2023-06-25 15:42:36

by YE Chengfeng

Subject: [PATCH] ALSA: dummy: Fix &dpcm->lock deadlock issues

The timer dummy_systimer_callback is executed under softirq
context, thus other process context code requiring the same lock
should disable interrupt. Otherwise there would be potential
deadlock issues when the code executing under process context
(i.e., dummy_systimer_pointer, dummy_systimer_start,
dummy_systimer_stop) is preempted by the timer while holding
the lock.

Deadlock scenario:
dummy_systimer_pointer
-> spin_lock(&dpcm->lock);
<timer interrupt>
-> dummy_systimer_callback
-> spin_lock_irqsave(&dpcm->lock, flags);

Fix the potential deadlock by using spin_lock_irqsave.

Signed-off-by: Chengfeng Ye <[email protected]>
---
sound/drivers/dummy.c | 17 +++++++++++------
1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/sound/drivers/dummy.c b/sound/drivers/dummy.c
index 9c17b49a2ae1..04fb4f17e05c 100644
--- a/sound/drivers/dummy.c
+++ b/sound/drivers/dummy.c
@@ -268,19 +268,23 @@ static void dummy_systimer_update(struct dummy_systimer_pcm *dpcm)
static int dummy_systimer_start(struct snd_pcm_substream *substream)
{
struct dummy_systimer_pcm *dpcm = substream->runtime->private_data;
- spin_lock(&dpcm->lock);
+ unsigned long flags;
+
+ spin_lock_irqsave(&dpcm->lock, flags);
dpcm->base_time = jiffies;
dummy_systimer_rearm(dpcm);
- spin_unlock(&dpcm->lock);
+ spin_unlock_irqrestore(&dpcm->lock, flags);
return 0;
}

static int dummy_systimer_stop(struct snd_pcm_substream *substream)
{
struct dummy_systimer_pcm *dpcm = substream->runtime->private_data;
- spin_lock(&dpcm->lock);
+ unsigned long flags;
+
+ spin_lock_irqsave(&dpcm->lock, flags);
del_timer(&dpcm->timer);
- spin_unlock(&dpcm->lock);
+ spin_unlock_irqrestore(&dpcm->lock, flags);
return 0;
}

@@ -320,11 +324,12 @@ dummy_systimer_pointer(struct snd_pcm_substream *substream)
{
struct dummy_systimer_pcm *dpcm = substream->runtime->private_data;
snd_pcm_uframes_t pos;
+ unsigned long flags;

- spin_lock(&dpcm->lock);
+ spin_lock_irqsave(&dpcm->lock, flags);
dummy_systimer_update(dpcm);
pos = dpcm->frac_pos / HZ;
- spin_unlock(&dpcm->lock);
+ spin_unlock_irqrestore(&dpcm->lock, flags);
return pos;
}

--
2.17.1

2023-06-25 18:21:39

by Takashi Iwai

Subject: Re: [PATCH] ALSA: dummy: Fix &dpcm->lock deadlock issues

On Sun, 25 Jun 2023 17:35:48 +0200,
YE Chengfeng wrote:
>
> The timer dummy_systimer_callback is executed under softirq
> context, thus other process context code requiring the same lock
> should disable interrupt. Otherwise there would be potential
> deadlock issues when the code executing under process context
> (i.e., dummy_systimer_pointer, dummy_systimer_start,
> dummy_systimer_stop) is preempted by the timer while holding
> the lock.
>
> Deadlock scenario:
> dummy_systimer_pointer
> -> spin_lock(&dpcm->lock);
> <timer interrupt>
> -> dummy_systimer_callback
> -> spin_lock_irqsave(&dpcm->lock, flags);
>
> Fix the potential deadlock by using spin_lock_irqsave.

Did you really trigger this deadlock, or is just your hypothesis?
I'm asking it because basically the deadlock above shouldn't happen;
those are called only via PCM trigger and pointer callbacks, and they
are always called inside the PCM stream lock, and already
irq-disabled.


thanks,

Takashi