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

From: Jiri Olsa <[email-masked]>
Date: Sat, 10 Dec 2022 01:06:16 +0100

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/[email-masked]/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;