netdev - Re: [PATCH net 0/5] net: xdp: execute xdp_do_flush() before napi_complete

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Date:   Tue, 17 Jan 2023 05:12:21 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     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,
        jasowang@...hat.com, ioana.ciornei@....com, madalin.bucur@....com,
        bpf@...r.kernel.org
Subject: Re: [PATCH net 0/5] net: xdp: execute xdp_do_flush() before
 napi_complete_done()

On Tue, Jan 17, 2023 at 10:25:28AM +0100, Magnus Karlsson wrote:
> 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 in [1].
> 
> 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 [2].
> 
> The drivers have only been compile-tested since I do not own any of
> the HW below. So if you are a manintainer, please make sure I did not
> mess something up. This is a lousy excuse for virtio-net though, but
> it should be much simpler for the vitio-net maintainers to test this,
> than me trying to find test cases, validation suites, instantiating a
> good setup, etc. Michael and Jason can likely do this in minutes.

This kind of thing doesn't scale though. There are more contributors
than maintainers. Also, I am not 100% sure what kind of XDP workload
do I need to be a good test.

> 
> Note that these were the drivers I found that violated the ordering by
> running a simple script and manually checking the ones that came up as
> potential offenders. But the script was not perfect in any way. There
> might still be offenders out there, since the script can generate
> false negatives.
> 
> [1] https://lore.kernel.org/r/20221220185903.1105011-1-sbohrer@cloudflare.com
> [2] https://lore.kernel.org/all/20210624160609.292325-1-toke@redhat.com/
> 
> Thanks: Magnus
> 
> Magnus Karlsson (5):
>   qede: execute xdp_do_flush() before napi_complete_done()
>   lan966x: execute xdp_do_flush() before napi_complete_done()
>   virtio-net: execute xdp_do_flush() before napi_complete_done()
>   dpaa_eth: execute xdp_do_flush() before napi_complete_done()
>   dpaa2-eth: execute xdp_do_flush() before napi_complete_done()
> 
>  drivers/net/ethernet/freescale/dpaa/dpaa_eth.c        | 6 +++---
>  drivers/net/ethernet/freescale/dpaa2/dpaa2-eth.c      | 9 ++++++---
>  drivers/net/ethernet/microchip/lan966x/lan966x_fdma.c | 6 +++---
>  drivers/net/ethernet/qlogic/qede/qede_fp.c            | 7 ++++---
>  drivers/net/virtio_net.c                              | 6 +++---
>  5 files changed, 19 insertions(+), 15 deletions(-)
> 
> 
> base-commit: 87b93b678e95c7d93fe6a55b0e0fbda26d8c7760
> --
> 2.34.1