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

From: Takashi Iwai
Date: Mon Mar 25 2019 - 12:58:45 EST

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


> 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
>
>