On Thu, Jul 9, 2020 at 7:06 PM Jonathan Lemon <[email protected]> wrote:
>
> On Thu, Jul 09, 2020 at 11:45:51AM +0200, Magnus Karlsson wrote:
> > In the skb Tx path, transmission of a packet is performed with
> > dev_direct_xmit(). When QUEUE_STATE_FROZEN is set in the transmit
> > routines, it returns NETDEV_TX_BUSY signifying that it was not
> > possible to send the packet now, please try later. Unfortunately, the
> > xsk transmit code discarded the packet and returned EBUSY to the
> > application. Fix this unnecessary packet loss, by not discarding the
> > packet and return EAGAIN. As EAGAIN is returned to the application, it
> > can then retry the send operation and the packet will finally be sent
> > as we will likely not be in the QUEUE_STATE_FROZEN state anymore. So
> > EAGAIN tells the application that the packet was not discarded from
> > the Tx ring and that it needs to call send() again. EBUSY, on the
> > other hand, signifies that the packet was not sent and discarded from
> > the Tx ring. The application needs to put the packet on the Tx ring
> > again if it wants it to be sent.
>
> Doesn't the original code leak the skb if NETDEV_TX_BUSY is returned?
> I'm not seeing where it was released. The new code looks correct.
You are correct. Should also have mentioned that in the commit message.
/Magnus
>
> > Fixes: 35fcde7f8deb ("xsk: support for Tx")
> > Signed-off-by: Magnus Karlsson <[email protected]>
> > Reported-by: Arkadiusz Zema <[email protected]>
> > Suggested-by: Arkadiusz Zema <[email protected]>
> > ---
> > net/xdp/xsk.c | 13 +++++++++++--
> > 1 file changed, 11 insertions(+), 2 deletions(-)
> >
> > diff --git a/net/xdp/xsk.c b/net/xdp/xsk.c
> > index 3700266..5304250 100644
> > --- a/net/xdp/xsk.c
> > +++ b/net/xdp/xsk.c
> > @@ -376,13 +376,22 @@ static int xsk_generic_xmit(struct sock *sk)
> > skb->destructor = xsk_destruct_skb;
> >
> > err = dev_direct_xmit(skb, xs->queue_id);
> > - xskq_cons_release(xs->tx);
> > /* Ignore NET_XMIT_CN as packet might have been sent */
> > - if (err == NET_XMIT_DROP || err == NETDEV_TX_BUSY) {
> > + if (err == NET_XMIT_DROP) {
> > /* SKB completed but not sent */
> > + xskq_cons_release(xs->tx);
> > err = -EBUSY;
> > goto out;
> > + } else if (err == NETDEV_TX_BUSY) {
>
> Should be "if (err == ..." here, no else.
>
>
> > + /* QUEUE_STATE_FROZEN, tell application to
> > + * retry sending the packet
> > + */
> > + skb->destructor = NULL;
> > + kfree_skb(skb);
> > + err = -EAGAIN;
> > + goto out;
> > }
> > + xskq_cons_release(xs->tx);
> >
> > sent_frame = true;
> > }
> > --
> > 2.7.4
> >
