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

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Date:   Fri, 19 Oct 2018 10:17:07 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Cc:     Toshiaki Makita <makita.toshiaki@....ntt.co.jp>,
        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 下午4:47, 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.
>
>>> Toshiaki Makita
>>
>> Or maybe NAPI is enabled unexpectedly somewhere?
>>
>> Btw, the schedule_delayed_work() in virtnet_open() is also suspicious, if
>> the work is executed before virtnet_napi_enable(), there will be a deadloop
>> for napi_disable().
> something like this? It is also likely if it runs OOM on queue 2, it
> will run OOM again on queue 3.
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index fbcfb4d272336..87d6ec4765270 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1263,22 +1263,22 @@ static void refill_work(struct work_struct *work)
>   {
>   	struct virtnet_info *vi =
>   		container_of(work, struct virtnet_info, refill.work);
> -	bool still_empty;
> +	int still_empty = 0;
>   	int i;
>   
>   	for (i = 0; i < vi->curr_queue_pairs; i++) {
>   		struct receive_queue *rq = &vi->rq[i];
>   
>   		napi_disable(&rq->napi);
> -		still_empty = !try_fill_recv(vi, rq, GFP_KERNEL);
> +		if (!try_fill_recv(vi, rq, GFP_KERNEL))
> +		    still_empty++;
>   		virtnet_napi_enable(rq->vq, &rq->napi);
> -
> -		/* In theory, this can happen: if we don't get any buffers in
> -		 * we will *never* try to fill again.
> -		 */
> -		if (still_empty)
> -			schedule_delayed_work(&vi->refill, HZ/2);
>   	}
> +	/* In theory, this can happen: if we don't get any buffers in
> +	 * we will *never* try to fill again.
> +	 */
> +	if (still_empty)
> +		schedule_delayed_work(&vi->refill, HZ/2);
>   }


I think this part is not a must or an independent optimization?


Thanks


>   
>   static int virtnet_receive(struct receive_queue *rq, int budget,
> @@ -1407,12 +1407,13 @@ static int virtnet_open(struct net_device *dev)
>   {
>   	struct virtnet_info *vi = netdev_priv(dev);
>   	int i, err;
> +	int need_refill = 0;
>   
>   	for (i = 0; i < vi->max_queue_pairs; i++) {
>   		if (i < vi->curr_queue_pairs)
>   			/* Make sure we have some buffers: if oom use wq. */
>   			if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
> -				schedule_delayed_work(&vi->refill, 0);
> +				need_refill++;
>   
>   		err = xdp_rxq_info_reg(&vi->rq[i].xdp_rxq, dev, i);
>   		if (err < 0)
> @@ -1428,6 +1429,8 @@ static int virtnet_open(struct net_device *dev)
>   		virtnet_napi_enable(vi->rq[i].vq, &vi->rq[i].napi);
>   		virtnet_napi_tx_enable(vi, vi->sq[i].vq, &vi->sq[i].napi);
>   	}
> +	if (need_refill)
> +		schedule_delayed_work(&vi->refill, 0);
>   
>   	return 0;
>   }
> @@ -2236,6 +2239,7 @@ static int virtnet_restore_up(struct virtio_device *vdev)
>   {
>   	struct virtnet_info *vi = vdev->priv;
>   	int err, i;
> +	int need_refill = 0;
>   
>   	err = init_vqs(vi);
>   	if (err)
> @@ -2246,13 +2250,15 @@ static int virtnet_restore_up(struct virtio_device *vdev)
>   	if (netif_running(vi->dev)) {
>   		for (i = 0; i < vi->curr_queue_pairs; i++)
>   			if (!try_fill_recv(vi, &vi->rq[i], GFP_KERNEL))
> -				schedule_delayed_work(&vi->refill, 0);
> +				need_refill++;
>   
>   		for (i = 0; i < vi->max_queue_pairs; i++) {
>   			virtnet_napi_enable(vi->rq[i].vq, &vi->rq[i].napi);
>   			virtnet_napi_tx_enable(vi, vi->sq[i].vq,
>   					       &vi->sq[i].napi);
>   		}
> +		if (need_refill)
> +			schedule_delayed_work(&vi->refill, 0);
>   	}
>   
>   	netif_device_attach(vi->dev);
>
>> Thanks
> Sebastian