ALSA: hda - Fix deadlock of controller device lock at unbinding

Imre Deak reported a deadlock of HD-audio driver at unbinding while
it's still in probing.  Since we probe the codecs asynchronously in a
work, the codec driver probe may still be kicked off while the
controller itself is being unbound.  And, azx_remove() tries to
process all pending tasks via cancel_work_sync() for fixing the other
races (see commit [0b8c82190c12: ALSA: hda - Cancel probe work instead
of flush at remove]), now we may meet a bizarre deadlock:

Unbind snd_hda_intel via sysfs:
  device_release_driver() ->
    device_lock(snd_hda_intel) ->
      azx_remove() ->
        cancel_work_sync(azx_probe_work)

azx_probe_work():
  codec driver probe() ->
     __driver_attach() ->
       device_lock(snd_hda_intel)

This deadlock is caused by the fact that both device_release_driver()
and driver_probe_device() take both the device and its parent locks at
the same time.  The codec device sets the controller device as its
parent, and this lock is taken before the probe() callback is called,
while the controller remove() callback gets called also with the same
lock.

In this patch, as an ugly workaround, we unlock the controller device
temporarily during cancel_work_sync() call.  The race against another
bind call should be still suppressed by the parent's device lock.

Reported-by: Imre Deak <imre.deak@intel.com>
Fixes: 0b8c82190c ("ALSA: hda - Cancel probe work instead of flush at remove")
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/hda/hda_intel.c

@@ -2155,7 +2155,20 @@ static void azx_remove(struct pci_dev *pci)
 		/* cancel the pending probing work */
 		chip = card->private_data;
 		hda = container_of(chip, struct hda_intel, chip);
+		/* FIXME: below is an ugly workaround.
+		 * Both device_release_driver() and driver_probe_device()
+		 * take *both* the device's and its parent's lock before
+		 * calling the remove() and probe() callbacks.  The codec
+		 * probe takes the locks of both the codec itself and its
+		 * parent, i.e. the PCI controller dev.  Meanwhile, when
+		 * the PCI controller is unbound, it takes its lock, too
+		 * ==> ouch, a deadlock!
+		 * As a workaround, we unlock temporarily here the controller
+		 * device during cancel_work_sync() call.
+		 */
+		device_unlock(&pci->dev);
 		cancel_work_sync(&hda->probe_work);
+		device_lock(&pci->dev);
 
 		snd_card_free(card);
 	}