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

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Message-ID: <87lem1ct2e.fsf@toke.dk>
Date:   Tue, 17 Jan 2023 12:13:29 +0100
From:   Toke Høiland-Jørgensen <toke@...hat.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, 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:     Magnus Karlsson <magnus.karlsson@...il.com>, bpf@...r.kernel.org
Subject: Re: [PATCH net 0/5] net: xdp: execute xdp_do_flush() before
 napi_complete_done()

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.

> 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>