内核同步机制——原子操作

内核为原子操作提供了两组接口。一组操作整数，一个组操作比特位。
1.整数原子操作
数据类型为：
typedefstruct {
volatileint counter;
}atomic_t;



为了保持内核在各个平台兼容，以前规定atomic_t的值不能超过24位（都是SPARC惹的祸），不过现在该规定已经不需要了。
相关操作如下：
voidatomic_set(atomic_t *v, int i);
atomic_tv = ATOMIC_INIT(0);//设置原子变量v的值为整数i。
intatomic_read(atomic_t *v);//返回原子变量当前的值
voidatomic_add(int i, atomic_t *v);//将i加到原子变量上
voidatomic_sub(int i, atomic_t *v)//从原子变量的值中减去i
voidatomic_inc(atomic_t *v);//增加原子变量的值
voidatomic_dec(atomic_t *v);//减少原子变量的值

执行相关的操作后测试原子变量的值是否为0
Performthe specified operation and test the result; if, after the operation,the atomic value is 0, then the return value is true; otherwise, itis false. Note that there is no atomic_add_and_test.

intatomic_inc_and_test(atomic_t *v);

intatomic_dec_and_test(atomic_t *v);

intatomic_sub_and_test(int i, atomic_t *v);

Addthe integer variable i to v. The return value is true if the resultis negative,false otherwise.
intatomic_add_negative(int i, atomic_t *v);

Behavejust like atomic_add and friends, with the exception that they returnthe new value of the atomic variable to the caller.

intatomic_add_return(int i, atomic_t *v);

intatomic_sub_return(int i, atomic_t *v);

intatomic_inc_return(atomic_t *v);

intatomic_dec_return(atomic_t *v);
最近的内核也提供了64位的版本，即atomic64_t，方法和用法与32位类似，方法名相应的地方换为atomic64。

2.位操作
Setsbit number nr in the data item pointed to by addr.
voidset_bit(nr, void *addr);
Clearsthe specified bit in the unsigned long datum that lives at addr. Itssemantics are otherwise the same as set_bit.
voidclear_bit(nr, void *addr);

voidchange_bit(nr, void *addr); // Toggles the bit.

Thisfunction is the only bit operation that doesn’t need to be atomic;it simply returns the current value of the bit.
test_bit(nr,void *addr);

Behaveatomically like those listed previously, except that they also returnthe previous value of the bit.
inttest_and_set_bit(nr, void *addr);

inttest_and_clear_bit(nr, void *addr);

inttest_and_change_bit(nr, void *addr);

使用场景：
/*try to set lock */

while(test_and_set_bit(nr, addr) != 0)
wait_for_a_while();

/*do your work */
/*release lock, and check... */
if(test_and_clear_bit(nr, addr) = = 0)
something_went_wrong(); /* already released: error */

内核也提供了一套非原子位操作函数，函数名就是原子版函数前面加两下划线。