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 <rusty@xxxxxxxxxxxxxxx> > >>>> Cc: Michael S. Tsirkin <mst@xxxxxxxxxx> > >>>> Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx> > >>> 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 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
