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