Re: [PATCH v2] vhost/net: Replace wait_queue with completion in ubufs reference

Hillf Danton
2025-07-18T12:47:19Z

On Fri, 18 Jul 2025 14:03:55 +0300 Nikolay Kuratov wrote:
>
> drivers/vhost/net.c | 9 +++++----
> 1 file changed, 5 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/vhost/net.c b/drivers/vhost/net.c
> index 7cbfc7d718b3..69e1bfb9627e 100644
> --- a/drivers/vhost/net.c
> +++ b/drivers/vhost/net.c
> @@ -94,7 +94,7 @@ struct vhost_net_ubuf_ref {
> * >1: outstanding ubufs
> */
> atomic_t refcount;
> - wait_queue_head_t wait;
> + struct completion wait;
> struct vhost_virtqueue *vq;
> };
>
> @@ -240,7 +240,7 @@ vhost_net_ubuf_alloc(struct vhost_virtqueue *vq, bool zcopy)
> if (!ubufs)
> return ERR_PTR(-ENOMEM);
> atomic_set(&ubufs->refcount, 1);
> - init_waitqueue_head(&ubufs->wait);
> + init_completion(&ubufs->wait);
> ubufs->vq = vq;
> return ubufs;
> }
> @@ -249,14 +249,14 @@ static int vhost_net_ubuf_put(struct vhost_net_ubuf_ref *ubufs)
> {
> int r = atomic_sub_return(1, &ubufs->refcount);
> if (unlikely(!r))
> - wake_up(&ubufs->wait);
> + complete_all(&ubufs->wait);
> return r;
> }
>
> static void vhost_net_ubuf_put_and_wait(struct vhost_net_ubuf_ref *ubufs)
> {
> vhost_net_ubuf_put(ubufs);
> - wait_event(ubufs->wait, !atomic_read(&ubufs->refcount));
> + wait_for_completion(&ubufs->wait);
> }
>
> static void vhost_net_ubuf_put_wait_and_free(struct vhost_net_ubuf_ref *ubufs)
> @@ -1381,6 +1381,7 @@ static void vhost_net_flush(struct vhost_net *n)
> mutex_lock(&n->vqs[VHOST_NET_VQ_TX].vq.mutex);
> n->tx_flush = false;
> atomic_set(&n->vqs[VHOST_NET_VQ_TX].ubufs->refcount, 1);
> + reinit_completion(&n->vqs[VHOST_NET_VQ_TX].ubufs->wait);
> mutex_unlock(&n->vqs[VHOST_NET_VQ_TX].vq.mutex);
> }
> }
> --
> 2.34.1
>
In the sequence below,
vhost_net_flush()
vhost_net_ubuf_put_and_wait(n->vqs[VHOST_NET_VQ_TX].ubufs);
wait_for_completion(&ubufs->wait);
reinit_completion(&n->vqs[VHOST_NET_VQ_TX].ubufs->wait);

reinit after wait, so the chance for missing wakeup still exists.