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

[Jia-Ju Bai <baijiaju1990@gmail.com>]

On 2022/1/29 12:27, Takashi Sakamoto wrote:
> 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 a lot for the quick reply :)
Your explanation is reasonable, because snd_shutdown_f_ops indeed has no 
value for .open.

However, my static analysis tool finds another possible deadlock in the 
mentioned code:

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()
   snd_card_file_remove()
     spin_lock(&card->files_lock); --> Line 962 (Lock A)
     wake_up_all(&card->remove_sleep); --> Line 976 (Wake X)
     spin_unlock(&card->files_lock); --> Line 977 (Unlock A)

When snd_card_disconnect_sync() is executed, "Wait X" is performed by 
holding "Lock A". If snd_hwdep_release() is executed at this time, "Wake 
X" cannot be performed to wake up "Wait X", because "Lock A" has been 
already hold by snd_card_disconnect_sync().

I am not quite sure whether this possible problem is real.
Any feedback would be appreciated, thanks :)


Best wishes,
Jia-Ju Bai