Linux 内核学习笔记系列，GCC 扩展语法和内核数据结构部分，简单介绍 Linux 内核位图。

内核位图的使用

下面只列举常用的内核位图函数/宏，头文件为 include/linux/bitops.h。

创建位图

DECLARE_BITMAP(name,bits);

定义一个名称为 name 且至少包含 bits 位的位图。

设置位

void set_bit(unsigned int nr, volatile unsigned long *addr);

清除位

void clear_bit(int nr, volatile unsigned long *addr);

改变位

void change_bit(int nr, volatile unsigned long *addr);

测试并设置位

int test_and_set_bit(int nr, volatile unsigned long *addr);

若被测试位为 0，返回 0，反之返回非 0 值。

测试并清除位

int test_and_clear_bit(int nr, volatile unsigned long *addr);

若被测试位为 0，返回 0，反之返回非 0 值。

测试并改变位

int test_and_change_bit(int nr, volatile unsigned long *addr);

若被测试位为 0，返回 0，反之返回非 0 值。

测试位

int test_bit(int nr, const volatile unsigned long *addr);

若被测试位为 0，返回 0，反之返回非 0 值。

内核位图的实现

定义内核位图的宏位于 include/linux/types.h。内核位操作的实现分为两部分，体系结构无关的实现位于 include/linux/bitops.h，x86 体系结构相关的部分位于 arch/x86/include/asm/bitops.h。

创建位图的实现

include/linux/types.h：

#define DECLARE_BITMAP(name,bits) \
        unsigned long name[BITS_TO_LONGS(bits)]

include/linux/bitops.h：

#define BITS_PER_BYTE     8
#define BITS_TO_LONGS(nr) DIV_ROUND_UP(nr, BITS_PER_BYTE * sizeof(long))

辅助宏的实现

#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 1)
/* Technically wrong, but this avoids compilation errors on some gcc
   versions. */
#define BITOP_ADDR(x) "=m" (*(volatile long *) (x))
#else
#define BITOP_ADDR(x) "+m" (*(volatile long *) (x))
#endif

#define ADDR                            BITOP_ADDR(addr)

/*
 * We do the locked ops that don't return the old value as
 * a mask operation on a byte.
 */
#define IS_IMMEDIATE(nr)                (__builtin_constant_p(nr))
#define CONST_MASK_ADDR(nr, addr)       BITOP_ADDR((void *)(addr) + ((nr)>>3))
#define CONST_MASK(nr)                  (1 << ((nr) & 7))

IS_IMMEDIATE 宏判断 nr 是否是整数常量。如果把位图看成 8 列的二维数组，那么 CONST_MASK_ADDR 宏负责计算行号，CONST_MASK 宏负责计算列号。

设置位的实现

arch/x86/include/asm/bitops.h：

/**
 * set_bit - Atomically set a bit in memory
 * @nr: the bit to set
 * @addr: the address to start counting from
 *
 * This function is atomic and may not be reordered.  See __set_bit()
 * if you do not require the atomic guarantees.
 *
 * Note: there are no guarantees that this function will not be reordered
 * on non x86 architectures, so if you are writing portable code,
 * make sure not to rely on its reordering guarantees.
 *
 * Note that @nr may be almost arbitrarily large; this function is not
 * restricted to acting on a single-word quantity.
 */
static __always_inline void
set_bit(unsigned int nr, volatile unsigned long *addr)
{
    if (IS_IMMEDIATE(nr)) {
        asm volatile(LOCK_PREFIX "orb %1,%0"
            : CONST_MASK_ADDR(nr, addr)
            : "iq" ((u8)CONST_MASK(nr))
            : "memory");
    } else {
        asm volatile(LOCK_PREFIX "bts %1,%0"
            : BITOP_ADDR(addr) : "Ir" (nr) : "memory");
    }
}

LOCK_PREFIX 的实现原理涉及锁，见 TODO。

查阅相关手册，可以知道这里 q 修饰符代表使用 8 位寄存器（例如 al、bl 等）。

bts 指令（Bit Test and Set）是位测试并置位指令，此处 bts %1,%0 即先测试 %0 内存处的 %1 位的值，存入 CF 标志寄存器，再将该位置 1。

