Function ifmp_ring_enqueue

freebsd/net/mp_ring.c:320–393

 */
#ifdef MP_RING_NO_64BIT_ATOMICS
int
ifmp_ring_enqueue(struct ifmp_ring *r, void **items, int n, int budget, int abdicate)
{
	union ring_state os, ns;
	uint16_t pidx_start, pidx_stop;
	int i;

	MPASS(items != NULL);
	MPASS(n > 0);

	mtx_lock(&r->lock);
	/*
	 * Reserve room for the new items.  Our reservation, if successful, is
	 * from 'pidx_start' to 'pidx_stop'.
	 */
	os.state = r->state;
	if (n >= space_available(r, os)) {
		counter_u64_add(r->drops, n);
		MPASS(os.flags != IDLE);
		mtx_unlock(&r->lock);
		if (os.flags == STALLED)
			ifmp_ring_check_drainage(r, 0);
		return (ENOBUFS);
	}
	ns.state = os.state;
	ns.pidx_head = increment_idx(r, os.pidx_head, n);
	r->state = ns.state;
	pidx_start = os.pidx_head;
	pidx_stop = ns.pidx_head;

	/*
	 * Wait for other producers who got in ahead of us to enqueue their
	 * items, one producer at a time.  It is our turn when the ring's
	 * pidx_tail reaches the beginning of our reservation (pidx_start).
	 */
	while (ns.pidx_tail != pidx_start) {
		cpu_spinwait();
		ns.state = r->state;
	}

	/* Now it is our turn to fill up the area we reserved earlier. */
	i = pidx_start;
	do {
		r->items[i] = *items++;
		if (__predict_false(++i == r->size))
			i = 0;
	} while (i != pidx_stop);

	/*
	 * Update the ring's pidx_tail.  The release style atomic guarantees
	 * that the items are visible to any thread that sees the updated pidx.
	 */
	os.state = ns.state = r->state;
	ns.pidx_tail = pidx_stop;
	if (abdicate) {
		if (os.flags == IDLE)
			ns.flags = ABDICATED;
	} else