Linux内核中的同步和互斥分析报告

struct semaphore
{
atomic_t count;
int sleepers;
wait_queue_head_t wait;
};

static inline void down(struct semaphore*sem)
{
/* 1 */sem->count--; //为原子操作
if(sem->count<0)
{
struct task_struct *tsk = current;
DECLARE_WAITQUEUE(wait, tsk);
tsk->state = TASK_UNINTERRUPTIBLE;
add_wait_queue_exclusive(&sem->wait, &wait);
spin_lock_irq(&semaphore_lock);
/* 2 */ sem->sleepers++;
for (;;) {
int sleepers = sem->sleepers;
/*
* Add "everybody else" into it. They aren't
* playing, because we own the spinlock.
*/
/* 3 */ if (!atomic_add_negative(sleepers - 1, &sem->count)) {
/* 4 */ sem->sleepers = 0; //这时sem->count=0
break;
}
/* 4 */ sem->sleepers = 1; /* us - see -1 above */ // 这时sem
->count
=-1
spin_unlock_irq(&semaphore_lock);
schedule();
tsk->state = TASK_UNINTERRUPTIBLE;
spin_lock_irq(&semaphore_lock);
}
spin_unlock_irq(&semaphore_lock);
remove_wait_queue(&sem->wait, &wait);
tsk->state = TASK_RUNNING;
wake_up(&sem->wait);
}
}

void up(struct semaphore*sem)
{
sem->count++; //为原子操作
if(sem->count<=0)
{
//唤醒等待队列中的一个符合条件的进程（因为每个进程都加了TASK_EXCLUSIVE标志）
。
};

typedef struct {
volatile unsigned int lock;
} spinlock_t;
static inline void spin_lock(spinlock_t *lock)
{
__asm__ __volatile__(
"\n1:\t"
"lock ; decb %0\n\t"
"js 2f\n" //lock->lock< 0 ,jmp 2 forward
".section .text.lock,\"ax\"\n"
"2:\t"
"cmpb $0,%0\n\t" //wait lock->lock==1
"rep;nop\n\t"
"jle 2b\n\t"
"jmp 1b\n"
".previous"
:"=m" (lock->lock) : : "memory");
}
static inline void spin_unlock(spinlock_t *lock)
{
__asm__ __volatile__(
"movb $1,%0"
:"=m" (lock->lock) : : "memory"); //lock->lock=1
}

#define SPIN_LOCK_UNLOCKED (spinlock_t) { 1 }
spinlock_t xxx_lock = SPIN_LOCK_UNLOCKED;
spin_lock_(&xxx_lock)
...
critical section
...
spin_unlock (&xxx_lock)