Re: [PATCH V2 5/6] vhost_net: poll vhost queue after marking DMA is done

Jason Wang
2014-02-12T10:06:15Z

On 02/12/2014 03:38 PM, Qin Chuanyu wrote:
> 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.
>

Good catch.
> 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.

Then you need another ref count to protect that spinlock? Care to send
patches?

Thanks
>
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