netdev - Re: [RFC] virtio_net: add local_bh_disable() around u64_stats_update

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Message-ID: <55f14915-744b-e11c-bc50-87a872218479@lab.ntt.co.jp>
Date:   Thu, 18 Oct 2018 18:19:21 +0900
From:   Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Jason Wang <jasowang@...hat.com>, netdev@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, tglx@...utronix.de,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "David S. Miller" <davem@...emloft.net>
Subject: Re: [RFC] virtio_net: add local_bh_disable() around
 u64_stats_update_begin

On 2018/10/18 18:08, Sebastian Andrzej Siewior wrote:
> On 2018-10-18 18:00:05 [+0900], Toshiaki Makita wrote:
>> On 2018/10/18 17:47, Sebastian Andrzej Siewior wrote:
>>> On 2018-10-17 14:48:02 [+0800], Jason Wang wrote:
>>>>
>>>> On 2018/10/17 上午9:13, Toshiaki Makita wrote:
>>>>> I'm not sure what condition triggered this warning.
>>>
>>> If the seqlock is acquired once in softirq and then in process context
>>> again it is enough evidence for lockdep to trigger this warning.
>>
>> No. As I said that should not happen because of NAPI guard.
> Again: lockdep saw the lock in softirq context once and in process
> context once and this is what triggers the warning. It does not matter
> if NAPI is enabled or not during the access in process context. If you
> want to allow this you need further lockdep annotation…
> 
> … but: refill_work() disables NAPI for &vi->rq[1] and refills + updates
> stats while NAPI is enabled for &vi->rq[0].

Do you mean this is false positive? rq[0] and rq[1] never race with each
other...

-- 
Toshiaki Makita