清除位的实现

/**
 * clear_bit - Clears a bit in memory
 * @nr: Bit to clear
 * @addr: Address to start counting from
 *
 * clear_bit() is atomic and may not be reordered.  However, it does
 * not contain a memory barrier, so if it is used for locking purposes,
 * you should call smp_mb__before_clear_bit() and/or smp_mb__after_clear_bit()
 * in order to ensure changes are visible on other processors.
 */
static __always_inline void
clear_bit(int nr, volatile unsigned long *addr)
{
    if (IS_IMMEDIATE(nr)) {
        asm volatile(LOCK_PREFIX "andb %1,%0"
            : CONST_MASK_ADDR(nr, addr)
            : "iq" ((u8)~CONST_MASK(nr)));
    } else {
        asm volatile(LOCK_PREFIX "btr %1,%0"
            : BITOP_ADDR(addr)
            : "Ir" (nr));
    }
}

btr 指令（Bit Test and Reset）是位测试并复位指令。

改变位的实现

/**
 * change_bit - Toggle a bit in memory
 * @nr: Bit to change
 * @addr: Address to start counting from
 *
 * change_bit() is atomic and may not be reordered.
 * Note that @nr may be almost arbitrarily large; this function is not
 * restricted to acting on a single-word quantity.
 */
static inline void change_bit(int nr, volatile unsigned long *addr)
{
    if (IS_IMMEDIATE(nr)) {
        asm volatile(LOCK_PREFIX "xorb %1,%0"
            : CONST_MASK_ADDR(nr, addr)
            : "iq" ((u8)CONST_MASK(nr)));
    } else {
        asm volatile(LOCK_PREFIX "btc %1,%0"
            : BITOP_ADDR(addr)
            : "Ir" (nr));
    }
}

btc 指令（Bit Test and Complement）是位测试并取反指令。

测试并设置位的实现

/**
 * test_and_set_bit - Set a bit and return its old value
 * @nr: Bit to set
 * @addr: Address to count from
 *
 * This operation is atomic and cannot be reordered.
 * It also implies a memory barrier.
 */
static inline int test_and_set_bit(int nr, volatile unsigned long *addr)
{
    int oldbit;

    asm volatile(LOCK_PREFIX "bts %2,%1\n\t"
             "sbb %0,%0" : "=r" (oldbit), ADDR : "Ir" (nr) : "memory");

    return oldbit;
}

这里很巧妙地借助了 sbb 指令执行带借位减法。如果 CF 标志被置位，函数返回 -1，反之返回 0。

测试并清除位的实现

/**
 * test_and_clear_bit - Clear a bit and return its old value
 * @nr: Bit to clear
 * @addr: Address to count from
 *
 * This operation is atomic and cannot be reordered.
 * It also implies a memory barrier.
 */
static inline int test_and_clear_bit(int nr, volatile unsigned long *addr)
{
    int oldbit;

    asm volatile(LOCK_PREFIX "btr %2,%1\n\t"
             "sbb %0,%0"
             : "=r" (oldbit), ADDR : "Ir" (nr) : "memory");

    return oldbit;
}

测试并改变位的实现

/**
 * test_and_change_bit - Change a bit and return its old value
 * @nr: Bit to change
 * @addr: Address to count from
 *
 * This operation is atomic and cannot be reordered.
 * It also implies a memory barrier.
 */
static inline int test_and_change_bit(int nr, volatile unsigned long *addr)
{
    int oldbit;

    asm volatile(LOCK_PREFIX "btc %2,%1\n\t"
             "sbb %0,%0"
             : "=r" (oldbit), ADDR : "Ir" (nr) : "memory");

    return oldbit;
}

测试位的实现

static __always_inline int constant_test_bit(unsigned int nr, const volatile unsigned long *addr)
{
    return ((1UL << (nr % BITS_PER_LONG)) &
        (((unsigned long *)addr)[nr / BITS_PER_LONG])) != 0;
}

static inline int variable_test_bit(int nr, volatile const unsigned long *addr)
{
    int oldbit;

    asm volatile("bt %2,%1\n\t"
             "sbb %0,%0"
             : "=r" (oldbit)
             : "m" (*(unsigned long *)addr), "Ir" (nr));

    return oldbit;
}

#if 0 /* Fool kernel-doc since it doesn't do macros yet */
/**
 * test_bit - Determine whether a bit is set
 * @nr: bit number to test
 * @addr: Address to start counting from
 */
static int test_bit(int nr, const volatile unsigned long *addr);
#endif

#define test_bit(nr, addr)               \
    (__builtin_constant_p((nr))          \
     ? constant_test_bit((nr), (addr))   \
     : variable_test_bit((nr), (addr)))

bt 指令（Bit Test）是位测试指令。