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Subject: Re: [PATCH -next v3] virtio_ring: Avoid loop when vq is broken in
 virtqueue_poll
To:     Mao Wenan <wenan.mao@linux.alibaba.com>, mst@redhat.com
Cc:     virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org
References: <20200802022549-mutt-send-email-mst@kernel.org>
 <1596354249-96204-1-git-send-email-wenan.mao@linux.alibaba.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <12bf457d-1283-62c6-6901-29e7a14831ca@redhat.com>
Date:   Tue, 4 Aug 2020 10:42:56 +0800
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On 2020/8/2 下午3:44, Mao Wenan wrote:
> The loop may exist if vq->broken is true,
> virtqueue_get_buf_ctx_packed or virtqueue_get_buf_ctx_split
> will return NULL, so virtnet_poll will reschedule napi to
> receive packet, it will lead cpu usage(si) to 100%.
>
> call trace as below:
> virtnet_poll
> 	virtnet_receive
> 		virtqueue_get_buf_ctx
> 			virtqueue_get_buf_ctx_packed
> 			virtqueue_get_buf_ctx_split
> 	virtqueue_napi_complete
> 		virtqueue_poll           //return true
> 		virtqueue_napi_schedule //it will reschedule napi
>
> to fix this, return false if vq is broken in virtqueue_poll.
>
> Signed-off-by: Mao Wenan <wenan.mao@linux.alibaba.com>
> Acked-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>   v2->v3: change subject, original is : "virtio_net: Avoid loop in virtnet_poll"
>   v1->v2: fix it in virtqueue_poll suggested by Michael S. Tsirkin <mst@redhat.com>
>   drivers/virtio/virtio_ring.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> index 58b96ba..4f7c73e 100644
> --- a/drivers/virtio/virtio_ring.c
> +++ b/drivers/virtio/virtio_ring.c
> @@ -1960,6 +1960,9 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned last_used_idx)
>   {
>   	struct vring_virtqueue *vq = to_vvq(_vq);
>   
> +	if (unlikely(vq->broken))
> +		return false;
> +
>   	virtio_mb(vq->weak_barriers);
>   	return vq->packed_ring ? virtqueue_poll_packed(_vq, last_used_idx) :
>   				 virtqueue_poll_split(_vq, last_used_idx);


Acked-by: Jason Wang <jasowang@redhat.com>