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

Timo Wischer
2019-03-26T08:12:50Z

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@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
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@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
>>
>>