• Takashi Iwai's avatar
    ALSA: usb-audio: Avoid nested autoresume calls · 47ab1545
    Takashi Iwai authored
    After the recent fix of runtime PM for USB-audio driver, we got a
    lockdep warning like:
    
      =============================================
      [ INFO: possible recursive locking detected ]
      4.2.0-rc8+ #61 Not tainted
      ---------------------------------------------
      pulseaudio/980 is trying to acquire lock:
       (&chip->shutdown_rwsem){.+.+.+}, at: [<ffffffffa0355dac>] snd_usb_autoresume+0x1d/0x52 [snd_usb_audio]
      but task is already holding lock:
       (&chip->shutdown_rwsem){.+.+.+}, at: [<ffffffffa0355dac>] snd_usb_autoresume+0x1d/0x52 [snd_usb_audio]
    
    This comes from snd_usb_autoresume() invoking down_read() and it's
    used in a nested way.  Although it's basically safe, per se (as these
    are read locks), it's better to reduce such spurious warnings.
    
    The read lock is needed to guarantee the execution of "shutdown"
    (cleanup at disconnection) task after all concurrent tasks are
    finished.  This can be implemented in another better way.
    
    Also, the current check of chip->in_pm isn't good enough for
    protecting the racy execution of multiple auto-resumes.
    
    This patch rewrites the logic of snd_usb_autoresume() & co; namely,
    - The recursive call of autopm is avoided by the new refcount,
      chip->active.  The chip->in_pm flag is removed accordingly.
    - Instead of rwsem, another refcount, chip->usage_count, is introduced
      for tracking the period to delay the shutdown procedure.  At
      the last clear of this refcount, wake_up() to the shutdown waiter is
      called.
    - The shutdown flag is replaced with shutdown atomic count; this is
      for reducing the lock.
    - Two new helpers are introduced to simplify the management of these
      refcounts; snd_usb_lock_shutdown() increases the usage_count, checks
      the shutdown state, and does autoresume.  snd_usb_unlock_shutdown()
      does the opposite.  Most of mixer and other codes just need this,
      and simply returns an error if it receives an error from lock.
    
    Fixes: 9003ebb1
    
     ('ALSA: usb-audio: Fix runtime PM unbalance')
    Reported-and-tested-by: default avatarAlexnader Kuleshov <kuleshovmail@gmail.com>
    Signed-off-by: default avatarTakashi Iwai <tiwai@suse.de>
    47ab1545