第5章：并发与竞态条件-16：Bit Operations

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In continuation of the previous text 第5章：并发与竞态条件-15：Atomic Variables, let's GO ahead.

Bit Operations

The atomic_t type is good for performing integer arithmetic. It doesn’t work as well,
however, when you need to manipulate individual bits in an atomic manner. For that
purpose, instead, the kernel offers a set of functions that modify or test single bits
atomically. Because the whole operation happens in a single step, no interrupt (or
other processor) can interfere.

atomic_t 类型适用于整数算术运算，但当你需要以原子方式操作单个比特位时，它就不再适用了。针对这类场景，内核提供了一组函数，可原子化地修改或测试单个比特位 —— 由于整个操作在单个步骤中完成，任何中断（或其他处理器）都无法干扰操作过程。

Atomic bit operations are very fast, since they perform the operation using a single
machine instruction without disabling interrupts whenever the underlying platform
can do that. The functions are architecture dependent and are declared in <asm/
bitops.h>. They are guaranteed to be atomic even on SMP computers and are useful
to keep coherence across processors.

原子位操作的执行速度极快：只要底层硬件平台支持，这些操作会通过单条机器指令完成，且无需禁用中断。这类函数与架构强相关，声明在 <asm/bitops.h> 头文件中；它们能保证在 SMP（对称多处理）系统中仍具备原子性，可有效维持多处理器间的数据一致性。

Unfortunately, data typing in these functions is architecture dependent as well. The
nr argument (describing which bit to manipulate) is usually defined as int but is
unsigned long for a few architectures. The address to be modified is usually a pointer
to unsigned long, but a few architectures use void * instead.

需要注意的是，这些函数的参数类型也与架构相关：

nr 参数（指定要操作的比特位序号）通常定义为 int 类型，但部分架构下为 unsigned long；
待修改数据的地址参数，通常为指向 unsigned long 的指针，但少数架构中使用 void * 类型。

The available bit operations are:

// 将addr指向的数据中第nr位设为1
void set_bit(nr, void *addr);

/*
 * 将addr指向的unsigned long类型数据中第nr位清零。
 * 其他语义与set_bit一致。
 */
void clear_bit(nr, void *addr);

// 翻转addr指向数据中第nr位的值（0变1，1变0）
void change_bit(nr, void *addr);

/*
 * 该函数是唯一无需原子执行的位操作函数；
 * 仅返回addr指向数据中第nr位的当前值。
 */
test_bit(nr, void *addr);

// 原子化设置第nr位，并返回该位操作前的原值
int test_and_set_bit(nr, void *addr);
// 原子化清零第nr位，并返回该位操作前的原值
int test_and_clear_bit(nr, void *addr);
/*
 * 原子化翻转第nr位，语义与上述函数一致，
 * 同时返回该位操作前的原值。
 */
int test_and_change_bit(nr, void *addr);

When these functions are used to access and modify a shared flag, you don’t have to
do anything except call them; they perform their operations in an atomic manner.
Using bit operations to manage a lock variable that controls access to a shared vari-
able, on the other hand, is a little more complicated and deserves an example. Most
modern code does not use bit operations in this way, but code like the following still
exists in the kernel.

当使用这些函数访问 / 修改共享标志位时，只需直接调用即可 —— 它们本身已保证操作的原子性。但如果想用位操作实现 “控制共享变量访问的锁”，逻辑会稍复杂，以下是一个示例说明（尽管现代内核代码已很少这样用，但内核中仍存在此类遗留代码）。

A code segment that needs to access a shared data item tries to atomically acquire a
lock using either test_and_set_bit or test_and_clear_bit. The usual implementation is
shown here; it assumes that the lock lives at bit nr of address addr. It also assumes
that the bit is 0 when the lock is free or nonzero when the lock is busy.

若代码段需要访问共享数据，可通过 test_and_set_bit 或 test_and_clear_bit 原子化获取锁。典型实现逻辑如下：

假设锁对应 addr 地址的第 nr 位；
约定：比特位为 0 表示锁空闲，非 0 表示锁被占用。

/* 尝试获取锁 */
while (test_and_set_bit(nr, addr) != 0)
    wait_for_a_while(); // 锁被占用，等待

/* 执行临界区操作 */

/* 释放锁，并检查状态 */
if (test_and_clear_bit(nr, addr) == 0)
    something_went_wrong(); // 锁已被释放：出现错误

If you read through the kernel source, you find code that works like this example. It
is, however, far better to use spinlocks in new code; spinlocks are well debugged,