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. > 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. _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
