netdev - Re: [PATCH net-next 9/9] net: enetc: add support for XDP

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Message-ID: <875z16nsiu.fsf@toke.dk>
Date:   Thu, 01 Apr 2021 13:39:05 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Vladimir Oltean <olteanv@...il.com>
Cc:     Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>, Yonghong Song <yhs@...com>,
        KP Singh <kpsingh@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, Alexander Duyck <alexander.duyck@...il.com>,
        Ioana Ciornei <ioana.ciornei@....com>,
        Alex Marginean <alexandru.marginean@....com>,
        Claudiu Manoil <claudiu.manoil@....com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Vladimir Oltean <vladimir.oltean@....com>
Subject: Re: [PATCH net-next 9/9] net: enetc: add support for XDP_REDIRECT

Vladimir Oltean <olteanv@...il.com> writes:

> On Thu, Apr 01, 2021 at 01:26:02PM +0200, Toke Høiland-Jørgensen wrote:
>> > +int enetc_xdp_xmit(struct net_device *ndev, int num_frames,
>> > +		   struct xdp_frame **frames, u32 flags)
>> > +{
>> > +	struct enetc_tx_swbd xdp_redirect_arr[ENETC_MAX_SKB_FRAGS] = {0};
>> > +	struct enetc_ndev_priv *priv = netdev_priv(ndev);
>> > +	struct enetc_bdr *tx_ring;
>> > +	int xdp_tx_bd_cnt, i, k;
>> > +	int xdp_tx_frm_cnt = 0;
>> > +
>> > +	tx_ring = priv->tx_ring[smp_processor_id()];
>> 
>> What mechanism guarantees that this won't overflow the array? :)
>
> Which array, the array of TX rings?

Yes.

> You mean that it's possible to receive a TC_SETUP_QDISC_MQPRIO or
> TC_SETUP_QDISC_TAPRIO with num_tc == 1, and we have 2 CPUs?

Not just that, this ndo can be called on arbitrary CPUs after a
redirect. The code just calls through from the XDP receive path so which
CPU it ends up on depends on the RSS+IRQ config of the other device,
which may not even be the same driver; i.e., you have no control over
that... :)

> Well, yeah, I don't know what's the proper way to deal with that. Ideas?

Well the obvious one is just:

tx_ring = priv->tx_ring[smp_processor_id() % num_ring_ids];

and then some kind of locking to deal with multiple CPUs accessing the
same TX ring...

-Toke