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    Subject[PATCH 4.19 34/71] virtio_ring: Avoid loop when vq is broken in virtqueue_poll
    Date
    From: Mao Wenan <wenan.mao@linux.alibaba.com>

    [ Upstream commit 481a0d7422db26fb63e2d64f0652667a5c6d0f3e ]

    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>
    Link: https://lore.kernel.org/r/1596354249-96204-1-git-send-email-wenan.mao@linux.alibaba.com
    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    Acked-by: Jason Wang <jasowang@redhat.com>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    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 6228b48d1e127..df7980aef927a 100644
    --- a/drivers/virtio/virtio_ring.c
    +++ b/drivers/virtio/virtio_ring.c
    @@ -828,6 +828,9 @@ 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 (u16)last_used_idx != virtio16_to_cpu(_vq->vdev, vq->vring.used->idx);
    }
    --
    2.25.1


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