Re: [PATCH 3/3] vhost-net: use lock_sock_fast() in peek_head_len()

Eric Dumazet
2011-03-13T15:53:00Z

Le dimanche 13 mars 2011 à 17:06 +0200, Michael S. Tsirkin a écrit :
> On Mon, Jan 17, 2011 at 04:11:17PM +0800, Jason Wang wrote:
> > We can use lock_sock_fast() instead of lock_sock() in order to get
> > speedup in peek_head_len().
> >
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> > drivers/vhost/net.c | 4 ++--
> > 1 files changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> > index c32a2e4..50b622a 100644
> > --- a/drivers/vhost/net.c
> > +++ b/drivers/vhost/net.c
> > @@ -211,12 +211,12 @@ static int peek_head_len(struct sock *sk)
> > {
> > struct sk_buff *head;
> > int len = 0;
> > + bool slow = lock_sock_fast(sk);
> >
> > - lock_sock(sk);
> > head = skb_peek(&sk->sk_receive_queue);
> > if (head)
> > len = head->len;
> > - release_sock(sk);
> > + unlock_sock_fast(sk, slow);
> > return len;
> > }
> >
>
> Wanted to apply this, but looking at the code I think the lock_sock here
> is wrong. What we really need is to handle the case where the skb is
> pulled from the receive queue after skb_peek. However this is not the
> right lock to use for that, sk_receive_queue.lock is.
> So I expect the following is the right way to handle this.
> Comments?
>
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 0329c41..5720301 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -213,12 +213,13 @@ static int peek_head_len(struct sock *sk)
> {
> struct sk_buff *head;
> int len = 0;
> + unsigned long flags;
>
> - lock_sock(sk);
> + spin_lock_irqsave(&sk->sk_receive_queue.lock, flags);
> head = skb_peek(&sk->sk_receive_queue);
> - if (head)
> + if (likely(head))
> len = head->len;
> - release_sock(sk);
> + spin_unlock_irqrestore(&sk->sk_receive_queue.lock, flags);
> return len;
> }
>

You may be right, only way to be sure is to check the other side.

If it uses skb_queue_tail(), then yes, your patch is fine.

If other side did not lock socket, then your patch is a bug fix.