Messages in this thread | | | Date | Wed, 12 Feb 2014 15:38:18 +0800 | From | Qin Chuanyu <> | Subject | Re: [PATCH V2 5/6] vhost_net: poll vhost queue after marking DMA is done |
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On 2013/8/30 12:29, Jason Wang wrote: > We used to poll vhost queue before making DMA is done, this is racy if vhost > thread were waked up before marking DMA is done which can result the signal to > be missed. Fix this by always poll the vhost thread before DMA is done. > > Signed-off-by: Jason Wang <jasowang@redhat.com> > --- > drivers/vhost/net.c | 9 +++++---- > 1 files changed, 5 insertions(+), 4 deletions(-) > > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c > index ff60c2a..d09c17c 100644 > --- a/drivers/vhost/net.c > +++ b/drivers/vhost/net.c > @@ -308,6 +308,11 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success) > struct vhost_virtqueue *vq = ubufs->vq; > int cnt = atomic_read(&ubufs->kref.refcount); > > + /* set len to mark this desc buffers done DMA */ > + vq->heads[ubuf->desc].len = success ? > + VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN; > + vhost_net_ubuf_put(ubufs); > + > /* > * Trigger polling thread if guest stopped submitting new buffers: > * in this case, the refcount after decrement will eventually reach 1 > @@ -318,10 +323,6 @@ static void vhost_zerocopy_callback(struct ubuf_info *ubuf, bool success) > */ > if (cnt <= 2 || !(cnt % 16)) > vhost_poll_queue(&vq->poll); > - /* set len to mark this desc buffers done DMA */ > - vq->heads[ubuf->desc].len = success ? > - VHOST_DMA_DONE_LEN : VHOST_DMA_FAILED_LEN; > - vhost_net_ubuf_put(ubufs); > } > > /* Expects to be always run from workqueue - which acts as > with this change, vq would lose protection that provided by ubufs->kref. if another thread is waiting at vhost_net_ubuf_put_and_wait called by vhost_net_release, then after vhost_net_ubuf_put, vq would been free by vhost_net_release soon, vhost_poll_queue(&vq->poll) may cause NULL pointer Exception.
another question is that vhost_zerocopy_callback is called by kfree_skb, it may called in different thread context. vhost_poll_queue is called decided by ubufs->kref.refcount, this may cause there isn't any thread call vhost_poll_queue, but at least one is needed. and this cause network break. We could repeat it by using 8 netperf thread in guest to xmit tcp to its host.
I think if using atomic_read to decide while do vhost_poll_queue or not, at least a spink_lock is needed.
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