On Wed, 2009-02-25 at 16:05 +0100, Jan-Bernd Themann wrote:

- When "open" is called for a registered network device, port->port_lock
is taken first,
  then ehea_fw_handles.lock
- When "open" is left these locks are released in a proper way (inverse
order)

So this has:

  port->port_lock
    ehea_fw_handles.lock

This would be the case that is generating the warning.

- In addition: ehea_fw_handles.lock is held by the function
"driver_probe_device"
  that registers all available network devices (register_netdev)
- When multiple network devices are registered, it is possible that
"open" is
  called on an already registered network device while further
netdevices are still registered
  in "driver_probe_device". ---> "open" will take port->port_lock, but
won't get ehea_fw_handles.lock

Right, so here you have 

  ehea_fw_handles.lock
    port->port_lock

Overlay these two cases and you have AB-BA deadlocks.

- However, ehea_fw_handles.lock is freed once all netdevices are registered.
- When the second netdevice is registered in "driver_probe_device", it
will also try to get
  the port->port_lock (which in fact is a different one, as there is one
per netdevice).
- Does the mutex debug mechanism distinguish between the different
port->port_lock instances?

Not unless you tell it to.

Are you really sure the port->port_lock in this AB-BA scenario are never
the same? The above explanation didn't convince me (also very hard to
read due to funny wrapping).

Suppose you do an open concurrently with a re-probe, which apparently
takes port->port_lock's of existing devices, in the above scenario that
deadlocks.