[dpdk-dev] [PATCH] vhost: fix deadlock on rte_vhost_driver_unregister()

[dpdk-dev] [PATCH] vhost: fix deadlock on rte_vhost_driver_unregister()

[Dariusz Stojaczyk]

Consider the following scenario, threads A and B:
(A)
 * fdset_event_dispatch() start
   * pfdentry->busy = 1;
   * vhost_user_read_cb() start
     * vhost_destroy_device() start
(B)
 * rte_vhost_driver_unregister() start
   * pthread_mutex_lock(&vsocket->conn_mutex);
   * fdset_del()
     * endless loop, waiting for pfdentry->busy == 0
(A)
     * vhost_destroy_device() end
     * pthread_mutex_lock(&vsocket->conn_mutex);
       (mutex already locked - deadlock at this point)

Thread B has locked vsocket->conn_mutex and is in while(1)
loop waiting for given fd to change it's busy flag to 0.
Thread A would have to finish vhost_user_read_cb() in order
to set busy flag back to 0, but that can't happen due to
the vsocket->conn_mutex lock.

This patch synchronizes rte_vhost_driver_unregister() with
vhost_user_read_cb() through vhost_user.mutex.

Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
---
 lib/librte_vhost/socket.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/lib/librte_vhost/socket.c b/lib/librte_vhost/socket.c
index c7f99b0..77e58fe 100644
--- a/lib/librte_vhost/socket.c
+++ b/lib/librte_vhost/socket.c
@@ -273,6 +273,8 @@ vhost_user_read_cb(int connfd, void *dat, int *remove)
 
 	ret = vhost_user_msg_handler(conn->vid, connfd);
 	if (ret < 0) {
+		pthread_mutex_lock(&vhost_user.mutex);
+
 		close(connfd);
 		*remove = 1;
 		vhost_destroy_device(conn->vid);
@@ -287,6 +289,8 @@ vhost_user_read_cb(int connfd, void *dat, int *remove)
 			create_unix_socket(vsocket);
 			vhost_user_start_client(vsocket);
 		}
+
+		pthread_mutex_unlock(&vhost_user.mutex);
 	}
 }
 
-- 
2.7.4

Re: [dpdk-dev] [PATCH] vhost: fix deadlock on rte_vhost_driver_unregister()

[Stojaczyk, DariuszX]

> -----Original Message-----
> From: Stojaczyk, DariuszX
> Sent: Tuesday, May 16, 2017 4:35 PM
> To: dev@dpdk.org
> Cc: Wodkowski, PawelX <pawelx.wodkowski@intel.com>; Stojaczyk, DariuszX
> <dariuszx.stojaczyk@intel.com>
> Subject: [PATCH] vhost: fix deadlock on rte_vhost_driver_unregister()
> 
> Consider the following scenario, threads A and B:
> (A)
>  * fdset_event_dispatch() start
>    * pfdentry->busy = 1;
>    * vhost_user_read_cb() start
>      * vhost_destroy_device() start
> (B)
>  * rte_vhost_driver_unregister() start
>    * pthread_mutex_lock(&vsocket->conn_mutex);
>    * fdset_del()
>      * endless loop, waiting for pfdentry->busy == 0
> (A)
>      * vhost_destroy_device() end
>      * pthread_mutex_lock(&vsocket->conn_mutex);
>        (mutex already locked - deadlock at this point)
> 
> Thread B has locked vsocket->conn_mutex and is in while(1) loop waiting for
> given fd to change it's busy flag to 0.
> Thread A would have to finish vhost_user_read_cb() in order to set busy flag
> back to 0, but that can't happen due to the vsocket->conn_mutex lock.
> 
> This patch synchronizes rte_vhost_driver_unregister() with
> vhost_user_read_cb() through vhost_user.mutex.
> 
> Signed-off-by: Dariusz Stojaczyk <dariuszx.stojaczyk@intel.com>
> ---
>  lib/librte_vhost/socket.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/lib/librte_vhost/socket.c b/lib/librte_vhost/socket.c index
> c7f99b0..77e58fe 100644
> --- a/lib/librte_vhost/socket.c
> +++ b/lib/librte_vhost/socket.c
> @@ -273,6 +273,8 @@ vhost_user_read_cb(int connfd, void *dat, int
> *remove)
> 
>  	ret = vhost_user_msg_handler(conn->vid, connfd);
>  	if (ret < 0) {
> +		pthread_mutex_lock(&vhost_user.mutex);
> +
>  		close(connfd);
>  		*remove = 1;
>  		vhost_destroy_device(conn->vid);
> @@ -287,6 +289,8 @@ vhost_user_read_cb(int connfd, void *dat, int
> *remove)
>  			create_unix_socket(vsocket);
>  			vhost_user_start_client(vsocket);
>  		}
> +
> +		pthread_mutex_unlock(&vhost_user.mutex);
>  	}
>  }
> 
> --
> 2.7.4

I've found other cases where rte_vhost_driver_unregister() can still deadlock. 
Please do not merge this patch now, I will try to come up with a different solution for this issue.