Subject: Re: [PATCH v5 net-next 6/6] net: ethernet: ti: cpsw: add XDP support

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 1 Jul 2019 18:19:01 +0200

On Sun, 30 Jun 2019 20:23:48 +0300
Ivan Khoronzhuk <[email-masked]> wrote:

> +static int cpsw_ndev_create_xdp_rxq(struct cpsw_priv *priv, int ch)
> +{
> +	struct cpsw_common *cpsw = priv->cpsw;
> +	int ret, new_pool = false;
> +	struct xdp_rxq_info *rxq;
> +
> +	rxq = &priv->xdp_rxq[ch];
> +
> +	ret = xdp_rxq_info_reg(rxq, priv->ndev, ch);
> +	if (ret)
> +		return ret;
> +
> +	if (!cpsw->page_pool[ch]) {
> +		ret =  cpsw_create_rx_pool(cpsw, ch);
> +		if (ret)
> +			goto err_rxq;
> +
> +		new_pool = true;
> +	}
> +
> +	ret = xdp_rxq_info_reg_mem_model(rxq, MEM_TYPE_PAGE_POOL,
> +					 cpsw->page_pool[ch]);
> +	if (!ret)
> +		return 0;
> +
> +	if (new_pool) {
> +		page_pool_free(cpsw->page_pool[ch]);
> +		cpsw->page_pool[ch] = NULL;
> +	}
> +
> +err_rxq:
> +	xdp_rxq_info_unreg(rxq);
> +	return ret;
> +}

Looking at this, and Ilias'es XDP-netsec error handling path, it might
be a mistake that I removed page_pool_destroy() and instead put the
responsibility on xdp_rxq_info_unreg().

As here, we have to detect if page_pool_create() was a success, and then
if xdp_rxq_info_reg_mem_model() was a failure, explicitly call
page_pool_free() because the xdp_rxq_info_unreg() call cannot "free"
the page_pool object given it was not registered.  

Ivan's patch in[1], might be a better approach, which forced all
drivers to explicitly call page_pool_free(), even-though it just
dec-refcnt and the real call to page_pool_free() happened via
xdp_rxq_info_unreg().

To better handle error path, I would re-introduce page_pool_destroy(),
as a driver API, that would gracefully handle NULL-pointer case, and
then call page_pool_free() with the atomic_dec_and_test().  (It should
hopefully simplify the error handling code a bit)

[1] https://lore.kernel.org/netdev/[email-masked]/

> +void cpsw_ndev_destroy_xdp_rxqs(struct cpsw_priv *priv)
> +{
> +	struct cpsw_common *cpsw = priv->cpsw;
> +	struct xdp_rxq_info *rxq;
> +	int i;
> +
> +	for (i = 0; i < cpsw->rx_ch_num; i++) {
> +		rxq = &priv->xdp_rxq[i];
> +		if (xdp_rxq_info_is_reg(rxq))
> +			xdp_rxq_info_unreg(rxq);
> +	}
> +}

Are you sure you need to test xdp_rxq_info_is_reg() here?

You should just call xdp_rxq_info_unreg(rxq), if you know that this rxq
should be registered.  If your assumption failed, you will get a
WARNing, and discover your driver level bug.  This is one of the ways
the API is designed to "detect" misuse of the API.  (I found this
rather useful, when I converted the approx 12 drivers using this
xdp_rxq_info API).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer