Re: [dpdk-dev] [PATCH v3 4/4] vhost: fix async register/unregister deadlock

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 9/29/20 11:29 AM, Patrick Fu wrote:
> When async register/unregister function is invoked in certain vhost
> event callbacks (e.g. vring state change), deadlock may occur due to
> recursive spinlock acquire. This patch removes unnecessary spinlock
> from register API and use trylock() primitive in the unregister API
> to avoid deadlock.

I am not convinced it is unnecessary in every cases.


> Fixes: 78639d54563a ("vhost: introduce async enqueue registration API")
> 
> Signed-off-by: Patrick Fu <patrick.fu@intel.com>
> ---
>  lib/librte_vhost/vhost.c      | 13 +++++++------
>  lib/librte_vhost/vhost_user.c |  4 ++--
>  2 files changed, 9 insertions(+), 8 deletions(-)
> 
> diff --git a/lib/librte_vhost/vhost.c b/lib/librte_vhost/vhost.c
> index 05b578c2f..ba504a00a 100644
> --- a/lib/librte_vhost/vhost.c
> +++ b/lib/librte_vhost/vhost.c
> @@ -1577,8 +1577,6 @@ int rte_vhost_async_channel_register(int vid, uint16_t queue_id,
>  		ops->transfer_data == NULL))
>  		return -1;
>  
> -	rte_spinlock_lock(&vq->access_lock);
> -

What if a virtqueue is being destroyed while this function is called?

>  	if (unlikely(vq->async_registered)) {
>  		VHOST_LOG_CONFIG(ERR,
>  			"async register failed: channel already registered "
> @@ -1627,8 +1625,6 @@ int rte_vhost_async_channel_register(int vid, uint16_t queue_id,
>  	vq->async_registered = true;
>  
>  reg_out:
> -	rte_spinlock_unlock(&vq->access_lock);
> -
>  	return 0;
>  }
>  
> @@ -1647,10 +1643,15 @@ int rte_vhost_async_channel_unregister(int vid, uint16_t queue_id)
>  		return ret;
>  
>  	ret = 0;
> -	rte_spinlock_lock(&vq->access_lock);
>  
>  	if (!vq->async_registered)
> -		goto out;
> +		return ret;
> +
> +	if (!rte_spinlock_trylock(&vq->access_lock)) {
> +		VHOST_LOG_CONFIG(ERR, "Failed to unregister async channel. "
> +			"virt queue busy.\n");
> +		return -1;
> +	}
>  
>  	if (vq->async_pkts_inflight_n) {
>  		VHOST_LOG_CONFIG(ERR, "Failed to unregister async channel. "
> diff --git a/lib/librte_vhost/vhost_user.c b/lib/librte_vhost/vhost_user.c
> index 67d2a2d43..c8d74bde6 100644
> --- a/lib/librte_vhost/vhost_user.c
> +++ b/lib/librte_vhost/vhost_user.c
> @@ -2043,9 +2043,9 @@ vhost_user_set_vring_enable(struct virtio_net **pdev,
>  		"set queue enable: %d to qp idx: %d\n",
>  		enable, index);
>  
> -	if (!enable && dev->virtqueue[index]->async_registered) {
> +	if (enable && dev->virtqueue[index]->async_registered) {
>  		if (dev->virtqueue[index]->async_pkts_inflight_n) {
> -			VHOST_LOG_CONFIG(ERR, "failed to disable vring. "
> +			VHOST_LOG_CONFIG(ERR, "failed to enable vring. "
>  			"async inflight packets must be completed first\n");
>  			return RTE_VHOST_MSG_RESULT_ERR;
>  		}
>