A potential data race in drivers/iio/adc/berlin2-adc.ko

A potential data race in drivers/iio/adc/berlin2-adc.ko

[Pavel Andrianov]

Hi,

berlin2_adc_probe [1] registers two interrupt handlers: berlin2_adc_irq [2]
and berlin2_adc_tsen_irq [3]. The interrupt handlers operate with the 
same data, for example, modify
priv->data with different masks:

priv->data &= BERLIN2_SM_ADC_MASK;
and
priv->data &= BERLIN2_SM_TSEN_MASK;

If the two interrupt handlers are executed simultaneously, a potential 
data race takes place. So, the question is if the situation is possible. 
For example, in the case of the handlers are executed on different CPU 
cores.

Best regards,
Pavel

[1] 
https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L283 

[2] 
https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L239 

[3] 
https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L259

Re: A potential data race in drivers/iio/adc/berlin2-adc.ko

[Lars-Peter Clausen]

On 3/18/21 9:07 AM, Pavel Andrianov wrote:
> Hi,
>
> berlin2_adc_probe [1] registers two interrupt handlers: 
> berlin2_adc_irq [2]
> and berlin2_adc_tsen_irq [3]. The interrupt handlers operate with the 
> same data, for example, modify
> priv->data with different masks:
>
> priv->data &= BERLIN2_SM_ADC_MASK;
> and
> priv->data &= BERLIN2_SM_TSEN_MASK;
>
> If the two interrupt handlers are executed simultaneously, a potential 
> data race takes place. So, the question is if the situation is 
> possible. For example, in the case of the handlers are executed on 
> different CPU cores.
>
> Best regards,
> Pavel
>
> [1] 
> https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L283 
>
> [2] 
> https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L239 
>
> [3] 
> https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L259
>
Looking at the code there are two functions. berlin2_adc_tsen_read() and 
berlin2_adc_read(). These two function are take the same mutex and can 
not run concurrently. At the beginning of the protected section the 
corresponding interrupt for that function is enabled and at the end 
disabled. So at least if the hardware works correctly those two 
interrupts will never fire at the same time.

Now, if the hardware misbehaves the two interrupts could still fire at 
the same time.

- Lars

Re: A potential data race in drivers/iio/adc/berlin2-adc.ko

[Lars-Peter Clausen]

On 3/18/21 9:27 AM, Lars-Peter Clausen wrote:
> On 3/18/21 9:07 AM, Pavel Andrianov wrote:
>> Hi,
>>
>> berlin2_adc_probe [1] registers two interrupt handlers: 
>> berlin2_adc_irq [2]
>> and berlin2_adc_tsen_irq [3]. The interrupt handlers operate with the 
>> same data, for example, modify
>> priv->data with different masks:
>>
>> priv->data &= BERLIN2_SM_ADC_MASK;
>> and
>> priv->data &= BERLIN2_SM_TSEN_MASK;
>>
>> If the two interrupt handlers are executed simultaneously, a 
>> potential data race takes place. So, the question is if the situation 
>> is possible. For example, in the case of the handlers are executed on 
>> different CPU cores.
>>
>> Best regards,
>> Pavel
>>
>> [1] 
>> https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L283 
>>
>> [2] 
>> https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L239 
>>
>> [3] 
>> https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L259
>>
> Looking at the code there are two functions. berlin2_adc_tsen_read() 
> and berlin2_adc_read(). These two function are take the same mutex and 
> can not run concurrently. At the beginning of the protected section 
> the corresponding interrupt for that function is enabled and at the 
> end disabled. So at least if the hardware works correctly those two 
> interrupts will never fire at the same time.
>
> Now, if the hardware misbehaves the two interrupts could still fire at 
> the same time.
>
> - Lars
>
Actually thinking a bit more about this the interrupt could still fire 
after it has been disabled since there is no synchronization between the 
disable and the interrupt handler. And the handler might be queued on 
one CPU, while the disable is running on another CPU.

Re: A potential data race in drivers/iio/adc/berlin2-adc.ko

[Jonathan Cameron]

On Thu, 18 Mar 2021 09:47:29 +0100
Lars-Peter Clausen <lars@metafoo.de> wrote:

> On 3/18/21 9:27 AM, Lars-Peter Clausen wrote:
> > On 3/18/21 9:07 AM, Pavel Andrianov wrote:  
> >> Hi,
> >>
> >> berlin2_adc_probe [1] registers two interrupt handlers: 
> >> berlin2_adc_irq [2]
> >> and berlin2_adc_tsen_irq [3]. The interrupt handlers operate with the 
> >> same data, for example, modify
> >> priv->data with different masks:
> >>
> >> priv->data &= BERLIN2_SM_ADC_MASK;
> >> and
> >> priv->data &= BERLIN2_SM_TSEN_MASK;
> >>
> >> If the two interrupt handlers are executed simultaneously, a 
> >> potential data race takes place. So, the question is if the situation 
> >> is possible. For example, in the case of the handlers are executed on 
> >> different CPU cores.

If we assume there is a race here, the reading into priv->data from
two different registers on the line above the masking is more of any issue.

> >>
> >> Best regards,
> >> Pavel
> >>
> >> [1] 
> >> https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L283 
> >>
> >> [2] 
> >> https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L239 
> >>
> >> [3] 
> >> https://elixir.bootlin.com/linux/latest/source/drivers/iio/adc/berlin2-adc.c#L259
> >>  
> > Looking at the code there are two functions. berlin2_adc_tsen_read() 
> > and berlin2_adc_read(). These two function are take the same mutex and 
> > can not run concurrently. At the beginning of the protected section 
> > the corresponding interrupt for that function is enabled and at the 
> > end disabled. So at least if the hardware works correctly those two 
> > interrupts will never fire at the same time.
> >
> > Now, if the hardware misbehaves the two interrupts could still fire at 
> > the same time.
> >
> > - Lars
> >  
> Actually thinking a bit more about this the interrupt could still fire 
> after it has been disabled since there is no synchronization between the 
> disable and the interrupt handler. And the handler might be queued on 
> one CPU, while the disable is running on another CPU.
> 
Superficially it looks like splitting the priv->data and related priv->data_available
into versions for the normal ADC and the touch screen ADC paths should solve
this at the trivial cost of a couple of elements in that structure.
Possibly also need to deal with the wait_queue but I think that's fine as is.
(haven't thought about it that much!)

Jonathan