Re: usb device removed from sysfs before input children devices

On Thu, 2013-01-24 at 13:58 +0000, Karl Relton wrote:

On Tue, 2013-01-15 at 20:27 +0000, Karl Relton wrote:

quoted

quoted

The end result (for the user) is that even when the bluetooth
mouse/keyboard is re-added, Xorg ignores it - thinking it is some

hoax

quoted

duplicate device. The keyboard/mouse is then non-operational.

Instrumenting the code suggests that the issue arises in a race
between:

hidp_session() in bluetooth/hidp/core.c

and

hci_unregister_dev() in bluetooth/hci_core.c

...

quoted

I can't figure out why there is such a delay for
input_unregister_device(). 

The power_supply system seems to be triggering a lookup of battery
'capacity' in the input device. This calls into the driver, which deep
down invokes more code in the 'hidp' module: hidp_get_raw_report()

This function tries to communicate with the bluetooth device (keyboard
in my case), but because the device is being 'torn down' such
communication fails. Accordingly hidp_get_raw_report has a 5 second
timeout ... so this function stalls for 5 seconds until that timeout
occurs.


I would guess that for some reason, removing the 'hid' or associated
'input' device is leading to the power_supply code wanting to lookup the
battery capacity which introduces a 5 second wait. This would delay
hidp_session by 5 seconds ... in the mean time allowing
hci_unregister_dev() to win the race and hence the hci device is removed
before the input & hid devices.

device_del is done on the battery device, called from
hidinput_cleanup_battery in hid-input.c.

This will generate a uevent which has the power_supply attributes added,
which invokes power_supply_show_property for 'capacity' which invokes
the hidp_get_raw_report (which stalls for 5 seconds).


Back in the days of linux 3.2, hid-input.c did not have any support for
batteries, so this chain of calls was never made, hence the race was won
by hidp_session() because it didn't have the 5 second delay injected.

Now with battery support added, the problem is manifest.

--

I think hidp_get_raw_report should abort if the devices is being 'torn
down' (and so not go into the 5 second wait). But also the race between
the two functions needs sorting, since sooner or later someone else will
get unlucky.