Re: [PATCH] net: dev: fix the incorrect hold of net namespace's lo device

in dst_dev_event,we get the still referenced dst entries
from dst_garbage list,and call dst_ifdown to change these
dst entries' device to the net namesapce's lo device.

when we moving a net device(A) to another net namespace,
because free_fib_info_rcu is called after a grace period,
we may call dst_dev_event before free_fib_info_rcu putting
dst_entry into the dst_garbage list.

so in dst_dev_event, we can't see these dst entries through
dst_garbage list, and without changing their device to the
old net namespace's lo device. after a grace period, these
dst entries which dst->dev is device A will in the dst_garbage
list, and the device A will belong to the new net namespcae.

then we exit from this new net namespace, the dst_dev_event
is called again,it will get these dst entries from dst_garbage
list,and call dst_ifdown to hold the new net namespace's lo
device incorrectly and put the device A.

so it will tigger the emg message in netdev_wait_allrefs like
below.
unregister_netdevice: waiting for lo to become free. Usage count = 1

fix this problem by adding rcu_barrier() in dst_dev_event
when event is NETDEV_UNREGISTER.
with this,dst_ifdown will be called after the dst_garbage list
beeing updated.

Signed-off-by: Gao feng <redacted>
---
 net/core/dst.c |    1 +
 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/net/core/dst.c b/net/core/dst.c
index 56d6361..38c2199 100644
--- a/net/core/dst.c
+++ b/net/core/dst.c

@@ -375,6 +375,7 @@ static int dst_dev_event(struct notifier_block *this, unsigned long event,
 
 	switch (event) {
 	case NETDEV_UNREGISTER:
+		rcu_barrier();
 	case NETDEV_DOWN:
 		mutex_lock(&dst_gc_mutex);
 		for (dst = dst_busy_list; dst; dst = dst->next) {