Re: [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....@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