RE: [PATCH net-next] hv_netvsc: Fix race condition on Multi-Send Data field

[Haiyang Zhang]

> -----Original Message-----
> From: Vitaly Kuznetsov [mailto:vkuznets@xxxxxxxxxx]
> Sent: Friday, December 11, 2015 8:53 AM
> To: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> Cc: davem@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> jasowang@xxxxxxxxxx; driverdev-devel@xxxxxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH net-next] hv_netvsc: Fix race condition on Multi-
> Send Data field
> 
> Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> writes:
> 
> > Haiyang Zhang <haiyangz@xxxxxxxxxxxxx> writes:
> >
> >> In commit 2a04ae8acb14 ("hv_netvsc: remove locking in netvsc_send()"),
> the
> >> locking for MSD (Multi-Send Data) field was removed. This could cause
> a
> >> race condition between RNDIS control messages and data packets
> processing,
> >> because these two types of traffic are not synchronized.
> >> This patch fixes this issue by sending control messages out directly
> >> without reading MSD field.
> >>
> >> Signed-off-by: Haiyang Zhang <haiyangz@xxxxxxxxxxxxx>
> >> Reviewed-by: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
> >> ---
> >>  drivers/net/hyperv/netvsc.c |    9 +++++++++
> >>  1 files changed, 9 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/drivers/net/hyperv/netvsc.c
> b/drivers/net/hyperv/netvsc.c
> >> index 02bab9a..059fc52 100644
> >> --- a/drivers/net/hyperv/netvsc.c
> >> +++ b/drivers/net/hyperv/netvsc.c
> >> @@ -867,6 +867,14 @@ int netvsc_send(struct hv_device *device,
> >>  	packet->send_buf_index = NETVSC_INVALID_INDEX;
> >>  	packet->cp_partial = false;
> >>
> >> +	/* Send control message directly without accessing msd (Multi-Send
> >> +	 * Data) field which may be changed during data packet processing.
> >> +	 */
> >> +	if (!skb) {
> >> +		cur_send = packet;
> >> +		goto send_now;
> >> +	}
> >> +
> >
> > Is is supposed to be applied on top of some other patches? It doesn't
> > compile on top of current net-next:
> >
> > drivers/net/hyperv/netvsc.c: In function ânetvsc_sendâ:
> > drivers/net/hyperv/netvsc.c:865:7: error: âskbâ undeclared (first use
> in
> > this function)
> >   if (!skb) {
> >          ^
> >
> > Did you mean to check rndis_msg instead (as skb is not defined here)?
> 
> Oh, my bad, it was me who was looking at the wrong branch... Sorry for
> the noise.
> 
> >
> >>  	msdp = &net_device->msd[q_idx];
> >>
> >>  	/* batch packets in send buffer if possible */
> >> @@ -939,6 +947,7 @@ int netvsc_send(struct hv_device *device,
> >>  		}
> >>  	}
> >>
> >> +send_now:
> >>  	if (cur_send)
> >>  		ret = netvsc_send_pkt(cur_send, net_device, pb, skb);
> >
> > I suppose we untangle these two pathes completely: let
> > rndis_filter_send_request() call netvsc_send_pkt() directly. Please
> see
> > my patch attached (note: it should be split in 3 patches if
> > submitted). If you like the idea I can send it.
> 
> This patch will need some changes but the suggestion still stands: let's
> untangle sending data and control messages.

Thanks for the suggestion.
I still prefer fixing this bug in the netvsc_send(), and keep the common
entry point for sending data & control packets. So, the total lines of
code is smaller. And, when we add more pre-processing steps for send in 
the future, we won't need to change multiple places.

Thanks,
- Haiyang