[PATCH 4.9 007/172] Fix an intermittent pr_emerg warning about lo becoming free.

From: Greg Kroah-Hartman
Date: Mon Jul 03 2017 - 10:37:15 EST

4.9-stable review patch. If anyone has any objections, please let me know.


From: Krister Johansen <kjlx@xxxxxxxxxxxxxxxxxx>

[ Upstream commit f186ce61bb8235d80068c390dc2aad7ca427a4c2 ]

It looks like this:

Message from syslogd@flamingo at Apr 26 00:45:00 ...
kernel:unregister_netdevice: waiting for lo to become free. Usage count = 4

They seem to coincide with net namespace teardown.

The message is emitted by netdev_wait_allrefs().

Forced a kdump in netdev_run_todo, but found that the refcount on the lo
device was already 0 at the time we got to the panic.

Used bcc to check the blocking in netdev_run_todo. The only places
where we're off cpu there are in the rcu_barrier() and msleep() calls.
That behavior is expected. The msleep time coincides with the amount of
time we spend waiting for the refcount to reach zero; the rcu_barrier()
wait times are not excessive.

After looking through the list of callbacks that the netdevice notifiers
invoke in this path, it appears that the dst_dev_event is the most
interesting. The dst_ifdown path places a hold on the loopback_dev as
part of releasing the dev associated with the original dst cache entry.
Most of our notifier callbacks are straight-forward, but this one a)
looks complex, and b) places a hold on the network interface in

I constructed a new bcc script that watches various events in the
liftime of a dst cache entry. Note that dst_ifdown will take a hold on
the loopback device until the invalidated dst entry gets freed.

[ __dst_free] on DST: ffff883ccabb7900 IF tap1008300eth0 invoked at 1282115677036183
Acked-by: Eric Dumazet <edumazet@xxxxxxxxxx>

Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
net/core/dst.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

--- a/net/core/dst.c
+++ b/net/core/dst.c
@@ -470,6 +470,20 @@ static int dst_dev_event(struct notifier
dst = dst_garbage.list;
dst_garbage.list = NULL;
+ /* The code in dst_ifdown places a hold on the loopback device.
+ * If the gc entry processing is set to expire after a lengthy
+ * interval, this hold can cause netdev_wait_allrefs() to hang
+ * out and wait for a long time -- until the the loopback
+ * interface is released. If we're really unlucky, it'll emit
+ * pr_emerg messages to console too. Reset the interval here,
+ * so dst cleanups occur in a more timely fashion.
+ */
+ if (dst_garbage.timer_inc > DST_GC_INC) {
+ dst_garbage.timer_inc = DST_GC_INC;
+ dst_garbage.timer_expires = DST_GC_MIN;
+ mod_delayed_work(system_wq, &dst_gc_work,
+ dst_garbage.timer_expires);
+ }

if (last)