netdev - Re: BUG: unable to handle kernel paging request in bpf_dispatcher

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Message-ID: <20221209153445.22182ca5@kernel.org>
Date:   Fri, 9 Dec 2022 15:34:45 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     Jiri Olsa <olsajiri@...il.com>, Yonghong Song <yhs@...a.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Song Liu <song@...nel.org>, Hao Sun <sunhao.th@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        bpf <bpf@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
        John Fastabend <john.fastabend@...il.com>,
        Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Yonghong Song <yhs@...com>, KP Singh <kpsingh@...nel.org>,
        Stanislav Fomichev <sdf@...gle.com>,
        Hao Luo <haoluo@...gle.com>,
        David Miller <davem@...emloft.net>,
        Jesper Dangaard Brouer <hawk@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Thorsten Leemhuis <regressions@...mhuis.info>
Subject: Re: BUG: unable to handle kernel paging request in
 bpf_dispatcher_xdp

On Sat, 10 Dec 2022 00:32:07 +0100 Daniel Borkmann wrote:
> fwiw, these should not be necessary, Documentation/RCU/checklist.rst :
> 
>    [...] One example of non-obvious pairing is the XDP feature in networking,
>    which calls BPF programs from network-driver NAPI (softirq) context. BPF
>    relies heavily on RCU protection for its data structures, but because the
>    BPF program invocation happens entirely within a single local_bh_disable()
>    section in a NAPI poll cycle, this usage is safe. The reason that this usage
>    is safe is that readers can use anything that disables BH when updaters use
>    call_rcu() or synchronize_rcu(). [...]

FWIW I sent a link to the thread to Paul and he confirmed 
the RCU will wait for just the BH.