ALSA: usb-audio: Avoid nested autoresume calls

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: 9003ebb13f61 ('ALSA: usb-audio: Fix runtime PM unbalance')
Reported-and-tested-by: Alexnader Kuleshov <kuleshovmail@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>

1 files changed, 2 insertions, 2 deletions

diff --git a/sound/usb/proc.c b/sound/usb/proc.c
index 5f761ab34c01..0ac89e294d31 100644
--- a/sound/usb/proc.c
+++ b/sound/usb/proc.c
@@ -46,14 +46,14 @@ static inline unsigned get_high_speed_hz(unsigned int usb_rate)
 static void proc_audio_usbbus_read(struct snd_info_entry *entry, struct snd_info_buffer *buffer)
 {
 	struct snd_usb_audio *chip = entry->private_data;
-	if (!chip->shutdown)
+	if (!atomic_read(&chip->shutdown))
 		snd_iprintf(buffer, "%03d/%03d\n", chip->dev->bus->busnum, chip->dev->devnum);
 }
 
 static void proc_audio_usbid_read(struct snd_info_entry *entry, struct snd_info_buffer *buffer)
 {
 	struct snd_usb_audio *chip = entry->private_data;
-	if (!chip->shutdown)
+	if (!atomic_read(&chip->shutdown))
 		snd_iprintf(buffer, "%04x:%04x\n", 
 			    USB_ID_VENDOR(chip->usb_id),
 			    USB_ID_PRODUCT(chip->usb_id));