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

[Takashi Iwai <tiwai@suse.de>]

On Thu, 06 Jul 2023 17:16:38 +0200,
Amadeusz Sławiński wrote:
> 
> On 7/5/2023 4:47 PM, Amadeusz Sławiński wrote:
> > 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.
> 
> I've run tests and see nothing worrying, so
> Tested-by: Amadeusz Sławiński <amadeuszx.slawinski@linux.intel.com>

Good to hear, I'll submit the proper patch.


thanks,

Takashi