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