qlock.c

著名操作系统Plan 9的第三版的部分核心源代码。现在很难找到了。Plan 9是bell实验室开发的Unix后继者。

#include <u.h>
#include <libc.h>

static struct {
	QLp	*p;
	QLp	x[1024];
} ql = {
	ql.x
};

enum
{
	Queuing,
	QueuingR,
	QueuingW,
};

static ulong	(*_rendezvousp)(ulong, ulong) = rendezvous;

/* this gets called by the thread library ONLY to get us to use its rendezvous */
void
_qlockinit(ulong (*r)(ulong, ulong))
{
	_rendezvousp = r;
}

/* find a free shared memory location to queue ourselves in */
static QLp*
getqlp(void)
{
	QLp *p, *op;

	op = ql.p;
	for(p = op+1; ; p++){
		if(p == &ql.x[nelem(ql.x)])
			p = ql.x;
		if(p == op)
			abort();
		if(_tas(&(p->inuse)) == 0){
			ql.p = p;
			p->next = nil;
			break;
		}
	}
	return p;
}

void
qlock(QLock *q)
{
	QLp *p, *mp;

	lock(&q->lock);
	if(!q->locked){
		q->locked = 1;
		unlock(&q->lock);
		return;
	}


	/* chain into waiting list */
	mp = getqlp();
	p = q->tail;
	if(p == nil)
		q->head = mp;
	else
		p->next = mp;
	q->tail = mp;
	mp->state = Queuing;
	unlock(&q->lock);

	/* wait */
	(*_rendezvousp)((ulong)mp, 1);
	mp->inuse = 0;
}

void
qunlock(QLock *q)
{
	QLp *p;

	lock(&q->lock);
	p = q->head;
	if(p != nil){
		/* wakeup head waiting process */
		q->head = p->next;
		if(q->head == nil)
			q->tail = nil;
		unlock(&q->lock);
		(*_rendezvousp)((ulong)p, 0x12345);
		return;
	}
	q->locked = 0;
	unlock(&q->lock);
}

int
canqlock(QLock *q)
{
	lock(&q->lock);
	if(!q->locked){
		q->locked = 1;
		unlock(&q->lock);
		return 1;
	}
	unlock(&q->lock);
	return 0;
}

void
rlock(RWLock *q)
{
	QLp *p, *mp;

	lock(&q->lock);
	if(q->writer == 0 && q->head == nil){
		/* no writer, go for it */
		q->readers++;
		unlock(&q->lock);
		return;
	}

	mp = getqlp();
	p = q->tail;
	if(p == 0)
		q->head = mp;
	else
		p->next = mp;
	q->tail = mp;
	mp->next = nil;
	mp->state = QueuingR;
	unlock(&q->lock);

	/* wait in kernel */
	(*_rendezvousp)((ulong)mp, 1);
	mp->inuse = 0;
}

void
runlock(RWLock *q)
{
	QLp *p;

	lock(&q->lock);
	p = q->head;
	if(--(q->readers) > 0 || p == nil){
		unlock(&q->lock);
		return;
	}

	/* start waiting writer */
	if(p->state != QueuingW)
		abort();
	q->head = p->next;
	if(q->head == 0)
		q->tail = 0;
	q->writer = 1;
	unlock(&q->lock);

	/* wakeup waiter */
	(*_rendezvousp)((ulong)p, 0);
}

void
wlock(RWLock *q)
{
	QLp *p, *mp;

	lock(&q->lock);
	if(q->readers == 0 && q->writer == 0){
		/* noone waiting, go for it */
		q->writer = 1;
		unlock(&q->lock);
		return;
	}

	/* wait */
	p = q->tail;
	mp = getqlp();
	if(p == nil)
		q->head = mp;
	else
		p->next = mp;
	q->tail = mp;
	mp->next = nil;
	mp->state = QueuingW;
	unlock(&q->lock);

	/* wait in kernel */
	(*_rendezvousp)((ulong)mp, 1);
	mp->inuse = 0;
}

void
wunlock(RWLock *q)
{
	QLp *p;

	lock(&q->lock);
	p = q->head;
	if(p == nil){
		q->writer = 0;
		unlock(&q->lock);
		return;
	}
	if(p->state == QueuingW){
		/* start waiting writer */
		q->head = p->next;
		if(q->head == nil)
			q->tail = nil;
		unlock(&q->lock);
		(*_rendezvousp)((ulong)p, 0);

		return;
	}

	if(p->state != QueuingR)
		abort();

	/* waken waiting readers */
	while(q->head != nil && q->head->state == QueuingR){
		p = q->head;
		q->head = p->next;
		q->readers++;
		(*_rendezvousp)((ulong)p, 0);
	}
	if(q->head == nil)
		q->tail = nil;
	q->writer = 0;
	unlock(&q->lock);
}