Linux Virtualization - Oct 2014 - [RFC PATCH net-next 1/6] virtio: make sure used event never go backwards

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 <rusty at rustcorp.com.au>
>> Cc: Michael S. Tsirkin <mst at redhat.com>
>> Signed-off-by: Jason Wang <jasowang at redhat.com>
> 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.
> 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?
>
> 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.
>
>>  	END_USE(vq);
>>  	return last_used_idx;
>>  }
>> -- 
>> 1.7.1

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 <rusty at rustcorp.com.au>
> >> Cc: Michael S. Tsirkin <mst at redhat.com>
> >> Signed-off-by: Jason Wang <jasowang at redhat.com>
> > 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

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 <rusty at rustcorp.com.au>
>>>> Cc: Michael S. Tsirkin <mst at redhat.com>
>>>> Signed-off-by: Jason Wang <jasowang at redhat.com>
>>> 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.
> 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.
>
>>> 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?
>
>>> 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.