From mboxrd@z Thu Jan  1 00:00:00 1970
To: 9fans@cse.psu.edu
From: "Russ Cox" <rsc@plan9.bell-labs.com>
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Message-Id: <20020116034426.F0943199EE@mail.cse.psu.edu>
Subject: [9fans] semaphores
Date: Tue, 15 Jan 2002 22:44:21 -0500
Topicbox-Message-UUID: 3ddfdfe4-eaca-11e9-9e20-41e7f4b1d025

I just wrote these for a porting project and
then discovered that I didn't need them.
However, they seem a little subtle to me
(I gave up the first couple times I tried)
so I figured I'd toss them out for others
and to see if anyone noticed any problems.

Russ

/*
 * Semaphore support.
 *
 * The data QLock could be a Lock if you care;
 * it protects the n and need values.
 *
 * The wait QLock manages the line of people waiting to
 * acquire the semaphore.  If you manage to qlock it,
 * it's your turn.  If n isn't big enough for you, record
 * your needs and rendezvous on the Sema; once incsema
 * makes n big enough, it will rendezvous with you.
 */
typedef struct Sema Sema;
struct Semaphore {
	QLock data;
	QLock wait;
	int n;
	int need;
};

void
incsema(Sema *s, int c)
{
	qlock(&s->data);
	s->n += c;
	if(s->need > 0 && s->n >= s->need){
		rendezvous(s, 0);
		s->need = 0;
	}
	qunlock(&s->data);
}

void
decsema(Sema *s, int c)
{
	qlock(&s->wait);
	qlock(&s->data);
	if(s->n < c){
		s->need = c;
		qunlock(&s->data);
		rendezvous(s, 0);
		qlock(&s->data);
	}
	assert(s->n >= c);
	s->n -= c;
	qunlock(&s->data);
	qunlock(&s->wait);
}

int
candecsema(Sema *s, int c)
{
	int ret;

	if(!canqlock(&s->wait))
		return 0;
	qlock(&s->data);	/* won't block for long */
	if(s->n >= c){
		ret = 1;
		s->n -= c;
	}else
		ret = 0;
	qunlock(&s->data);
	qunlock(&s->wait);
	return ret;
}