Re: [dpdk-dev] [PATCH] kni: fix kernel deadlock when using mlx devices

[Ferruh Yigit <ferruh.yigit@intel.com>]

On 12/22/2019 5:55 PM, Stephen Hemminger wrote:
> This fixes a deadlock when using KNI with bifurcated drivers.
> Bringing kni device up always times out when using Mellanox
> devices.
> 
> The kernel KNI driver sends message to userspace to complete
> the request. For the case of bifurcated driver, this may involve
> an additional request to kernel to change state. This request
> would deadlock because KNI was holding the RTNL mutex.
> 
> This was a bad design which goes back to the original code.
> A workaround is for KNI driver to drop RTNL while waiting.
> To prevent the device from disappearing while the operation
> is in progress, it needs to hold reference to network device
> while waiting.
> 
> As an added benefit, an useless error check can also be removed.
> 
> Fixes: 3fc5ca2f6352 ("kni: initial import")
> Cc: stable@dpdk.org
> Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
> ---
>  kernel/linux/kni/kni_net.c | 34 ++++++++++++++++++----------------
>  1 file changed, 18 insertions(+), 16 deletions(-)
> 
> diff --git a/kernel/linux/kni/kni_net.c b/kernel/linux/kni/kni_net.c
> index 1ba9b1b99f66..b7337c1410b8 100644
> --- a/kernel/linux/kni/kni_net.c
> +++ b/kernel/linux/kni/kni_net.c
> @@ -17,6 +17,7 @@
>  #include <linux/skbuff.h>
>  #include <linux/kthread.h>
>  #include <linux/delay.h>
> +#include <linux/rtnetlink.h>
>  
>  #include <rte_kni_common.h>
>  #include <kni_fifo.h>
> @@ -102,17 +103,15 @@ get_data_kva(struct kni_dev *kni, void *pkt_kva)
>   * It can be called to process the request.
>   */
>  static int
> -kni_net_process_request(struct kni_dev *kni, struct rte_kni_request *req)
> +kni_net_process_request(struct net_device *dev, struct rte_kni_request *req)
>  {
> +	struct kni_dev *kni = netdev_priv(dev);
>  	int ret = -1;
>  	void *resp_va;
>  	uint32_t num;
>  	int ret_val;
>  
> -	if (!kni || !req) {
> -		pr_err("No kni instance or request\n");
> -		return -EINVAL;
> -	}
> +	ASSERT_RTNL();
>  
>  	mutex_lock(&kni->sync_lock);
>  
> @@ -125,8 +124,17 @@ kni_net_process_request(struct kni_dev *kni, struct rte_kni_request *req)
>  		goto fail;
>  	}
>  
> +	/* Since we need to wait and RTNL mutex is held
> +	 * drop the mutex and hold refernce to keep device
> +	 */
> +	dev_hold(dev);
> +	rtnl_unlock();
> +
>  	ret_val = wait_event_interruptible_timeout(kni->wq,
>  			kni_fifo_count(kni->resp_q), 3 * HZ);
> +	rtnl_lock();
> +	dev_put(dev);
> +
>  	if (signal_pending(current) || ret_val <= 0) {
>  		ret = -ETIME;
>  		goto fail;

<...>

This patch cause a hang on my server, not sure what exactly was the problem but
kernel log was continuously printing "Cannot send to req_q". Will dig more.