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From:   Mao Wenan <wenan.mao@linux.alibaba.com>
To:     mst@redhat.com, jasowang@redhat.com
Cc:     virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, kernel-janitors@vger.kernel.org,
        Mao Wenan <wenan.mao@linux.alibaba.com>
Subject: [PATCH -next v2] virtio_net: Avoid loop in virtnet_poll
Date:   Sun,  2 Aug 2020 13:56:33 +0800
Message-Id: <1596347793-55894-1-git-send-email-wenan.mao@linux.alibaba.com>
In-Reply-To: <20200802003818-mutt-send-email-mst@kernel.org>
References: <20200802003818-mutt-send-email-mst@kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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>
---
 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);
-- 
1.8.3.1