面再解释：

/*
* Prevent the compiler from merging or refetching accesses. The compiler
* is also forbidden from reordering successive instances of ACCESS_ONCE(),
* but only when the compiler is aware of some particular ordering. One way
* to make the compiler aware of ordering is to put the two invocations of
* ACCESS_ONCE() in different C statements.
*
* This macro does absolutely -nothing- to prevent the CPU from reordering,
* merging, or refetching absolutely anything at any time. Its main intended
* use is to mediate communication between process-level code and irq/NMI
* handlers, all running on the same CPU.
*/

这个宏看起来很奇怪，先取地址，再类型转换，最后又还原成数据。这个宏用来干什么呢？简单点解释就是防止编译器优化代码，它的关键就在于volatile关键字，请看下面的代码段（arch/x86/include/asm/spinlock.h）：

do {    if (ACCESS_ONCE(lock->tickets.head) == inc.tail)        goto out;    cpu_relax();} while (--count);

如果不使用ACCESS_ONCE()宏，代码就会变成：

do {    if (lock->tickets.head == inc.tail)        goto out;    cpu_relax();} while (--count);

由于编译器进行编译时并不会考虑多线程的环境，因此认为在这个循环中lock->tickets.head的值不会发生改变，也就没必要没一次循环都访问一次，因此可能会优化成这个样子：

head = lock->tickets.head;do {    if (head == inc.tail)        goto out;    cpu_relax();} while (--count);

这样子就可能会出现问题，因为在多线程环境下，lock->tickets.head的值是有可能被其他线程修改的，因此每次循环都访问一次是有必要的。volatile关键字告诉编译器不要做出上面那样的优化，而要每一次循环都访问一次内存。
Linus在一封邮件中认为这样做还能防止另一种形式的优化，将：

    if (a > MEMORY) {        do1;        do2;        do3;    } else {        do2;    }

优化成：

    if (a > MEMORY)        do1;    do2;    if (a > MEMORY)        do3;

如果do2部分对a做了修改，就有可能出现问题。但是我认为Linus多虑了，我觉得编译器的设计者和维护者并不会进行如此激进的优化，这种优化很明显在单线程环境下也是可能出现问题的。
至于网上说的volatile关键字是为了防止处理器从缓存中得到错误的数据就大错特错了，任何设计正确的计算机系统都应该能确认cache中是有效数据还是无效数据。
再引用上http://lwn.net/Articles/233479/的一段话：

Your editor’s copy of The C Programming Language, Second Edition (copyright 1988, still known as “the new C book”) has the following to say about the volatile keyword:

The purpose of volatile is to force an implementation to suppress optimization that could otherwise occur. For example, for a machine with memory-mapped input/output, a pointer to a device register might be declared as a pointer to volatile, in order to prevent the compiler from removing apparently redundant references through the pointer.

最后回到开头的注释，上面已经讲得比较多了，注释中的第一段话说到：

Prevent the compiler from merging or refetching accesses.

第二段话又说：

This macro does absolutely -nothing- to prevent the CPU from reordering, merging, or refetching absolutely anything at any time.

所以说，volatile只会对编译器的行为有影响，并不会对CPU产生直接影响，在汇编代码中是不可能看到volatile的。它只在编译期产生作用，在运行期不起作用。
另一篇参考资料：http://lwn.net/Articles/508991/。