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

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Date:   Thu, 18 Oct 2018 11:08:12 +0200
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>
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 18:00:05 [+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].

Sebastian