Subject: Re: [PATCH v2 1/1] net: fec: add initial XDP support

From: Andrew Lunn <[email-masked]>
Date: Mon, 31 Oct 2022 18:10:07 +0100

> +static int fec_enet_bpf(struct net_device *dev, struct netdev_bpf *bpf)
> +{
> +	struct fec_enet_private *fep = netdev_priv(dev);
> +	bool is_run = netif_running(dev);
> +	struct bpf_prog *old_prog;
> +	unsigned int dsize;
> +	int i;
> +
> +	switch (bpf->command) {
> +	case XDP_SETUP_PROG:
> +		if (is_run) {
> +			napi_disable(&fep->napi);
> +			netif_tx_disable(dev);
> +		}
> +
> +		old_prog = xchg(&fep->xdp_prog, bpf->prog);
> +
> +		/* Update RX ring size */
> +		dsize = fep->bufdesc_ex ? sizeof(struct bufdesc_ex) :
> +			sizeof(struct bufdesc);
> +		for (i = 0; i < fep->num_rx_queues; i++) {
> +			struct fec_enet_priv_rx_q *rxq = fep->rx_queue[i];
> +			struct bufdesc *cbd_base;
> +			unsigned int size;
> +
> +			cbd_base = rxq->bd.base;
> +			if (bpf->prog)
> +				rxq->bd.ring_size = XDP_RX_RING_SIZE;
> +			else
> +				rxq->bd.ring_size = RX_RING_SIZE;
> +			size = dsize * rxq->bd.ring_size;
> +			cbd_base = (struct bufdesc *)(((void *)cbd_base) + size);
> +			rxq->bd.last = (struct bufdesc *)(((void *)cbd_base) - dsize);

This does not look safe. netif_tx_disable(dev) will stop new
transmissions, but the hardware can be busy receiving, DMAing frames,
using the descriptors, etc. Modifying rxq->bd.last in particular seems
risky. I think you need to disable the receiver, wait for it to
indicate it really has stopped, and only then make these
modifications.

	Andrew