Many devices use userspace bluetooth stacks like BlueZ or Bluedroid in combination
with uhid. If any of these stacks is used with a HID device for which the driver
performs a HID request as part .probe (or technically another HID operation),
this results in a deadlock situation. The deadlock results in a 5 second timeout
for I/O operations in HID drivers, so isn't fatal, but none of the I/O operations
have a chance of succeeding.

The root cause for the problem is that uhid only allows for one request to be
processed at a time per uhid instance and locks out other operations. This means
that if a user space is creating a new HID device through 'UHID_CREATE', which
ultimately triggers '.probe' through the HID layer. Then any HID request e.g. a
read for calibration data would trigger a HID operation on uhid again, but it
won't go out to userspace, because it is still stuck in UHID_CREATE.
In addition bluetooth stacks are typically single threaded, so they wouldn't be
able to handle any requests while waiting on uhid.

Lucikly the UHID spec is somewhat flexible and allows for fixing the issue,
without breaking user space. The idea which the patch implements as discussed
with David Herrmann is to decouple adding of a hid device (which triggers .probe)
from UHID_CREATE. The work will kick off roughly once UHID_CREATE completed (or
else will wait a tiny bit of time in .probe for a lock). A HID driver has to call
HID to call 'hid_hw_start()' as part of .probe once it is ready for I/O, which
triggers UHID_START to user space. Any HID operations should function now within
.probe and won't deadlock because userspace is stuck on UHID_CREATE.

We verified this patch on Bluedroid with Android 6.0 and on desktop Linux with
BlueZ stacks. Prior to the patch they had the deadlock issue.

[jkosina@suse.cz: reword subject]
Signed-off-by: Roderick Colenbrander <roderick.colenbrander@sony.com>
Cc: stable@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>