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 14:38:59 +0300

On Wed, Oct 15, 2014 at 06:44:41PM +0800, Jason Wang wrote:
> On 10/15/2014 06:32 PM, Michael S. Tsirkin wrote:
> > 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.
> 
> But it that is what we want to reduce.

It's all about correctness. I don't think mixing enable_cb
and enable_cb_delayed makes sense, let's just make
virtio behave correctly if that happens, no need to
optimize for that.

> > 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.
> 
> So something like this patch should be done in virtio core somewhere
> else. Virtio-net can not do this since it does not have the knowledge of
> event index.

Take a look at my patch - no calls to enable_cb, only
enable_cb_delayed, so we should be fine.

> >
> >>> 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.
> 
> This is just what we want I think. The interrupt was not lost but fired
> after 3/4 pending buffers were consumed. Do you see any real issue on this?

Yes, this violates the API. For example device might never
consume the rest of buffers.

> >
> >>> 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?
> >
> 
> In the previous reply you said you're using
> virtuqueue_enable_cb_delayed(), so no race in your patch.

OK so you think my patch is also correct, but that yours gives better
efficiency?

-- 
MST