用户空间原子操作的实现

使用lock指令前缀:
#if __GNUC__ < 4 || (__GNUC__ == 4 && __GNUC_MINOR__ < 1)
/* Technically wrong, but this avoids compilation errors on some gcc
   versions. */
#define ADDR "=m" (*(volatile long *) addr)
#else
#define ADDR "+m" (*(volatile long *) addr)
#endif

#ifdef CONFIG_SMP
#define LOCK_PREFIX "lock; "
#else /* ! CONFIG_SMP */
#define LOCK_PREFIX ""
#endif

typedef struct {
    int counter;
} atomic_t;

#define atomic_read(v)        ((v)->counter)
#define atomic_set(v, i)        (((v)->counter) = (i))

static inline void atomic_inc(atomic_t *v)
{
    asm volatile(LOCK_PREFIX "incl %0"
             : "=m" (v->counter)
             : "m" (v->counter));
}

static inline void atomic_dec(atomic_t *v)
{
    asm volatile(LOCK_PREFIX "decl %0"
             : "=m" (v->counter)
             : "m" (v->counter));
}

static inline void set_bit(int nr, volatile void *addr)
{
    asm volatile(LOCK_PREFIX "bts %1,%0" : ADDR : "Ir" (nr) : "memory");
}

static inline int test_and_set_bit(int nr, volatile void *addr)
{
    int oldbit;

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

    return oldbit;
}

static inline void clear_bit(int nr, volatile void *addr)
{
    asm volatile(LOCK_PREFIX "btr %1,%0" : ADDR : "Ir" (nr));
}

static inline int test_and_clear_bit(int nr, volatile void *addr)
{
    int oldbit;

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

    return oldbit;
}

使用gcc的内建函数来进行原子访问:

#ifndef _ATOMIC_H
#define _ATOMIC_H
 
/**
 * Atomic type.
 */
 
typedef struct {
volatile int counter;
} atomic_t;
 
#define ATOMIC_INIT(i)  { (i) }
 
/**
 * Read atomic variable
 * @param v pointer of type atomic_t
 *
 * Atomically reads the value of @v.
 */
#define atomic_read(v) ((v)->counter)
 
/**
 * Set atomic variable
 * @param v pointer of type atomic_t
 * @param i required value
 */
#define atomic_set(v,i) (((v)->counter) = (i))
 
/**
 * Add to the atomic variable
 * @param i integer value to add
 * @param v pointer of type atomic_t
 */
static inline void atomic_add( int i, atomic_t *v ) 
{
         (void)__sync_add_and_fetch(&v->counter, i);
}
 
/**
 * Subtract the atomic variable
 * @param i integer value to subtract
 * @param v pointer of type atomic_t
 *
 * Atomically subtracts @i from @v.
 */
static inline void atomic_sub( int i, atomic_t *v ) 
{
        (void)__sync_sub_and_fetch(&v->counter, i);
}
 
/**
 * Subtract value from variable and test result
 * @param i integer value to subtract
 * @param v pointer of type atomic_t
 *
 * Atomically subtracts @i from @v and returns
 * true if the result is zero, or false for all
 * other cases.
 */
static inline int atomic_sub_and_test( int i, atomic_t *v ) 
{
        return !(__sync_sub_and_fetch(&v->counter, i));
}
 
/**
 * Increment atomic variable
 * @param v pointer of type atomic_t
 *
 * Atomically increments @v by 1.
 */
static inline void atomic_inc( atomic_t *v ) 
{
       (void)__sync_fetch_and_add(&v->counter, 1);
}
 
/**
 * @brief decrement atomic variable
 * @param v: pointer of type atomic_t
 *
 * Atomically decrements @v by 1.  Note that the guaranteed
 * useful range of an atomic_t is only 24 bits.
 */
static inline void atomic_dec( atomic_t *v ) 
{
       (void)__sync_fetch_and_sub(&v->counter, 1);
}
 
/**
 * @brief Decrement and test
 * @param v pointer of type atomic_t
 *
 * Atomically decrements @v by 1 and
 * returns true if the result is 0, or false for all other
 * cases.
 */
static inline int atomic_dec_and_test( atomic_t *v ) 
{
       return !(__sync_sub_and_fetch(&v->counter, 1));
}
 
/**
 * @brief Increment and test
 * @param v pointer of type atomic_t
 *
 * Atomically increments @v by 1
 * and returns true if the result is zero, or false for all
 * other cases.
 */
static inline int atomic_inc_and_test( atomic_t *v ) 
{
      return !(__sync_add_and_fetch(&v->counter, 1));
}
 
/**
 * @brief add and test if negative
 * @param v pointer of type atomic_t
 * @param i integer value to add
 *
 * Atomically adds @i to @v and returns true
 * if the result is negative, or false when
 * result is greater than or equal to zero.
 */
static inline int atomic_add_negative( int i, atomic_t *v ) 
{
       return (__sync_add_and_fetch(&v->counter, i) < 0);
}
 
#endif

参考:
http://golubenco.org/blog/atomic-operations/
http://gcc.gnu.org/onlinedocs/gcc-4.1.0/gcc/Atomic-Builtins.html
http://hi.baidu.com/wrap/blog/item/d8e40f2e7fb022554fc226a0.html
http://bugs.mysql.com/bug.php?id=34175