Re: [bug report] ALSA: jack: Access input_dev under mutex

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On 7/4/2023 10:07 AM, Takashi Iwai wrote:
On Mon, 03 Jul 2023 16:18:27 +0200,
Dan Carpenter wrote:

Hello Amadeusz Sławiński,

The patch 1b6a6fc5280e: "ALSA: jack: Access input_dev under mutex"
from Apr 12, 2022, leads to the following Smatch static checker
warning:

	sound/core/jack.c:673 snd_jack_report()
	warn: sleeping in atomic context

sound/core/jack.c
     663         jack->hw_status_cache = status;
     664
     665         list_for_each_entry(jack_kctl, &jack->kctl_list, list)
     666                 if (jack_kctl->sw_inject_enable)
     667                         mask_bits |= jack_kctl->mask_bits;
     668                 else
     669                         snd_kctl_jack_report(jack->card, jack_kctl->kctl,
     670                                              status & jack_kctl->mask_bits);
     671
     672 #ifdef CONFIG_SND_JACK_INPUT_DEV
--> 673         mutex_lock(&jack->input_dev_lock);
                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

That patch adds this mutex but we can't take mutex because we're already
holding a spinlock.  The problematic call trees are:

virtsnd_event_notify_cb() <- disables preempt
virtsnd_disable_event_vq() <- disables preempt
-> virtsnd_event_dispatch()
    -> virtsnd_jack_event()
       -> snd_jack_report()

The virtsnd_event_notify_cb() and virtsnd_disable_event_vq() functions
take the spin_lock_irqsave(&queue->lock, flags);

Indeed it was no good choice to use the mutex there inside the report
function.  It's supposed to be callable from an irq-disabled context,
too.

How about simply using the device refcount like below?

Although we may drop the mutex from snd_jack, it can can be left, as
it's still useful for protecting a potential race between creation and
deletion.


thanks,

Takashi

-- 8< --
--- a/sound/core/jack.c
+++ b/sound/core/jack.c
@@ -654,6 +654,7 @@ void snd_jack_report(struct snd_jack *jack, int status)
  	struct snd_jack_kctl *jack_kctl;
  	unsigned int mask_bits = 0;
  #ifdef CONFIG_SND_JACK_INPUT_DEV
+	struct input_dev *idev;
  	int i;
  #endif
@@ -670,17 +671,15 @@ void snd_jack_report(struct snd_jack *jack, int status)
  					     status & jack_kctl->mask_bits);
#ifdef CONFIG_SND_JACK_INPUT_DEV
-	mutex_lock(&jack->input_dev_lock);
-	if (!jack->input_dev) {
-		mutex_unlock(&jack->input_dev_lock);
+	idev = input_get_device(jack->input_dev);
+	if (!idev)
  		return;
-	}
for (i = 0; i < ARRAY_SIZE(jack->key); i++) {
  		int testbit = ((SND_JACK_BTN_0 >> i) & ~mask_bits);
if (jack->type & testbit)
-			input_report_key(jack->input_dev, jack->key[i],
+			input_report_key(idev, jack->key[i],
  					 status & testbit);
  	}
@@ -688,13 +687,13 @@ void snd_jack_report(struct snd_jack *jack, int status)
  		int testbit = ((1 << i) & ~mask_bits);
if (jack->type & testbit)
-			input_report_switch(jack->input_dev,
+			input_report_switch(idev,
  					    jack_switch_types[i],
  					    status & testbit);
  	}
- input_sync(jack->input_dev);
-	mutex_unlock(&jack->input_dev_lock);
+	input_sync(idev);
+	input_put_device(idev);
  #endif /* CONFIG_SND_JACK_INPUT_DEV */
  }
  EXPORT_SYMBOL(snd_jack_report);


Looking at code it looks like it should also work. Will schedule test run tomorrow to see if it causes any problems.



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