Re: [PATCH 2/4] ALSA: dice: postpone card registration

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

On Dec 24 Stefan Richter wrote:
> On Dec 24 Takashi Sakamoto wrote:
> [...]
> > Between bootstrap, firmware loading and post configuration, some bus resets
> > are observed.
> > 
> > This commit offloads most processing of probe function into workqueue and
> > schedules the workqueue after successive bus resets. This has an effect to
> > get correct hardware information and avoid involvement to bus reset storm.
> > 
> > For code simplicity, this change effects all of Dice-based models, i.e.
> > Dice II, Dice Jr., Dice Mini and Dice III. Due to the same reason, sound
> > card instance is kept till card free function even if some errors are
> > detected in the workqueue.  
> [...]
> > --- a/sound/firewire/dice/dice.c
> > +++ b/sound/firewire/dice/dice.c
> > @@ -18,6 +18,8 @@ MODULE_LICENSE("GPL v2");
> >  #define WEISS_CATEGORY_ID	0x00
> >  #define LOUD_CATEGORY_ID	0x10
> >  
> > +#define PROBE_DELAY_MS		(2 * MSEC_PER_SEC)
> > +
> >  static int check_dice_category(struct fw_unit *unit)
> >  {
> >  	struct fw_device *device = fw_parent_device(unit);
> > @@ -185,6 +187,9 @@ static void dice_card_free(struct snd_card *card)
> >  {
> >  	struct snd_dice *dice = card->private_data;
> >  
> > +	/* The workqueue for registration uses the memory block. */
> > +	cancel_work_sync(&dice->dwork.work);  
> 
> According to the kerneldoc comment at cancel_work_sync, this should
> actually be cancel_delayed_work_sync(&dice->dwork);.
> 
> > +
> >  	snd_dice_stream_destroy_duplex(dice);
> >  	snd_dice_transaction_destroy(dice);
> >  	fw_unit_put(dice->unit);
> > @@ -192,6 +197,66 @@ static void dice_card_free(struct snd_card *card)
> >  	mutex_destroy(&dice->mutex);
> >  }

Another comment to dice_card_free() which is aside from this patch:

You are calling mutex_destroy(&dice->mutex) here.  But if I am right in my
understanding that dice_card_free() is called from dice_remove(), then
this is wrong because dice_card_free() still holding the mutex.

I am not sure what the proper fix is.  Perhaps just never perform
mutex_destroy().

> > +static void do_registration(struct work_struct *work)
> > +{
> > +	struct snd_dice *dice = container_of(work, struct snd_dice, dwork.work);
> > +	int err;
> > +
> > +	mutex_lock(&dice->mutex);  
> 
> So the worker needs to obtain &dice->mutex.  But...
> 
> [...]
> > @@ -263,14 +307,25 @@ static void dice_remove(struct fw_unit *unit)
> >  {
> >  	struct snd_dice *dice = dev_get_drvdata(&unit->device);
> >  
> > +	/* For a race condition of struct snd_card.shutdown. */
> > +	mutex_lock(&dice->mutex);
> > +
> >  	/* No need to wait for releasing card object in this context. */
> >  	snd_card_free_when_closed(dice->card);
> > +
> > +	mutex_unlock(&dice->mutex);
> >  }  
> 
> ...if I read snd_card_free_when_closed() and its surrounding code
> correctly, then dice_card_free() is called from within this, which can
> cause a deadlock.  Right?
> 
> If so, then it seems to me that cancel_delayed_work_sync(&dice->dwork)
> should be moved out of dice_card_free() and be put at the beginning of
> dice_remove().

-- 
Stefan Richter
-=====-===== ==-- ==---
http://arcgraph.de/sr/