netdev - Re: [PATCH net 2/5] lan966x: execute xdp_do_flush() before napi_complete

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Date:   Tue, 17 Jan 2023 12:53:13 +0100
From:   Steen Hegelund <steen.hegelund@...rochip.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>,
        <magnus.karlsson@...el.com>, <bjorn@...nel.org>, <ast@...nel.org>,
        <daniel@...earbox.net>, <netdev@...r.kernel.org>,
        <jonathan.lemon@...il.com>, <maciej.fijalkowski@...el.com>,
        <kuba@...nel.org>, <toke@...hat.com>, <pabeni@...hat.com>,
        <davem@...emloft.net>, <aelior@...vell.com>, <manishc@...vell.com>,
        <horatiu.vultur@...rochip.com>, <UNGLinuxDriver@...rochip.com>,
        <mst@...hat.com>, <jasowang@...hat.com>, <ioana.ciornei@....com>,
        <madalin.bucur@....com>
CC:     <bpf@...r.kernel.org>
Subject: Re: [PATCH net 2/5] lan966x: execute xdp_do_flush() before
 napi_complete_done()

Hi Magnus,

This looks good to me.

Acked-by: Steen Hegelund <Steen.Hegelund@...rochip.com>

BR
Steen

On Tue, 2023-01-17 at 10:25 +0100, Magnus Karlsson wrote:
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> 
> From: Magnus Karlsson <magnus.karlsson@...el.com>
> 
> Make sure that xdp_do_flush() is always executed before
> napi_complete_done(). This is important for two reasons. First, a
> redirect to an XSKMAP assumes that a call to xdp_do_redirect() from
> napi context X on CPU Y will be follwed by a xdp_do_flush() from the
> same napi context and CPU. This is not guaranteed if the
> napi_complete_done() is executed before xdp_do_flush(), as it tells
> the napi logic that it is fine to schedule napi context X on another
> CPU. Details from a production system triggering this bug using the
> veth driver can be found following the first link below.
> 
> The second reason is that the XDP_REDIRECT logic in itself relies on
> being inside a single NAPI instance through to the xdp_do_flush() call
> for RCU protection of all in-kernel data structures. Details can be
> found in the second link below.
> 
> Fixes: a825b611c7c1 ("net: lan966x: Add support for XDP_REDIRECT")
> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
> Link: https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@cloudflare.com
> Link: https://lore.kernel.org/all/20210624160609.292325-1-toke@redhat.com/
> ---
>  drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> index 5314c064ceae..55b484b10562 100644
> --- a/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> +++ b/drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c
> @@ -608,12 +608,12 @@ static int lan966x_fdma_napi_poll(struct napi_struct *napi, int weight)
>                 lan966x_fdma_rx_reload(rx);
>         }
> 
> -       if (counter < weight && napi_complete_done(napi, counter))
> -               lan_wr(0xff, lan966x, FDMA_INTR_DB_ENA);
> -
>         if (redirect)
>                 xdp_do_flush();
> 
> +       if (counter < weight && napi_complete_done(napi, counter))
> +               lan_wr(0xff, lan966x, FDMA_INTR_DB_ENA);
> +
>         return counter;
>  }
> 
> --
> 2.34.1
>