netdev - Re: virtio_net: suspicious RCU usage with xdp

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Date:   Fri, 26 Apr 2019 16:42:58 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Jesper Dangaard Brouer <brouer@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     David Ahern <dsahern@...il.com>,
        Toke Høiland-Jørgensen 
        <toke@...e.dk>, "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: virtio_net: suspicious RCU usage with xdp

On 2019/04/26 2:41, Jesper Dangaard Brouer wrote:
> On Thu, 25 Apr 2019 13:03:39 -0400
> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
>> On Thu, Apr 25, 2019 at 01:58:48PM +0900, Toshiaki Makita wrote:
>>> On 2019/04/25 2:37, Michael S. Tsirkin wrote:  
>>>> On Wed, Apr 24, 2019 at 11:13:42AM -0600, David Ahern wrote:  
>>>>> seeing an RCU warning testing xdp with virtio net. net-next as of commit
>>>>> b2f97f7de2f6a4df8e431330cf467576486651c5. No obvious changes so hoping
>>>>> this rings a bell with someone else.
>>>>>
>>>>>
>>>>> [  121.990304] =============================
>>>>> [  121.991488] WARNING: suspicious RCU usage
>>>>> [  121.992392] 5.1.0-rc5+ #60 Not tainted
>>>>> [  121.993220] -----------------------------
>>>>> [  121.994158] /home/dsa/kernel-3.git/drivers/net/virtio_net.c:516
>>>>> suspicious rcu_dereference_check() usage!
>>>>> [  121.996284]
>>>>>                other info that might help us debug this:
>>>>>
>>>>> [  121.997988]
>>>>>                rcu_scheduler_active = 2, debug_locks = 1
>>>>> [  121.999321] no locks held by swapper/1/0.
>>>>> [  122.000328]
>>>>>                stack backtrace:
>>>>> [  122.001253] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.1.0-rc5+ #60
>>>>> [  122.002474] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996),
>>>>> BIOS 1.11.1-1 04/01/2014
>>>>> [  122.004141] Call Trace:
>>>>> [  122.004651]  <IRQ>
>>>>> [  122.005082]  dump_stack+0x7e/0xbb
>>>>> [  122.005757]  lockdep_rcu_suspicious+0x102/0x10b
>>>>> [  122.006654]  virtnet_xdp_xmit+0x104/0x4fe
>>>>> [  122.007447]  ? kasan_check_read+0x11/0x13
>>>>> [  122.008267]  ? mergeable_rx_buffer_size_show+0x163/0x163
>>>>> [  122.009299]  ? __asan_loadN+0xf/0x11
>>>>> [  122.010010]  ? pvclock_clocksource_read+0xfa/0x189
>>>>> [  122.010975]  bq_xmit_all+0xdc/0x358
>>>>> [  122.011699]  __dev_map_flush+0xc2/0xef
>>>>> [  122.012472]  xdp_do_flush_map+0x5b/0x74
>>>>> [  122.013238]  virtnet_poll+0x58f/0x679  
>>>>
>>>> Well virtnet_xdp_xmit seems to be called from .ndo_xdp_xmit
>>>> and that isn't in an RCU read-side critical section.
>>>>
>>>> Looks like we just need to add RCU read lock/unlock.
>>>> Like the below perhaps?
>>>>
>>>> This issue was introduced by 8dcc5b0ab0 however I find it  
>>>
>>> Probably not 8dcc5b0ab0, but 5d053f9da431 ("bpf: devmap prepare xdp
>>> frames for bulking").
>>>   
>>>> inelegant that we need to do checks in each driver,
>>>> and add RCU locks just for a startup initialization issue.
>>>> Can't XDP core make sure the callback isn't invoked
>>>> at an inappropriate time instead?  
>>>
>>> Before commit 5d053f9da431, .ndo_xdp_xmit() should have always been
>>> called under RCU. After the commit, xdp_do_flush_map() also can trigger
>>> .ndo_xdp_xmit() but we forgot to add RCU read lock there?
>>> I guess veth has the same problem and I feel like it should be fixed in
>>> __dev_map_flush(). dev_map_flush_old() needs to be cared too.  
>>
>> I don't have a problem either way.  Jesper, what do you think?
> 
> 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, is that still satisifed for veth?
> (even when invoked via xdp_do_flush_map()).

veth_xdp_xmit() assumes it's called from NAPI handler, and veth calls
xdp_do_flush_map() only from within NAPI context. Not sure what happens
when .ndo_xdp_xmit is called from dev_map_flush_old().

-- 
Toshiaki Makita