Subject: Re: [RFC PATCH net-next 1/6] virtio: make sure used event never go backwards

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 15 Oct 2014 13:32:32 +0300

On Wed, Oct 15, 2014 at 06:13:19PM +0800, Jason Wang wrote:
> On 10/15/2014 05:34 PM, Michael S. Tsirkin wrote:
> > On Wed, Oct 15, 2014 at 03:25:25PM +0800, Jason Wang wrote:
> >> This patch checks the new event idx to make sure used event idx never
> >> goes back. This is used to synchronize the calls between
> >> virtqueue_enable_cb_delayed() and virtqueue_enable_cb().
> >>
> >> Cc: Rusty Russell <[email-masked]>
> >> Cc: Michael S. Tsirkin <[email-masked]>
> >> Signed-off-by: Jason Wang <[email-masked]>
> > the implication being that moving event idx back might cause some race
> > condition?  
> 
> This will cause race condition when tx interrupt is enabled. Consider
> the following cases
> 
> 1) tx napi was scheduled
> 2) start_xmit() call virtqueue_enable_cb_delayed() and disable cb, [used
> event is vq->last_used_idx + 3/4 pendg bufs]
> 3) tx napi enable the callback by virtqueue_enable_cb() [ used event is
> vq->last_used_idx ]
>  
> After step 3, used event was moved back, unnecessary tx interrupt was
> triggered.

Well unnecessary interrupts are safe.
With your patch caller of virtqueue_enable_cb will not get an
interrupt on the next buffer which is not safe.

If you don't want an interrupt on the next buffer, don't
call virtqueue_enable_cb.

> > If yes but please describe the race explicitly.
> > Is there a bug we need to fix on stable?
> 
> Looks not, current code does not have such race condition.
> > Please also explicitly describe a configuration that causes event idx
> > to go back.
> >
> > All this info should go in the commit log.
> 
> Will do this.
> >> ---
> >>  drivers/virtio/virtio_ring.c |    7 +++++--
> >>  1 files changed, 5 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> >> index 3b1f89b..1b3929f 100644
> >> --- a/drivers/virtio/virtio_ring.c
> >> +++ b/drivers/virtio/virtio_ring.c
> >> @@ -559,14 +559,17 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> >>  	u16 last_used_idx;
> >>  
> >>  	START_USE(vq);
> >> -
> >> +	last_used_idx = vq->last_used_idx;
> >>  	/* We optimistically turn back on interrupts, then check if there was
> >>  	 * more to do. */
> >>  	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> >>  	 * either clear the flags bit or point the event index at the next
> >>  	 * entry. Always do both to keep code simple. */
> >>  	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;
> >> -	vring_used_event(&vq->vring) = last_used_idx = vq->last_used_idx;
> >> +	/* Make sure used event never go backwards */
> > s/go/goes/
> >
> >> +	if (!vring_need_event(vring_used_event(&vq->vring),
> >> +			      vq->vring.avail->idx, last_used_idx))
> >> +		vring_used_event(&vq->vring) = last_used_idx;
> > The result will be that driver will *not* get an interrupt
> > on the next consumed buffer, which is likely not what driver
> > intended when it called virtqueue_enable_cb.
> 
> This will only happen when we want to delay the interrupt for next few
> consumed buffers (virtqueue_enable_cb_delayed() was called). For the
> other case, vq->last_used_idx should be ahead of previous used event. Do
> you see any other case?

Call virtqueue_enable_cb_delayed, later call virtqueue_enable_cb.  If
event index is not updated in virtqueue_enable_cb, driver will not get
an interrupt on the next buffer.

> >
> > Instead, how about we simply document the requirement that drivers either
> > always call virtqueue_enable_cb_delayed or virtqueue_enable_cb
> > but not both?
> 
> We need call them both when tx interrupt is enabled I believe.

Can you pls reply to my patch and document issues you see?

> >>  	END_USE(vq);
> >>  	return last_used_idx;
> >>  }
> >> -- 
> >> 1.7.1