hda codec unbind refcount hang

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

Hi Takashi,

commit 7206998f578d ("ALSA: hda: Fix potential deadlock at codec
unbinding") introduced a problem on at least one of my older machines.

The problem happens when hda_codec_driver_remove() encounters a
codec without any pcms (and thus the refcount is 1) and tries to
call refcount_dec(). Turns out refcount_dec() doesn't like to be
used for dropping the refcount to 0, and instead if spews a warning
and does its saturate thing. The subsequent wait_event() is then
permanently stuck waiting on the saturated refcount.

I've definitely seen the same kind of pattern used elsewhere
in the kernel as well, so the fact that refcount_t can't be used
to implement it is a bit of surprise to me. I guess most other
places still use atomic_t instead.

-- 
Ville Syrjälä
Intel