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

[Alexei Starovoitov]

On Fri, Dec 9, 2022 at 4:06 PM Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
>
> On Fri, Dec 09, 2022 at 03:34:45PM -0800, Jakub Kicinski wrote:
> > 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.
>
> so IIUC we can omit the rcu_read_lock/unlock on bpf_prog_run_xdp side
>
> Paul,
> any thoughts on what we can use in here to synchronize bpf_dispatcher_change_prog
> with bpf_prog_run_xdp callers?
>
> with synchronize_rcu_tasks I'm getting splats like:
>   https://lore.kernel.org/bpf/20221209153445.22182ca5@xxxxxxxxxx/T/#m0a869f93404a2744884d922bc96d497ffe8f579f
>
> synchronize_rcu_tasks_rude seems to work (patch below), but it also sounds special ;-)

Jiri,

I haven't tried to repro this yet, but I feel you're on
the wrong path here. The splat has this:
? bpf_prog_run_xdp include/linux/filter.h:775 [inline]
? bpf_test_run+0x2ce/0x990 net/bpf/test_run.c:400
that test_run logic takes rcu_read_lock.
See bpf_test_timer_enter.
I suspect the addition of synchronize_rcu_tasks_rude
only slows down the race.
The synchronize_rcu_tasks_trace also behaves like synchronize_rcu.
See our new and fancy rcu_trace_implies_rcu_gp(),
but I'm not sure it applies to synchronize_rcu_tasks_rude.
Have you tried with just synchronize_rcu() ?
If your theory about the race is correct then
the vanila sync_rcu should help.
If not, the issue is some place else.