struct plock {
	struct percpu	*pl_percpu; /* struct plock_cpu */
	kmutex_t	pl_lock;
	kcondvar_t	pl_cv;
	unsigned	pl_count;
	struct lwp	*pl_writer;
};

struct plock_cpu {
#define PLC_RD	0x01
#define PLC_WR	0x02
	int		plc_flags;
};

int
plock_read_enter(struct plock *plock)
{
	struct plock_cpu *plc;
	int s, error;

	s = splsoftserial();
	plc = percpu_getref(plock->pl_percpu);
	KASSERT(!ISSET(plc->plc_flags, PLC_RD));
	if (ISSET(plc->plc_flags, PLC_WR)) {
		error = EBUSY;
	} else {
		plc->plc_flags |= PLC_RD;
		error = 0;
	}
	percpu_putref(plock->pl_percpu);
	splx(s);

	return error;
}

void
plock_read_exit(struct plock *plock)
{
	struct plock_cpu *plc;
	int s, error;

	s = splsoftserial();
	plc = percpu_getref(plock->pl_percpu);
	KASSERT(ISSET(plc->plc_flags, PLC_RD));
	plc->plc_flags &= ~PLC_RD;
	if (ISSET(plc->plc_flags, PLC_WR)) {
		mutex_enter(&plock->pl_lock);
		KASSERT(plock->pl_writer != NULL);
		KASSERT(0 < plock->pl_count);
		plock->pl_count--;
		cv_broadcast(&plock->pl_cv);
		mutex_exit(&plock->pl_count);
	}
	percpu_putref(plock->pl_percpu);
	splx(s);
}

void
plock_write_enter(struct plock *plock)
{

	mutex_enter(&plock->pl_lock);
	while (plock->pl_writer != NULL)
		cv_wait(&plock->pl_cv, &plock->pl_lock);
	plock->pl_writer = curlwp;
	KASSERT(plock->pl_count == 0);
	mutex_exit(&plock->pl_lock);
	xc_wait(xc_broadcast(0, &plock_write_enter_xc, plock, NULL));
	mutex_enter(&plock->pl_lock);
	while (0 < plock->pl_count)
		cv_wait(&plock->pl_cv, &plock->pl_lock);
	mutex_exit(&plock->pl_lock);
}

static void
plock_write_enter_xc(void *cookie0, void *cookie1 __unused)
{
	struct plock *plock = cookie0;
	struct plock_cpu *plc;
	int s;

	s = splsoftserial();
	plc = percpu_getref(plock->pl_percpu);
	plc->plc_flags |= PLC_WR;
	percpu_putref(plock->pl_percpu);
	splx(s);

	mutex_enter(&plock->pl_lock);
	plock->pl_count++;
	cv_broadcast(&plock->pl_cv);
	mutex_exit(&plock->pl_lock);
}

void
plock_write_exit(struct plock *plock)
{

	KASSERT(plock->pl_writer == curlwp);

	percpu_foreach(plock->pl_percpu, &plock_write_exit_cb, plock);

	mutex_enter(&plock->pl_lock);
	KASSERT(plock->pl_writer == curlwp);
	plock->pl_writer = NULL;
	cv_broadcast(&plock->pl_cv);
	mutex_exit(&plock->pl_lock);
}

static  void
plock_write_exit_cb(void *cpu, void *cookie, struct cpu_info *ci __unused)
{
	struct plock_cpu *plc = cpu;

	KASSERT(ISSET(plc->plc_flags, PLC_WR));
	plc->plc_flags &= ~PLC_WR;
}