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

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Date:   Thu, 18 Oct 2018 11:30:35 +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 18:19:21 [+0900], Toshiaki Makita wrote:
> On 2018/10/18 18:08, Sebastian Andrzej Siewior wrote:
> > 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...

Why? So you can't refill rq[1] and then be interrupted and process NAPI
for rq[0]?

But as I said. If lockdep saw the lock in acquired in softirq (what it
did) _and_ in process context (what it did as well) _once_ then this is
enough evidence for the warning.
If you claim that this can not happen due to NAPI guard [0] then this is
something lockdep does not know about.

[0] which I currently don't understand and therefore sent the patch [1]
    as Jason pointed out that in the ->ndo_open case the work is
    scheduled and then NAPI is enabled (which means the worker could
    disable NAPI and refill but before it finishes, ->ndo_open would
    continue and enable NAPI)).
[1] 20181018084753.wefvsypdevbzoadg@...utronix.de

Sebastian