[dpdk-dev] [PATCH 2/2] kni: fix rtnl deadlocks and race conditions v3

Elad Nachman eladv6 at gmail.com
Tue Feb 23 14:45:04 CET 2021

This part of the series includes my fixes for the issues reported
by Ferruh and Igor on top of part 1 of the patch series:

A. KNI sync lock is being locked while rtnl is held.
If two threads are calling kni_net_process_request() ,
then the first one will take the sync lock, release rtnl lock then sleep.
The second thread will try to lock sync lock while holding rtnl.
The first thread will wake, and try to lock rtnl, resulting in a deadlock.
The remedy is to release rtnl before locking the KNI sync lock.
Since in between nothing is accessing Linux network-wise,
no rtnl locking is needed.

B. There is a race condition in __dev_close_many() processing the
close_list while the application terminates.
It looks like if two vEth devices are terminating,
and one releases the rtnl lock, the other takes it,
updating the close_list in an unstable state,
causing the close_list to become a circular linked list,
hence list_for_each_entry() will endlessly loop inside
__dev_close_many() .
Since the description for the original patch indicate the
original motivation was bringing the device up,
I have changed kni_net_process_request() to hold the rtnl mutex
in case of bringing the device down since this is the path called
from __dev_close_many() , causing the corruption of the close_list. 

Signed-off-by: Elad Nachman <eladv6 at gmail.com>
* Include original patch and new patch as a series of patch, added a
  comment to the new patch
* rebuild the patch as increment from patch 64106
* fix comment and blank lines
 kernel/linux/kni/kni_net.c | 29 +++++++++++++++++++++--------
 1 file changed, 21 insertions(+), 8 deletions(-)

diff --git a/kernel/linux/kni/kni_net.c b/kernel/linux/kni/kni_net.c
index f0b6e9a8d..017e44812 100644
--- a/kernel/linux/kni/kni_net.c
+++ b/kernel/linux/kni/kni_net.c
@@ -110,9 +110,26 @@ kni_net_process_request(struct net_device *dev, struct rte_kni_request *req)
 	void *resp_va;
 	uint32_t num;
 	int ret_val;
+	int req_is_dev_stop = 0;
+	/* For configuring the interface to down,
+	 * rtnl must be held all the way to prevent race condition
+	 * inside __dev_close_many() between two netdev instances of KNI
+	 */
+	if (req->req_id == RTE_KNI_REQ_CFG_NETWORK_IF &&
+			req->if_up == 0)
+		req_is_dev_stop = 1;
+	/* Since we need to wait and RTNL mutex is held
+	 * drop the mutex and hold reference to keep device
+	 */
+	if (!req_is_dev_stop) {
+		dev_hold(dev);
+		rtnl_unlock();
+	}
 	/* Construct data */
@@ -124,16 +141,8 @@ kni_net_process_request(struct net_device *dev, 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;
@@ -152,6 +161,10 @@ kni_net_process_request(struct net_device *dev, struct rte_kni_request *req)
+	if (!req_is_dev_stop) {
+		rtnl_lock();
+		dev_put(dev);
+	}
 	return ret;

More information about the dev mailing list