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

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

[Jia-Ju Bai]

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 :)


Best wishes,
Jia-Ju Bai

Re: [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

Re: [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

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

[Jia-Ju Bai]

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

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

[Takashi Iwai]

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.


Takashi

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

[Takashi Iwai]

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.


Takashi

Re: [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

Re: [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