???????
?????????д??????μ???????????????漰?????????????????-??????????????????????????????????????????????????????????????Щ??????????????????????????????????Linux?μ??????-??Щ“??”????????????????????????????????????????????????
???????
??????????????Linux????У???????????????????????????????????spin_lock????????????mutex????????????semaphore????????????
??????????
???????????????????????????????????ò?????????????????????κ????????????????????????У???????????С?????С????е?λ????????????????????????????????????API???????????嶼????????????include/asm/atomic.h????У?????????????????
???????????????????????????????????ü???????????????????
?????????????壺
????typedef struct{
????volatile int counter;
????}atomic_t;
????volatile????????ε???ο?????gcc???????????????????????????????????????????????????????????????
????????????API??
????1??atomic_read(atomic_t *v);
?????????????????????????????????????????????????????v?????
????2??atomic_set(atomic_t *v??int i);
????????????????????????v????i??
????3??atomic_add(int i??atomic_t *v);
??????????????????????v?????i??
????4??atomic_sub(int i??atomic_t *v);
??????????????????????v?м??i??
????5??atomic_sub_and_test(int i?? atomic_t *v);
??????????????????????v?м??i?????ж???????0??????0???????棬????????
????6??atomic_inc(atomic_t *v);
?????????????????????v????????1??
????7??atomic_dec(atomic_t *v);
??????????????????????v?????1??
????8??atomic_dec_and_test(atomic_t *v);
??????????????????????v?????1?????ж???????0??????0???????棬????????
????9??int atomic_inc_and_test(atomic_t *v);
??????????????????????v????????1?????ж???????0??????0???????棬????????
????10??atomic_add_negative(int i?? atomic_t*v);
??????????????????????v????????i?????ж????????????????????????棬????????
????11??atomic_add_return(int i?? atomic_t *v);
??????????????????????v????????i????????????v?????
????12??int atomic_sub_return(int i?? atomic_t *v);
??????????????????????v?м??i????????????v?????
????13??int atomic_inc_return(atomic_t * v);
??????????????????????v????????1??????????v?????
????14??int atomic_dec_return(atomic_t * v);
??????????????????????v?????1??????????v?????
????????????????????????????ü???????TCP/IPЭ?????IP????????У?????????ü???????????н?structipq?????????IP????????refcnt?????ü????????????????atomic_t????????IP???????????ip_frag_create?У??????atomic_set?????????????1?????????IP????????ú???atomic_inc?????ü?????1??????????????IP????????ú???ipq_put??????IP?????ipq_put??ú???atomic_dec_and_test?????ü?????1???ж????ü???????0??????????Ip?????????ipq_kill??IP?????ipq?????????????????????IP????????ü?????1???????ú???atomic_dec??????
?????????
??????????????????????????????????????????????繲????棩?????????????Э??????????????????????????????????淽???????????
???????????????????????????????????????2??裺
????1??????????????????????
????2?????????????????????????????????????????????
????3??????0?????????????????????sleep???????????????0?????????????????1???????У?
????4???????????????????????????????????????1???????????н?????sleep??????????????????????y????
????????????????????/usr/src/linux/include/linux/sem.h ?У????????????????????????????????????????е????????
????Linux2.6.26??????????????壺
????struct semaphore {
????spinlock_t                lock;
????unsigned int             count;
????struct list_head        wait_list;
????};
?????????????????????У??????????????????????spinlock??????????????spinlock????????????count??????????????(count--)?????(count > 0)??
???????????pv????
???????????P????
??????1??void down(struct semaphore *sem);
??????2??int down_interruptible(struct semaphore *sem);
??????3??int down_trylock(struct semaphore *sem);
????(1)?е????????2.6.26?е???????????????????out??(Use of this function is deprecated)?????????y??????????????ɡ?
????(2)?????????????
/**
* down_interruptible - acquire the semaphore unless interrupted
* @sem: the semaphore to be acquired
*
* Attempts to acquire the semaphore.  If no more tasks are allowed to
* acquire the semaphore?? calling this function will put the task to sleep.
* If the sleep is interrupted by a signal?? this function will return -EINTR.
* If the semaphore is successfully acquired?? this function returns 0.
*/
int down_interruptible(struct semaphore *sem)
{
unsigned long flags;
int result = 0;
spin_lock_irqsave(&sem->lock?? flags);
if (likely(sem->count > 0))
sem->count--;
else
result = __down_interruptible(sem);
spin_unlock_irqrestore(&sem->lock?? flags);
return result;
}
?????????????????????????????￡??????count??????0??????????????????????????????????????count--?????????????????????????????????????????????????????????????л????????????????????struct semaphore ??wait_list?????????У????????
???????????????????????__down_interruptible()?????У????????struct semaphore_waiter????????struct semaphore_waiter???????￡?
????struct semaphore_waiter
????{
????struct list_head list;
????struct task_struct *task;
????int up;
????};
????????????????task??????????list???????????????????sem?е?wait_list??????????б??У??????ж????????sem?????down_interruptible????sem??wait_list???γ????????????

????(???????????????????????????????????????????У??????????????????????TASK_INTERRUPTIBLE??????????????schedule()?????????????????cpu?????ж????????)