From f186ce61bb8235d80068c390dc2aad7ca427a4c2 Mon Sep 17 00:00:00 2001 From: Krister Johansen Date: Thu, 8 Jun 2017 13:12:38 -0700 Subject: Fix an intermittent pr_emerg warning about lo becoming free. 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 question. 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 __dst_free rcu_nocb_kthread kthread ret_from_fork Acked-by: Eric Dumazet Signed-off-by: David S. Miller --- net/core/dst.c | 14 ++++++++++++++ 1 file changed, 14 insertions(+) (limited to 'net') diff --git a/net/core/dst.c b/net/core/dst.c index 6192f11beec9..13ba4a090c41 100644 --- a/net/core/dst.c +++ b/net/core/dst.c @@ -469,6 +469,20 @@ static int dst_dev_event(struct notifier_block *this, unsigned long event, spin_lock_bh(&dst_garbage.lock); 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); + } spin_unlock_bh(&dst_garbage.lock); if (last) -- cgit v1.2.3