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

From: Jiri Olsa <olsajiri@gmail.com>
To: Jakub Kicinski <kuba@kernel.org>,
	"Paul E. McKenney" <paulmck@kernel.org>
Cc: Daniel Borkmann <daniel@iogearbox.net>,
	Jiri Olsa <olsajiri@gmail.com>, Yonghong Song <yhs@meta.com>,
	Alexei Starovoitov <alexei.starovoitov@gmail.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
Date: Sat, 10 Dec 2022 01:06:16 +0100	[thread overview]
Message-ID: <Y5PNeFYJrC6D4P9p@krava> (raw)
In-Reply-To: <20221209153445.22182ca5@kernel.org>

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 ;-)

thanks,
jirka


---

diff --git a/kernel/bpf/dispatcher.c b/kernel/bpf/dispatcher.c
index c19719f48ce0..e6126f07e85b 100644
--- a/kernel/bpf/dispatcher.c
+++ b/kernel/bpf/dispatcher.c
@@ -124,6 +124,7 @@ static void bpf_dispatcher_update(struct bpf_dispatcher *d, int prev_num_progs)
 	}
 
 	__BPF_DISPATCHER_UPDATE(d, new ?: (void *)&bpf_dispatcher_nop_func);
+	synchronize_rcu_tasks_rude();
 
 	if (new)
 		d->image_off = noff;
