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? If yes but please describe the race explicitly. Is there a bug we need to fix on stable? Please also explicitly describe a configuration that causes event idx to go back. All this info should go in the commit log. > --- > 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. Instead, how about we simply document the requirement that drivers either always call virtqueue_enable_cb_delayed or virtqueue_enable_cb but not both? > END_USE(vq); > return last_used_idx; > } > -- > 1.7.1 _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
