[PATCH] vhost: use try_lock in rte_vhost_vring_call

[Maxime Coquelin]

On 9/6/22 04:22, Changpeng Liu wrote:
> Note that this function is in data path, so the thread context
> may not same as socket messages processing context, by using
> try_lock here, users can have another try in case of VQ's access
> lock is held by `vhost-events` thread.
> 
> Signed-off-by: Changpeng Liu <changpeng.liu at intel.com>
> ---
>   lib/vhost/vhost.c | 6 +++++-
>   1 file changed, 5 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/vhost/vhost.c b/lib/vhost/vhost.c
> index 60cb05a0ff..072d2acb7b 100644
> --- a/lib/vhost/vhost.c
> +++ b/lib/vhost/vhost.c
> @@ -1329,7 +1329,11 @@ rte_vhost_vring_call(int vid, uint16_t vring_idx)
>   	if (!vq)
>   		return -1;
>   
> -	rte_spinlock_lock(&vq->access_lock);
> +	if (!rte_spinlock_trylock(&vq->access_lock)) {
> +		VHOST_LOG_CONFIG(dev->ifname, DEBUG,
> +			"failed to kick guest, virtqueue busy.\n");
> +		return -1;
> +	}
>   
>   	if (vq_is_packed(dev))
>   		vhost_vring_call_packed(dev, vq);

I think that's problematic, because it will break other applications
that currently rely on the API to block until the call is done.

Just some internal DPDK usage of this API:
./drivers/vdpa/ifc/ifcvf_vdpa.c:871:	rte_vhost_vring_call(internal->vid, 
qid);
./examples/vhost/virtio_net.c:236:	rte_vhost_vring_call(dev->vid, queue_id);
./examples/vhost/virtio_net.c:446:	rte_vhost_vring_call(dev->vid, queue_id);
./examples/vhost_blk/vhost_blk.c:99: 
rte_vhost_vring_call(task->ctrlr->vid, vq->id);
./examples/vhost_blk/vhost_blk.c:134: 
rte_vhost_vring_call(task->ctrlr->vid, vq->id);

This change will break all the above uses.

And that's not counting external projects.

ou should better introduce a new API that does not block.

Regards,
Maxime