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

[Jia-Ju Bai]

On 2022/1/29 16:20, Takashi Iwai wrote:
> On Sat, 29 Jan 2022 09:07:05 +0100,
> Jia-Ju Bai wrote:
>>
>>
>> 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".
> No, it's wait_event_lock_irq(), and this helper unlocks the given lock
> during waiting and re-locks it after schedule().  See the macro
> expansion in include/linux/wait.h.

Oh, yes, you are right.
Sorry for this false positive...
I will improve my tool, thanks.


Best wishes,
Jia-Ju Bai