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    SubjectRe: [PATCH net V2 2/4] vhost_net: switch to use mutex_trylock() in vhost_net_busy_poll()
    On Wed, Dec 12, 2018 at 06:08:17PM +0800, Jason Wang wrote:
    > We used to hold the mutex of paired virtqueue in
    > vhost_net_busy_poll(). But this will results an inconsistent lock
    > order which may cause deadlock if we try to bring back the protection
    > of device IOTLB with vq mutex that requires to hold mutex of all
    > virtqueues at the same time.
    >
    > Fix this simply by switching to use mutex_trylock(), when fail just
    > skip the busy polling. This can happen when device IOTLB is under
    > updating which should be rare.
    >
    > Fixes: commit 78139c94dc8c ("net: vhost: lock the vqs one by one")
    > Cc: Tonghao Zhang <xiangxia.m.yue@gmail.com>
    > Signed-off-by: Jason Wang <jasowang@redhat.com>

    Acked-by: Michael S. Tsirkin <mst@redhat.com>

    and I think we should try to put this fix in 4.20 too.


    > ---
    > drivers/vhost/net.c | 8 +++++++-
    > 1 file changed, 7 insertions(+), 1 deletion(-)
    >
    > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
    > index ab11b2bee273..ad7a6f475a44 100644
    > --- a/drivers/vhost/net.c
    > +++ b/drivers/vhost/net.c
    > @@ -513,7 +513,13 @@ static void vhost_net_busy_poll(struct vhost_net *net,
    > struct socket *sock;
    > struct vhost_virtqueue *vq = poll_rx ? tvq : rvq;
    >
    > - mutex_lock_nested(&vq->mutex, poll_rx ? VHOST_NET_VQ_TX: VHOST_NET_VQ_RX);
    > + /* Try to hold the vq mutex of the paired virtqueue. We can't
    > + * use mutex_lock() here since we could not guarantee a
    > + * consistenet lock ordering.
    > + */
    > + if (!mutex_trylock(&vq->mutex))
    > + return;
    > +
    > vhost_disable_notify(&net->dev, vq);
    > sock = rvq->private_data;
    >
    > --
    > 2.17.1

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