[alsa-devel] ALSA: usb-audio: fix oops due to cleanup race when disconnecting

[pl bossart]

>> Ah, I forgot to put my sign-off since I was waiting for any test
>> result...  Has anyone tried it?
>
> Doesn't work for me. same oops as before. Either I made a mistake when
> merging this patch manually (there were some rejects), or the problem
> is still there.

Added some additional traces, the dereference takes place before the
mutex is unlocked.

usb 3-1: USB disconnect, address 5
ALSA card.c:556: locking USB shutdown mutex
ALSA urb.c:492: frame 0 active: -84
ALSA urb.c:197: cannot submit urb (err = -19)
ALSA urb.c:492: frame 0 active: -84
ALSA urb.c:197: cannot submit urb (err = -19)
ALSA urb.c:492: frame 0 active: -84
ALSA urb.c:197: cannot submit urb (err = -19)
ALSA urb.c:492: frame 0 active: -84
ALSA urb.c:197: cannot submit urb (err = -19)
ALSA urb.c:186: NULL substream (subs->running 0)
ALSA urb.c:186: NULL substream (subs->running 0)
ALSA card.c:575: UNlocking USB shutdown mutex