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

From: Jason Wang <[email-masked]>
Date: Wed, 15 Oct 2014 18:44:41 +0800

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.
> 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.