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