Linux内核源码(asm/atomic.h)学习

由于现在正在看Linux下的内核同步方法，其中第一个提到的就是原子变量，其中会有原子操作．其中原子变量被定义在linux/types.h头文件中，在这一篇博客中，主要学习原子操作，这些原子操作的函数被定义在asm/atomic.h文件中，其中包括，初始化，原子读，原子更改等操作，下面我们来看一下内核源码，其中，有我的一些注释，这个是比较简单的，因为，该原子变量的操作是由体系结构的指令操作的．只因为他支持这样的原子操作，才使得使用起来比较简单．

话不多说，直接上代码：

/* * Generic C implementation of atomic counter operations * Originally implemented for MN10300. * * Copyright (C) 2007 Red Hat, Inc. All Rights Reserved. * Written by David Howells ([email protected]) * * This program is free software; you can redistribute it and/or * modify it under the terms of the GNU General Public Licence * as published by the Free Software Foundation; either version * 2 of the Licence, or (at your option) any later version. * * 该程序是自由软件；你可以重新发布它或者是在GNU协议下修改它． 　* 该程序实现了一些原子操作的函数． */
#ifndef __ASM_GENERIC_ATOMIC_H
#define __ASM_GENERIC_ATOMIC_H

#ifdef CONFIG_SMP
#error not SMP safe
#endif

/* * Atomic operations that C can't guarantee us. Useful for * resource counting etc.. 　* C不能保证给我们的原子操作．对于资源计数等是有用的． */

/* * 初始化atomic_t的值 　* 举例： 　* atomic_t v = ATOMIC_INIT(0); //定义v并把它初始化为０ */
#define ATOMIC_INIT(i)  { (i) }

#ifdef __KERNEL__

/** * atomic_read - read atomic variable * @v: pointer of type atomic_t 　* atomic_read - 读取atomic变量 　*　@v: 指向类型atomic_t的指针 * * Atomically reads the value of @v. Note that the guaranteed * useful range of an atomic_t is only 24 bits. 　* 原子的读取v的值．注意，一个atomic_t保证的有用的范围只有２４位　 * */
#define atomic_read(v)  ((v)->counter)

/** * atomic_set - set atomic variable * @v: pointer of type atomic_t * @i: required value 　* * atomic_set - 设置atomic变量 　* @v: 指向类型atomic_t的指针 * @i: 需要的值 * * Atomically sets the value of @v to @i. Note that the guaranteed * useful range of an atomic_t is only 24 bits. 　* 原子的设置v的值为i． 注意，一个atomic_t保证的有用的范围只有２４位　 * */
#define atomic_set(v, i) (((v)->counter) = (i))

#include <asm/system.h>

/** * atomic_add_return - add integer to atomic variable * @i: integer value to add * @v: pointer of type atomic_t 　* aotmic_Add_return - 向atomic变量中加一个整数 　* @i: 要添加的整数 *　@v: 指向类型atomic_t的指针 * * Atomically adds @i to @v and returns the result * Note that the guaranteed useful range of an atomic_t is only 24 bits. * 原子的将i加到v中，返回结果 *　注意，一个atomic_t保证的有用的范围只有２４位　 */
static inline int atomic_add_return(int i, atomic_t *v)
{
    unsigned long flags;
    int temp;

    local_irq_save(flags);
    temp = v->counter;
    temp += i;
    v->counter = temp;
    local_irq_restore(flags);

    return temp;
}

/** * atomic_sub_return - subtract integer from atomic variable * @i: integer value to subtract * @v: pointer of type atomic_t 　* atomic_sub_return - 从原子变量中减去一个整数 　*　@i: 要被减去的整数 * @v: 指向类型atomic_t的指针 　* * Atomically subtracts @i from @v and returns the result * Note that the guaranteed useful range of an atomic_t is only 24 bits. */
static inline int atomic_sub_return(int i, atomic_t *v)
{
    unsigned long flags;
    int temp;

    local_irq_save(flags);
    temp = v->counter;
    temp -= i;
    v->counter = temp;
    local_irq_restore(flags);

    return temp;
}

static inline int atomic_add_negative(int i, atomic_t *v)
{
    return atomic_add_return(i, v) < 0;
}

static inline void atomic_add(int i, atomic_t *v)
{
    atomic_add_return(i, v);
}

static inline void atomic_sub(int i, atomic_t *v)
{
    atomic_sub_return(i, v);
}

//自加１
static inline void atomic_inc(atomic_t *v)
{
    atomic_add_return(1, v);
}

//自减１
static inline void atomic_dec(atomic_t *v)
{
    atomic_sub_return(1, v);
}

#define atomic_dec_return(v)        atomic_sub_return(1, (v))
#define atomic_inc_return(v)        atomic_add_return(1, (v))

#define atomic_sub_and_test(i, v)   (atomic_sub_return((i), (v)) == 0)
#define atomic_dec_and_test(v)      (atomic_sub_return(1, (v)) == 0)
#define atomic_inc_and_test(v)      (atomic_add_return(1, (v)) == 0)

#define atomic_add_unless(v, a, u)              \
({                              \
    int c, old;                     \
    c = atomic_read(v);                 \
    while (c != (u) && (old = atomic_cmpxchg((v), c, c + (a))) != c) \
        c = old;                    \
    c != (u);                       \
})

#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)

static inline void atomic_clear_mask(unsigned long mask, unsigned long *addr)
{
    unsigned long flags;

    mask = ~mask;
    local_irq_save(flags);
    *addr &= mask;
    local_irq_restore(flags);
}

#define atomic_xchg(ptr, v)     (xchg(&(ptr)->counter, (v)))
#define atomic_cmpxchg(v, old, new) (cmpxchg(&((v)->counter), (old), (new)))

#define cmpxchg_local(ptr, o, n)                           \
    ((__typeof__(*(ptr)))__cmpxchg_local_generic((ptr), (unsigned long)(o),\
            (unsigned long)(n), sizeof(*(ptr))))

#define cmpxchg64_local(ptr, o, n) __cmpxchg64_local_generic((ptr), (o), (n))

/* Assume that atomic operations are already serializing */
/* 假设原子操作已经被序列化了　*/
#define smp_mb__before_atomic_dec() barrier()
#define smp_mb__after_atomic_dec()  barrier()
#define smp_mb__before_atomic_inc() barrier()
#define smp_mb__after_atomic_inc()  barrier()

#include <asm-generic/atomic-long.h>

#endif /* __KERNEL__ */
#endif /* __ASM_GENERIC_ATOMIC_H */

通过这个，基本上就可以使用原子操作了．下面列举一个原子操作的例子．

#include <asm/atomic.h>
#include <linux/types.h>

int main()
{
  atomic_t v = ATOMIC_INIT(0); //新建一个atomic_t变量，并且初始化为０

　int i = atomic_sub_return(2, &v);

　return 0;
}

这个仅仅就是一个例子程序，不一定能运行的起来，因为这个是在内核中使用的代码．