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

Michael S. Tsirkin
2011-03-13T16:19:33Z

On Sun, Mar 13, 2011 at 04:52:50PM +0100, Eric Dumazet wrote:
> 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.
>
>

Other side is in drivers/net/tun.c and net/packet/af_packet.c
At least wrt tun it seems clear socket is not locked.
Besides queue, dequeue seems to be done without socket locked.

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MST