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

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Message-ID: <CAJ8uoz2-tg2QWvaON2vJ9TrTQQO5K9NFvu=m-YXzYCAJcgB6Fg@mail.gmail.com>
Date:   Tue, 17 Jan 2023 12:34:05 +0100
From:   Magnus Karlsson <magnus.karlsson@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.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, 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, 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 12:13 PM Toke Høiland-Jørgensen <toke@...hat.com> wrote:
>
> Magnus Karlsson <magnus.karlsson@...il.com> writes:
>
> > 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
>
> Typo in 'followed' here (and in all the copy-pasted commit messages).
>
> > 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
>
> And another typo in 'maintainer' here.

Thanks for spotting. Will fix these spelling errors in a v2.

> > 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.
> >
> > 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/
>
> Otherwise LGTM!
>
> For the series:
>
> Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>
>