原子操作的一些函数，在文件 include\asm-x86\Atomic_32.h中
#ifndef __ARCH_I386_ATOMIC__
#define __ARCH_I386_ATOMIC__
#include
#include
#include /*
* Atomic operations that C can't guarantee us. Useful for
* resource counting etc..
*/
/*
* Make sure gcc doesn't try to be clever and move things around
* on us. We need to use _exactly_ the address the user gave us,
* not some alias that contains the same information.
*/
typedef struct { int counter; } atomic_t;//原子变量的结构体定义
#define ATOMIC_INIT(i) { (i) }
/**
* atomic_read - read atomic variable
* @v: pointer of type atomic_t
*
* Atomically reads the value of @v.
*/
#define atomic_read(v) ((v)->counter)//原子读
/**
* atomic_set - set atomic variable
* @v: pointer of type atomic_t
* @i: required value
*
* Atomically sets the value of @v to @i.
*/
#define atomic_set(v,i) (((v)->counter) = (i))//原子写
/**
* atomic_add - add integer to atomic variable
* @i: integer value to add
* @v: pointer of type atomic_t
*
* Atomically adds @i to @v.
*/
static __inline__ void atomic_add(int i, atomic_t *v)
{
__asm__ __volatile__(
LOCK_PREFIX "addl %1,%0"
:"+m" (v->counter)
:"ir" (i));
}//原子加，LOCK_PREFIX展开就是lock指令，作用是锁住总线，在SMP下，在addl指令周期期间，锁住总线，只允许一个cpu通过这个总线访问，另一个cpu访问不了。
/**
* atomic_sub - subtract integer from atomic variable
* @i: integer value to subtract
* @v: pointer of type atomic_t
*
* Atomically subtracts @i from @v.
*/
static __inline__ void atomic_sub(int i, atomic_t *v)
{
__asm__ __volatile__(
LOCK_PREFIX "subl %1,%0"
:"+m" (v->counter)
:"ir" (i));
}//原子减，同上
/**
* atomic_sub_and_test - subtract value from variable and test result
* @i: integer value to subtract
* @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)
{//减并测试，先减i，然后测试是否为0，如果为0，返回1，不为0，返回1
unsigned char c;
__asm__ __volatile__(
LOCK_PREFIX "subl %2,%0; sete %1"
:"+m" (v->counter), "=qm" (c)
:"ir" (i) : "memory");
return c;
}
//指令sete的作用是，根据ZF标志位，将sete的操作数设置成与ZF同样的值，即0或者1，如果结果为0，那么ZF置位为1，其他为0。在subl和sete之间，可能被中断，被调度，被另一个cpu访问，但是无论如何，ZF值属于当前进程的，不会由于被中断，被调度，被另一个cpu访问而改变，所以，ZF值不受到任何影响，于是可以认为，还是原子的。
/**
* atomic_inc - increment atomic variable
* @v: pointer of type atomic_t
*
* Atomically increments @v by 1.
*/
static __inline__ void atomic_inc(atomic_t *v)
{
__asm__ __volatile__(
LOCK_PREFIX "incl %0"
:"+m" (v->counter));
}//原子加1
/**
* atomic_dec - decrement atomic variable
* @v: pointer of type atomic_t
*
* Atomically decrements @v by 1.
*/
static __inline__ void atomic_dec(atomic_t *v)
{
__asm__ __volatile__(
LOCK_PREFIX "decl %0"
:"+m" (v->counter));
}//原子减1
/**
* atomic_dec_and_test - decrement and test
* @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)
{
unsigned char c;
__asm__ __volatile__(
LOCK_PREFIX "decl %0; sete %1"
:"+m" (v->counter), "=qm" (c)
: : "memory");
return c != 0;
}
/**
* atomic_inc_and_test - increment and test
* @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)
{
unsigned char c;
__asm__ __volatile__(
LOCK_PREFIX "incl %0; sete %1"
:"+m" (v->counter), "=qm" (c)
: : "memory");
return c != 0;
}
/**
* atomic_add_negative - add and test if negative
* @v: pointer of type atomic_t
* @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)
{
unsigned char c;
__asm__ __volatile__(
LOCK_PREFIX "addl %2,%0; sets %1"
:"+m" (v->counter), "=qm" (c)
:"ir" (i) : "memory");
return c;
}
/**
* atomic_add_return - add integer and return
* @v: pointer of type atomic_t
* @i: integer value to add
*
* Atomically adds @i to @v and returns @i + @v
*/
static __inline__ int atomic_add_return(int i, atomic_t *v)
{
int __i;
#ifdef CONFIG_M386
unsigned long flags;
if(unlikely(boot_cpu_data.x86 <= 3))
goto no_xadd;
#endif
/* Modern 486+ processor */
__i = i;
__asm__ __volatile__(
LOCK_PREFIX "xaddl %0, %1"
:"+r" (i), "+m" (v->counter)
: : "memory");
return i + __i;
//xadd m1,m2，作用是，先将m1和m2互换值，然后将和保存在m2中，
那么，xaddl后，i=v->counter,而v->counter=i+v->counter
这个时候，即使发生中断调度或者被其他cpu改写，i把原来的counter保存了，而真正的counter也已经加完了i。
调度回来，i+__i就是原来的i+counter的值了，也就是说，无论如何，这个结果，不会受到影响，所以近似认为是原子的。
#ifdef CONFIG_M386
no_xadd: /* Legacy 386 processor */
local_irq_save(flags);
__i = atomic_read(v);
atomic_set(v, i + __i);
local_irq_restore(flags);
return i + __i;
#endif
}
/**
* atomic_sub_return - subtract integer and return
* @v: pointer of type atomic_t
* @i: integer value to subtract
*
* Atomically subtracts @i from @v and returns @v - @i
*/
static __inline__ int atomic_sub_return(int i, atomic_t *v)
{
return atomic_add_return(-i,v);
}
#define atomic_cmpxchg(v, old, new) (cmpxchg(&((v)->counter), (old), (new)))
#define atomic_xchg(v, new) (xchg(&((v)->counter), (new)))
/**
* atomic_add_unless - add unless the number is already a given value
* @v: pointer of type atomic_t
* @a: the amount to add to v...
* @u: ...unless v is equal to u.
*
* Atomically adds @a to @v, so long as @v was not already @u.
* Returns non-zero if @v was not @u, and zero otherwise.
*/
static __inline__ int atomic_add_unless(atomic_t *v, int a, int u)
{
int c, old;
c = atomic_read(v);
for (;;) {
if (unlikely(c == (u)))
break;
old = atomic_cmpxchg((v), c, c + (a));
if (likely(old == c))
break;
c = old;
}
return c != (u);
}
#define atomic_inc_not_zero(v) atomic_add_unless((v), 1, 0)
#define atomic_inc_return(v) (atomic_add_return(1,v))
#define atomic_dec_return(v) (atomic_sub_return(1,v))
/* These are x86-specific, used by some header files */
#define atomic_clear_mask(mask, addr) \
__asm__ __volatile__(LOCK_PREFIX "andl %0,%1" \
: : "r" (~(mask)),"m" (*addr) : "memory")
#define atomic_set_mask(mask, addr) \
__asm__ __volatile__(LOCK_PREFIX "orl %0,%1" \
: : "r" (mask),"m" (*(addr)) : "memory")
/* Atomic operations are already serializing on x86 */
#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()
