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SubjectRe: [PATCH net-next 8/8] vhost_net: use lockless peeking for skb array during busy polling
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On 2017年03月30日 10:33, Michael S. Tsirkin wrote:
> On Thu, Mar 30, 2017 at 10:16:15AM +0800, Jason Wang wrote:
>>
>> On 2017年03月29日 20:07, Michael S. Tsirkin wrote:
>>> On Tue, Mar 21, 2017 at 12:04:47PM +0800, Jason Wang wrote:
>>>> For the socket that exports its skb array, we can use lockless polling
>>>> to avoid touching spinlock during busy polling.
>>>>
>>>> Signed-off-by: Jason Wang <jasowang@redhat.com>
>>>> ---
>>>> drivers/vhost/net.c | 7 +++++--
>>>> 1 file changed, 5 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
>>>> index 53f09f2..41153a3 100644
>>>> --- a/drivers/vhost/net.c
>>>> +++ b/drivers/vhost/net.c
>>>> @@ -551,10 +551,13 @@ static int peek_head_len(struct vhost_net_virtqueue *rvq, struct sock *sk)
>>>> return len;
>>>> }
>>>> -static int sk_has_rx_data(struct sock *sk)
>>>> +static int sk_has_rx_data(struct vhost_net_virtqueue *rvq, struct sock *sk)
>>>> {
>>>> struct socket *sock = sk->sk_socket;
>>>> + if (rvq->rx_array)
>>>> + return !__skb_array_empty(rvq->rx_array);
>>>> +
>>>> if (sock->ops->peek_len)
>>>> return sock->ops->peek_len(sock);
>>> I don't see which patch adds __skb_array_empty.
>> This is not something new, it was introduced by ad69f35d1dc0a ("skb_array:
>> array based FIFO for skbs").
>>
>> Thanks
> Same comment about a compiler barrier applies then.

Ok, rethink about this, since skb_array could be resized, using lockless
version seems wrong.

For the comment of using compiler barrier, caller
(vhost_net_rx_peek_head_len) uses cpu_relax(). But I haven't figured out
why a compiler barrier is needed here. Could you please explain?

Thanks

>
>>>> @@ -579,7 +582,7 @@ static int vhost_net_rx_peek_head_len(struct vhost_net *net,
>>>> endtime = busy_clock() + vq->busyloop_timeout;
>>>> while (vhost_can_busy_poll(&net->dev, endtime) &&
>>>> - !sk_has_rx_data(sk) &&
>>>> + !sk_has_rx_data(rvq, sk) &&
>>>> vhost_vq_avail_empty(&net->dev, vq))
>>>> cpu_relax();
>>>> --
>>>> 2.7.4

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