On Fri, Sep 13, 2013 at 6:46 PM, Eric W. Biederman
[off-list ref] wrote:

Francesco Ruggeri [off-list ref] writes:

quoted

On Thu, Sep 12, 2013 at 10:50 PM, Eric W. Biederman
[off-list ref] wrote:

I do have some concerns about both correctness and feasibility of this approach.

About 2), namespace dependent operations triggered by unregistering
the virtual devices (eg rt_flush_dev, dst_dev_event/dst_ifdown and
probably more) would not be done in the namespaces where they should.

Yes they will.  That is what dev_change_net_namespace does.

You are right, I don't know what I was thinking.

It was worth a second look.  I can not find anything wrong with your
patch but I can not convince myself that it is correct either.  The
moving around the loopback device in the net dev todo list to prevent
deadlock I can't imagine why you are doing that.

That is in order not to introduce a potential deadlock when multiple
namespaces are destroyed in default_device_exit_batch.
Take the same veth scenario as before:
v0 in namespace ns0 (loopback_dev lo0) and similarly for v1, ns1 and lo1.
Assume two processes destroy ns0 and ns1. cleanup_net is executed in a
workqueue and the two namespaces can end up being processed in the
same instance of cleanup_net/ops_exit_list/default_device_exit_batch.
default_device_exit_batch traverses each namespace's dev_base list and
unregister_netdevice_queue is executed in this order:
v0 v1 lo0 v1 v0 lo1.
unregister_netdevice_queue is executed twice on v0 and v1 but that is
ok because it uses list_move_tail and only the last one sticks.
The resulting list for unregister_netdevice_many is:
lo0 v1 v0 lo1.
If v0 holds a reference to lo0 there will be a deadlock in
netdev_run_todo if v0 does not come before lo0 in net_todo_list. By
pushing all loopback_devs to the end of net_todo_list this situation
is avoided.

This is the sequence with today's (actually 3.4) code:

unregister_netdevice_queue: v0 (ns ffff880037aecd00)
unregister_netdevice_queue: v1 (ns ffff880037aed800)
unregister_netdevice_queue: lo (ns ffff880037aecd00)
unregister_netdevice_queue: v1 (ns ffff880037aed800)
unregister_netdevice_queue: v0 (ns ffff880037aecd00)
unregister_netdevice_queue: lo (ns ffff880037aed800)
unregister_netdevice_many: lo (ns ffff880037aecd00) v1 (ns
ffff880037aed800) v0 (ns ffff880037aecd00) lo (ns ffff880037aed800)
netdev_run_todo: lo (ns ffff880037aecd00) v1 (ns ffff880037aed800) v0
(ns ffff880037aecd00) lo (ns ffff880037aed800)

and this is the sequence after pushing the loopback_devs to the tail
of net_todo_list:

unregister_netdevice_queue: v0 (ns ffff8800379f8000)
unregister_netdevice_queue: v1 (ns ffff8800378c0b00)
unregister_netdevice_queue: lo (ns ffff8800379f8000)
unregister_netdevice_queue: v1 (ns ffff8800378c0b00)
unregister_netdevice_queue: v0 (ns ffff8800379f8000)
unregister_netdevice_queue: lo (ns ffff8800378c0b00)
unregister_netdevice_many: lo (ns ffff8800379f8000) v1 (ns
ffff8800378c0b00) v0 (ns ffff8800379f8000) lo (ns ffff8800378c0b00)
netdev_run_todo: v1 (ns ffff8800378c0b00) v0 (ns ffff8800379f8000) lo
(ns ffff8800379f8000) lo (ns ffff8800378c0b00)

Should we take this discussion offline?
I do appreciate your spending time on this.

Thanks,
Francesco

Eric