[dpdk-dev] [PATCH] vhost: return -EAGAIN during unregistering vhost if it is busy.

Zhike Wang wangzhike at jd.com
Thu Mar 12 10:57:19 CET 2020


The vhost_user_read_cb() and rte_vhost_driver_unregister()
can be called at the same time by 2 threads, and may lead to deadlock.
Eg thread1 calls vhost_user_read_cb()->vhost_user_get_vring_base()->destroy_device(),
then thread2 calls rte_vhost_driver_unregister(), and will retry the fdset_try_del() in loop.

Some application implements destroy_device() as a blocking function, eg
OVS calls ovsrcu_synchronize() insides destroy_device(). As a result,
thread1(eg vhost_events) is blocked to wait quiesce of thread2(eg ovs-vswitchd),
and thread2 is in a loop to wait thread1 to give up the use of the vhost fd,
then leads to deadlock.

It is better to return -EAGAIN to application, who will decide how to handle
(eg OVS can call ovsrcu_quiesce() and then retry).

Signed-off-by: Zhike Wang <wangzhike at jd.com>
---
 lib/librte_vhost/rte_vhost.h | 4 +++-
 lib/librte_vhost/socket.c    | 8 ++++----
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/lib/librte_vhost/rte_vhost.h b/lib/librte_vhost/rte_vhost.h
index c7b619a..276db11 100644
--- a/lib/librte_vhost/rte_vhost.h
+++ b/lib/librte_vhost/rte_vhost.h
@@ -389,7 +389,9 @@ void rte_vhost_log_used_vring(int vid, uint16_t vring_idx,
  */
 int rte_vhost_driver_register(const char *path, uint64_t flags);
 
-/* Unregister vhost driver. This is only meaningful to vhost user. */
+/* Unregister vhost driver. This is only meaningful to vhost user.
+ * Return -EAGAIN if device is busy, and leave it to be handled by application.
+ */
 int rte_vhost_driver_unregister(const char *path);
 
 /**
diff --git a/lib/librte_vhost/socket.c b/lib/librte_vhost/socket.c
index 7c80121..a75a3f6 100644
--- a/lib/librte_vhost/socket.c
+++ b/lib/librte_vhost/socket.c
@@ -1027,7 +1027,8 @@ struct vhost_user_reconnect_list {
 }
 
 /**
- * Unregister the specified vhost socket
+ * Unregister the specified vhost socket.
+ * Return -EAGAIN if device is busy, and leave it to be handled by application.
  */
 int
 rte_vhost_driver_unregister(const char *path)
@@ -1039,7 +1040,6 @@ struct vhost_user_reconnect_list {
 	if (path == NULL)
 		return -1;
 
-again:
 	pthread_mutex_lock(&vhost_user.mutex);
 
 	for (i = 0; i < vhost_user.vsocket_cnt; i++) {
@@ -1063,7 +1063,7 @@ struct vhost_user_reconnect_list {
 					pthread_mutex_unlock(
 							&vsocket->conn_mutex);
 					pthread_mutex_unlock(&vhost_user.mutex);
-					goto again;
+					return -EAGAIN;
 				}
 
 				VHOST_LOG_CONFIG(INFO,
@@ -1085,7 +1085,7 @@ struct vhost_user_reconnect_list {
 				if (fdset_try_del(&vhost_user.fdset,
 						vsocket->socket_fd) == -1) {
 					pthread_mutex_unlock(&vhost_user.mutex);
-					goto again;
+					return -EAGAIN;
 				}
 
 				close(vsocket->socket_fd);
-- 
1.8.3.1




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