netdev - Re: [BUG] Inconsistent lock state in virtnet poll

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Date:   Tue, 5 May 2020 15:40:09 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     netdev@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [BUG] Inconsistent lock state in virtnet poll



On 5/5/20 3:30 PM, Thomas Gleixner wrote:
> "Michael S. Tsirkin" <mst@...hat.com> 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

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