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

Paul E. McKenney
2022-12-10T00:38:50Z

On Sat, Dec 10, 2022 at 01:06:16AM +0100, Jiri Olsa 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 ;-)

It sounds like we are all talking past each other, leaving me no
choice but to supply a wall of text:

It is quite true that synchronize_rcu_tasks_rude() will wait
for bh-disabled regions of code, just like synchronize_rcu()
and synchronize_rcu_tasks() will. However, please note that
synchronize_rcu_tasks() never waits on any of the idle tasks. So the
usual approach in tracing is to do both a synchronize_rcu_tasks() and
synchronize_rcu_tasks_rude(). One way of overlapping the resulting
pair of grace periods is to use synchronize_rcu_mult().

But none of these permit readers to sleep. That is what
synchronize_rcu_tasks_trace() is for, but unlike both
synchronize_rcu_tasks() and synchronize_rcu_tasks_rude(),
you must explicitly mark the readers with rcu_read_lock_trace()
and rcu_read_unlock_trace(). This is used to protect sleepable
BPF programs.

Now, synchronize_rcu() will also wait on bh-disabled lines of code, with
the exception of such code in the exception path, way deep in the idle
loop, early in the CPU-online process, or late in the CPU-offline process.
You can recognize the first two categories of code by the noinstr tags
on the functions.

And yes, synchronize_rcu_rude() is quite special. ;-)

Does this help, or am I simply adding to the confusion?

Thanx, Paul

> 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;