Subject: Re: BUG: unable to handle kernel paging request in bpf_dispatcher_xdp

From: Jakub Kicinski <[email-masked]>
Date: Fri, 9 Dec 2022 15:34:45 -0800

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.