Re: [PATCH 2/2] virtio_net: fix race in RX VQ processing

[Asias He <asias@redhat.com>]

On Tue, Jul 09, 2013 at 11:28:34AM +0800, Jason Wang wrote:
> On 07/08/2013 05:04 PM, Michael S. Tsirkin wrote:
> > virtio net called virtqueue_enable_cq on RX path after napi_complete, so
> > with NAPI_STATE_SCHED clear - outside the implicit napi lock.
> > This violates the requirement to synchronize virtqueue_enable_cq wrt
> > virtqueue_add_buf.  In particular, used event can move backwards,
> > causing us to lose interrupts.
> > In a debug build, this can trigger panic within START_USE.
> >
> > Jason Wang reports that he can trigger the races artificially,
> > by adding udelay() in virtqueue_enable_cb() after virtio_mb().
> >
> > However, we must call napi_complete to clear NAPI_STATE_SCHED before
> > polling the virtqueue for used buffers, otherwise napi_schedule_prep in
> > a callback will fail, causing us to lose RX events.
> >
> > To fix, call virtqueue_enable_cb_prepare with NAPI_STATE_SCHED
> > set (under napi lock), later call virtqueue_poll with
> > NAPI_STATE_SCHED clear (outside the lock).
> >
> > Reported-by: Jason Wang <jasowang@redhat.com>
> > Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

Acked-by: Asias He <asias@redhat.com>

> > ---
> 
> Tested-by: Jason Wang <jasowang@redhat.com>
> Acked-by: Jason Wang <jasowang@redhat.com>
> >  drivers/net/virtio_net.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > index 5305bd1..fbdd79a 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -622,8 +622,9 @@ again:
> >  
> >  	/* Out of packets? */
> >  	if (received < budget) {
> > +		unsigned r = virtqueue_enable_cb_prepare(rq->vq);
> >  		napi_complete(napi);
> > -		if (unlikely(!virtqueue_enable_cb(rq->vq)) &&
> > +		if (unlikely(virtqueue_poll(rq->vq, r)) &&
> >  		    napi_schedule_prep(napi)) {
> >  			virtqueue_disable_cb(rq->vq);
> >  			__napi_schedule(napi);
> 
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-- 
Asias