Re: [PATCH 04/10] ALSA: aloop: Use always spin_lock_irqsave() for cable->lock

[Takashi Iwai <tiwai@suse.de>]

On Mon, 25 Mar 2019 17:40:23 +0100,
Timo Wischer wrote:
> 
> Best regards
> Timo Wischer
> Engineering Software Base (ADITG/ESB)
> 
> Tel. +49 5121 49 6938
> On 3/25/19 17:07, Takashi Iwai wrote:
> 
>     On Mon, 25 Mar 2019 17:00:38 +0100,
>     <twischer@de.adit-jv.com> wrote:
>     
>         From: Timo Wischer <twischer@de.adit-jv.com>
>         
>         to allow the usage of timer callbacks from interrupt context.
>         For example the sound timer.
>         
>     The trigger callback is already irq-disabled.  And, open/close must
>     not be irq-disabled OTOH.  So these changes must be superfluous.
>     
> Hello Takashi,
> 
> could you explain why open/close must not be irq-disabled?

The open/close callbacks get called always in sleepable context where
you can use mutex & co gracefully.  The only non-sleepable (atomic)
context is pointer and trigger callbacks.  So it's safe to use
spin_lock() without irq in these callbacks.
OTOH, the rest (open, close,hw_params, hw_free, prepare) callbacks are
always sleepable, hence it's safe to use spin_lock_irq() (not
save/restore) variant.

When a driver declares as non-atomic via PCM flag, all callbacks
become sleepable, but this isn't the case for aloop driver.

> I see a potential deadlock in case of free_cable() uses only spin_lock()
> instead of spin_lock_irqsave().
> For example the following will be executed:
> 
> loopback_close()
> free_cable()
> spin_lock(&dpcm->cable->lock)

Which spin_lock() call?  The free_cable() calls spin_lock_irq().

> >> Interrupted by jiffies timer IRQ before calling spin_unlock()
> 
> loopback_jiffies_timer_function()
> spin_lock_irqsave(&dpcm->cable->lock)
> >> DEADLOCK due to dpcm->cable->lock is already locked
> 
> Do you also see this deadlock or do you see any reason why this could not
> happen?

If spin_lock_irq() is used in free_cable(), it must be OK.


