Subject: Re: [BUG] Inconsistent lock state in virtnet poll

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 5 May 2020 20:43:16 -0400

On Tue, May 05, 2020 at 03:40:09PM -0700, Eric Dumazet wrote:
> 
> 
> On 5/5/20 3:30 PM, Thomas Gleixner wrote:
> > "Michael S. Tsirkin" <[email-masked]> writes:
> >> On Tue, May 05, 2020 at 02:08:56PM +0200, Thomas Gleixner wrote:
> >>>
> >>> The following lockdep splat happens reproducibly on 5.7-rc4
> >>
> >>> ================================
> >>> WARNING: inconsistent lock state
> >>> 5.7.0-rc4+ #79 Not tainted
> >>> --------------------------------
> >>> inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
> >>> ip/356 [HC0[0]:SC1[1]:HE1:SE0] takes:
> >>> f3ee4cd8 (&syncp->seq#2){+.?.}-{0:0}, at: net_rx_action+0xfb/0x390
> >>> {SOFTIRQ-ON-W} state was registered at:
> >>>   lock_acquire+0x82/0x300
> >>>   try_fill_recv+0x39f/0x590
> >>
> >> Weird. Where does try_fill_recv acquire any locks?
> > 
> >   u64_stats_update_begin(&rq->stats.syncp);
> > 
> > That's a 32bit kernel which uses a seqcount for this. sequence counts
> > are "lock" constructs where you need to make sure that writers are
> > serialized.
> > 
> > Actually the problem at hand is that try_fill_recv() is called from
> > fully preemptible context initialy and then from softirq context.
> > 
> > Obviously that's for the open() path a non issue, but lockdep does not
> > know about that. OTOH, there is other code which calls that from
> > non-softirq context.
> > 
> > The hack below made it shut up. It's obvioulsy not ideal, but at least
> > it let me look at the actual problem I was chasing down :)
> > 
> > Thanks,
> > 
> >         tglx
> > 
> > 8<-----------
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -1243,9 +1243,11 @@ static bool try_fill_recv(struct virtnet
> >  			break;
> >  	} while (rq->vq->num_free);
> >  	if (virtqueue_kick_prepare(rq->vq) && virtqueue_notify(rq->vq)) {
> > +		local_bh_disable();
> 
> Or use u64_stats_update_begin_irqsave() whic is a NOP on 64bit kernels

I applied this, but am still trying to think of something that
is 0 overhead for all configs.
Maybe we can select a lockdep class depending on whether napi
is enabled?

> >  		u64_stats_update_begin(&rq->stats.syncp);
> >  		rq->stats.kicks++;
> >  		u64_stats_update_end(&rq->stats.syncp);
> > +		local_bh_enable();
> >  	}
> >  
> >  	return !oom;
> >