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

Michael S. Tsirkin
2011-03-13T21:11:50Z

On Sun, Mar 13, 2011 at 06:41:32PM +0100, Eric Dumazet wrote:
> Le dimanche 13 mars 2011 à 18:43 +0200, Michael S. Tsirkin a écrit :
> > On Sun, Mar 13, 2011 at 05:32:07PM +0100, Eric Dumazet wrote:
> > > Le dimanche 13 mars 2011 à 18:19 +0200, Michael S. Tsirkin a écrit :
> > >
> > > > 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.
> > >
> > > Yes (assuming you refer to tun_net_xmit())
> > >
> > > > Besides queue, dequeue seems to be done without socket locked.
> > > >
> > >
> > > It seems this code (assuming you speak of drivers/vhost/net.c ?) has
> > > some races indeed.
> > >
> >
> > Hmm. Any more besides the one fixed here?
> >
>
> If writers and readers dont share a common lock, how can they reliably
> synchronize states ?

They are all supposed to use sk_receive_queue.lock I think.

> For example, the check at line 420 seems unsafe or useless.
>
> skb_queue_empty(&sock->sk->sk_receive_queue)
>

It's mostly useless: code that is called after this
does skb_peek and checks the result under the spinlock.
This was supposed to be an optimization: quickly check
that queue is not empty before we bother disabling notifications
etc, but I dont' remember at this point whether it actually gives any gain.
Thanks for pointing this out, I'll take it out I think (below).
Note: there are two places of this call in upstream: handle_rx_bug and
handle_rx_mergeable, but they are merged into a single
handle_rx by a patch by Jason Wang.
The below patch is on top.
If you like to look at the latest code,
it's here master.kernel.org:/home/mst/pub/vhost.git
branch vhost-net-next has it all.

Eric, thanks very much for pointing out these.
Is there anything else that you see in this driver?

Thanks!

vhost-net: remove unlocked use of receive_queue

Use of skb_queue_empty(&sock->sk->sk_receive_queue)
without taking the sk_receive_queue.lock is unsafe
or useless. Take it out.

Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
index 5720301..2f7c76a 100644
--- a/drivers/vhost/net.c
+++ b/drivers/vhost/net.c
@@ -311,7 +311,7 @@ static void handle_rx(struct vhost_net *net)
/* TODO: check that we are running from vhost_worker? */
struct socket *sock = rcu_dereference_check(vq->private_data, 1);

- if (!sock || skb_queue_empty(&sock->sk->sk_receive_queue))
+ if (!sock)
return;

mutex_lock(&vq->mutex);