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From:   Jiri Olsa <olsajiri@gmail.com>
Date:   Sat, 10 Dec 2022 14:11:34 +0100
To:     Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc:     Jiri Olsa <olsajiri@gmail.com>, Jakub Kicinski <kuba@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Yonghong Song <yhs@meta.com>, Song Liu <song@kernel.org>,
        Hao Sun <sunhao.th@gmail.com>,
        Peter Zijlstra <peterz@infradead.org>,
        bpf <bpf@vger.kernel.org>, Alexei Starovoitov <ast@kernel.org>,
        John Fastabend <john.fastabend@gmail.com>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Yonghong Song <yhs@fb.com>, KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>,
        David Miller <davem@davemloft.net>,
        Jesper Dangaard Brouer <hawk@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Thorsten Leemhuis <regressions@leemhuis.info>
Subject: Re: BUG: unable to handle kernel paging request in bpf_dispatcher_xdp
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On Fri, Dec 09, 2022 at 05:12:03PM -0800, Alexei Starovoitov wrote:
> On Fri, Dec 9, 2022 at 4:06 PM Jiri Olsa <olsajiri@gmail.com> 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@kernel.org/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.

synchronize_rcu seems to work as well, I'll keep the test
running for some time

thanks,
jirka