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

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On 3/25/19 17:58, Takashi Iwai wrote:

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@xxxxxxxxxxxxxx> wrote:
From: Timo Wischer <twischer@xxxxxxxxxxxxxx> 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

Thanks for the detailed explanation.

@all:
Please simple ignore this patch.

Best regards

Timo





Best regards

Timo

     thanks,
Takashi 

         Signed-off-by: Timo Wischer <twischer@xxxxxxxxxxxxxx>
         ---
          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



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