Valve reported a kernel crash on Ubuntu 18.04 when disconnecting a DS4
gamepad while rumble is enabled. This issue is reproducible with a
frequency of 1 in 3 times in the game Borderlands 2 when using an
automatic weapon, which triggers many rumble operations.

We found the issue to be a race condition between sony_remove and the
final device destruction by the HID / input system. The problem was
that sony_remove didn't clean some of its work_item state in
"struct sony_sc". After sony_remove work, the corresponding evdev
node was around for sufficient time for applications to still queue
rumble work after "sony_remove".

On pre-4.19 kernels the race condition caused a kernel crash due to a
NULL-pointer dereference as "sc->output_report_dmabuf" got freed during
sony_remove. On newer kernels this crash doesn't happen due the buffer
now being allocated using devm_kzalloc. However we can still queue work,
while the driver is an undefined state.

This patch fixes the described problem, by guarding the work_item
"state_worker" with an initialized variable, which we are setting back
to 0 on cleanup.
Signed-off-by: Roderick Colenbrander <roderick.colenbrander@sony.com>
CC: stable@vger.kernel.org
Signed-off-by: Jiri Kosina <jkosina@suse.cz>