static inline unsigned int
atomic_dec_uint_lock_if_zero(volatile unsigned int *ptr, kmutex_t interlock)
{
	unsigned int old, new;

	do {
		old = *ptr;
		KASSERT(0 < old);
		if (old == 1) {
			mutex_enter(interlock);
			new = atomic_add_int_nv(ptr, -1);
			if (new == 0)
				return 0;
			mutex_exit(interlock);
			return new;
		}
		new = (old - 1);
	} while (atomic_cas_uint(ptr, old, new) != old);

	return new;
}

#if examples
void
vref(struct vnode *vp)
{
	const unsigned int usecnt __unused =
	    atomic_inc_uint_nv(&vp->v_usecount);

	KASSERT(1 < usecnt);
}

void
vrele(struct vnode *vp)
{

	if (atomic_dec_uint_lock_if_zero(&vp->v_usecnt, vp->v_interlock))
		return;
	KASSERT(mutex_owned(vp->v_interlock));

	/* ...rest of vrelel... */
}
#endif