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

Michael S. Tsirkin
2011-03-13T15:07:27Z

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;
}