netdev - Re: virtio_net: suspicious RCU usage with xdp

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Date:   Sun, 28 Apr 2019 11:06:56 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        David Ahern <dsahern@...il.com>,
        Toke Høiland-Jørgensen <toke@...e.dk>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: virtio_net: suspicious RCU usage with xdp


On 2019/4/26 下午7:05, Jesper Dangaard Brouer wrote:
> On Fri, 26 Apr 2019 16:00:28 +0800
> Jason Wang <jasowang@...hat.com> wrote:
>
>> On 2019/4/26 上午1:41, Jesper Dangaard Brouer wrote:
>>> It does sound like my commit 5d053f9da431 ("bpf: devmap prepare xdp
>>> frames for bulking") introduced this issue.  I guess we can add the RCU
>>> section to xdp_do_flush_map(), and then also verify that the devmap
>>> (and cpumap) take-down code also have appropriate RCU sections (which
>>> they should have).
>>>
>>> Another requirement for calling .ndo_xdp_xmit is running under NAPI
>>> protection,
>>
>> May I know the reason for this? I'm asking since if the packet was
>> redirected from tuntap, ndo_xdp_xmit()  won't be called under the
>> protection of NAPI (but bh is disabled).
> There are a number of things that rely on this NAPI/softirq protection.
>
> One is preempt-free access per-cpu struct bpf_redirect_info. Which is
> at the core of the XDP and TC redirect feature.
>
>    DEFINE_PER_CPU(struct bpf_redirect_info, bpf_redirect_info);
>    EXPORT_PER_CPU_SYMBOL_GPL(bpf_redirect_info);
>    struct bpf_redirect_info *ri = this_cpu_ptr(&bpf_redirect_info);
>
> And devmap and cpumap also have per-cpu variables, that we don't use
> preempt-disable around.
>
> Another is xdp_return_frame_rx_napi() that when page_pool is active,
> can store frames to be recycled directly into an array, in function
> __page_pool_recycle_direct() (but as I don't trust every driver getting
> this correct I've added a safe-guard in page-pool via
> in_serving_softirq().


I see, if I want to use page pool for tap for VM2VM traffic, this 
probably means I can only recycle through ptr_ring.


>
> I guess, disable_bh is sufficient protection, as we are mostly
> optimizing away a preempt-disable when accessing per-cpu variables.
>

Yes, that's what I want for confirm.

Thanks