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Date:   Tue, 5 May 2020 18:19:09 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, 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 5:43 PM, Michael S. Tsirkin wrote:
> 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" <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
> 
> 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?


Do you _really_ need 64bit counter for stats.kicks on 32bit kernels ?

Adding 64bit counters just because we can might be overhead anyway.

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

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