[BUG] ALSA: core: possible deadlock involving waiting and locking operations

[Takashi Sakamoto]

Hi,

On Sat, Jan 29, 2022 at 11:33:26AM +0800, Jia-Ju Bai wrote:
> Hello,
> 
> My static analysis tool reports a possible deadlock in the sound driver
> in Linux 5.10:
> 
> snd_card_disconnect_sync()
>   spin_lock_irq(&card->files_lock); --> Line 461 (Lock A)
>   wait_event_lock_irq(card->remove_sleep, ...); --> Line 462 (Wait X)
>   spin_unlock_irq(&card->files_lock); --> Line 465 (Unlock A)
> 
> snd_hwdep_release()
>   mutex_lock(&hw->open_mutex); --> Line 152 (Lock B)
>   mutex_unlock(&hw->open_mutex); --> Line 157 (Unlock B)
>   snd_card_file_remove()
>     wake_up_all(&card->remove_sleep); --> Line 976 (Wake X)
> 
> snd_hwdep_open()
>   mutex_lock(&hw->open_mutex); --> Line 95 (Lock B)
>   snd_card_file_add()
>     spin_lock(&card->files_lock); --> Line 932 (Lock A)
>     spin_unlock(&card->files_lock); --> Line 940 (Unlock A)
>   mutex_unlock(&hw->open_mutex); --> Line 139 (Unlock B)
> 
> When snd_card_disconnect_sync() is executed, "Wait X" is performed by
> holding "Lock A". If snd_hwdep_open() is executed at this time, it holds
> "Lock B" and then waits for acquiring "Lock A". If snd_hwdep_release()
> is executed at this time, it waits for acquiring "Lock B", and thus
> "Wake X" cannot be performed to wake up "Wait X" in
> snd_card_disconnect_sync(), causing a possible deadlock.
> 
> I am not quite sure whether this possible problem is real and how to fix
> it if it is real.
> Any feedback would be appreciated, thanks :)

I'm interested in your report about the deadlock, and seek the cause
of issue. Then I realized that we should take care of the replacement of
file_operation before acquiring spinlock in snd_card_disconnect_sync().

```
snd_card_disconnect_sync()
->snd_card_disconnect()
  ->spin_lock()
  ->list_for_each_entry()
    mfile->file->f_op = snd_shutdown_f_ops
  ->spin_unlock()
->spin_lock_irq()
->wait_event_lock_irq()
->spin_unlock_irq()
```

The implementation of snd_shutdown_f_ops has no value for .open, therefore
snd_hwdep_open() is not called anymore when waiting the event. The mutex
(Lock B) is not acquired in process context of ALSA hwdep application.

The original .release function can be called by snd_disconnect_release()
via replaced snd_shutdown_f_ops. In the case, as you can see, the spinlock
(Lock A) is not acquired.

I think there are no race conditions against Lock A and B in process
context of ALSA hwdep application after card disconnection. But it would
be probable to overlook the other case. I would be glad to receive your
check for the above procedure.


Thanks

Takashi Sakamoto