Takashi


> Best regards
> 
> Timo
> 
>     thanks,
>     
>     Takashi
> 
>         Signed-off-by: Timo Wischer <twischer@de.adit-jv.com>
>         ---
>          sound/drivers/aloop.c | 33 +++++++++++++++++++--------------
>          1 file changed, 19 insertions(+), 14 deletions(-)
>         
>         diff --git a/sound/drivers/aloop.c b/sound/drivers/aloop.c
>         index 11e8ed6..c6217c4 100644
>         --- a/sound/drivers/aloop.c
>         +++ b/sound/drivers/aloop.c
>         @@ -272,6 +272,7 @@ static int loopback_trigger(struct snd_pcm_substream *substream, int cmd)
>               struct loopback_pcm *dpcm = runtime->private_data;
>               struct loopback_cable *cable = dpcm->cable;
>               int err = 0, stream = 1 << substream->stream;
>         +     unsigned long flags;
>          
>               switch (cmd) {
>               case SNDRV_PCM_TRIGGER_START:
>         @@ -281,39 +282,39 @@ static int loopback_trigger(struct snd_pcm_substream *substream, int cmd)
>                                     dpcm->last_jiffies = jiffies;
>                                     dpcm->pcm_rate_shift = 0;
>                                     dpcm->last_drift = 0;
>         -                           spin_lock(&cable->lock);                    
>         +                           spin_lock_irqsave(&cable->lock, flags);
>                                     cable->running |= stream;
>                                     cable->pause &= ~stream;
>                                     err = loopback_timer_start(dpcm);
>         -                           spin_unlock(&cable->lock);
>         +                           spin_unlock_irqrestore(&cable->lock, flags);
>                                     if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
>                                                           loopback_active_notify(dpcm);
>                                     break;
>               case SNDRV_PCM_TRIGGER_STOP:
>         -                           spin_lock(&cable->lock);                    
>         +                           spin_lock_irqsave(&cable->lock, flags);
>                                     cable->running &= ~stream;
>                                     cable->pause &= ~stream;
>                                     err = loopback_timer_stop(dpcm);
>         -                           spin_unlock(&cable->lock);
>         +                           spin_unlock_irqrestore(&cable->lock, flags);
>                                     if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
>                                                           loopback_active_notify(dpcm);
>                                     break;
>               case SNDRV_PCM_TRIGGER_PAUSE_PUSH:
>               case SNDRV_PCM_TRIGGER_SUSPEND:
>         -                           spin_lock(&cable->lock);                    
>         +                           spin_lock_irqsave(&cable->lock, flags);
>                                     cable->pause |= stream;
>                                     err = loopback_timer_stop(dpcm);
>         -                           spin_unlock(&cable->lock);
>         +                           spin_unlock_irqrestore(&cable->lock, flags);
>                                     if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
>                                                           loopback_active_notify(dpcm);
>                                     break;
>               case SNDRV_PCM_TRIGGER_PAUSE_RELEASE:
>               case SNDRV_PCM_TRIGGER_RESUME:
>         -                           spin_lock(&cable->lock);
>         +                           spin_lock_irqsave(&cable->lock, flags);
>                                     dpcm->last_jiffies = jiffies;
>                                     cable->pause &= ~stream;
>                                     err = loopback_timer_start(dpcm);
>         -                           spin_unlock(&cable->lock);
>         +                           spin_unlock_irqrestore(&cable->lock, flags);
>                                     if (substream->stream == SNDRV_PCM_STREAM_PLAYBACK)
>                                                           loopback_active_notify(dpcm);
>                                     break;
>         @@ -557,12 +558,13 @@ static snd_pcm_uframes_t loopback_pointer(struct snd_pcm_substream *substream)
>          {
>               struct snd_pcm_runtime *runtime = substream->runtime;
>               struct loopback_pcm *dpcm = runtime->private_data;
>         +     unsigned long flags;
>               snd_pcm_uframes_t pos;
>          
>         -     spin_lock(&dpcm->cable->lock);
>         +     spin_lock_irqsave(&dpcm->cable->lock, flags);
>               loopback_pos_update(dpcm->cable);
>               pos = dpcm->buf_pos;
>         -     spin_unlock(&dpcm->cable->lock);
>         +     spin_unlock_irqrestore(&dpcm->cable->lock, flags);
>               return bytes_to_frames(runtime, pos);
>          }
>          
>         @@ -679,10 +681,12 @@ static void free_cable(struct snd_pcm_substream *substream)
>               if (!cable)
>                                     return;
>               if (cable->streams[!substream->stream]) {
>         +                           unsigned long flags;
>         +
>                                     /* other stream is still alive */
>         -                           spin_lock_irq(&cable->lock);
>         +                           spin_lock_irqsave(&cable->lock, flags);
>                                     cable->streams[substream->stream] = NULL;
>         -                           spin_unlock_irq(&cable->lock);
>         +                           spin_unlock_irqrestore(&cable->lock, flags);
>               } else {
>                                     /* free the cable */
>                                     loopback->cables[substream->number][dev] = NULL;
>         @@ -698,6 +702,7 @@ static int loopback_open(struct snd_pcm_substream *substream)
>               struct loopback_cable *cable = NULL;
>               int err = 0;
>               int dev = get_cable_index(substream);
>         +     unsigned long flags;
>          
>               mutex_lock(&loopback->cable_lock);
>               dpcm = kzalloc(sizeof(*dpcm), GFP_KERNEL);
>         @@ -753,9 +758,9 @@ static int loopback_open(struct snd_pcm_substream *substream)
>               else
>                                     runtime->hw = cable->hw;
>          
>         -     spin_lock_irq(&cable->lock);
>         +     spin_lock_irqsave(&cable->lock, flags);
>               cable->streams[substream->stream] = dpcm;
>         -     spin_unlock_irq(&cable->lock);
>         +     spin_unlock_irqrestore(&cable->lock, flags);
>          
>           unlock:
>               if (err < 0) {
>         -- 
>         2.7.4
> 
>