linux-kernel - Re: [PATCH 11/16] iio: adc: at91-sama5d2_adc: add locking parameter to at91_adc_read_info

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Message-ID: <2989a8a5-b350-aac1-dcfb-249ac5bb23aa@microchip.com>
Date:   Tue, 14 Jun 2022 08:50:14 +0000
From:   <Claudiu.Beznea@...rochip.com>
To:     <jic23@...nel.org>
CC:     <Eugen.Hristev@...rochip.com>, <lars@...afoo.de>,
        <Nicolas.Ferre@...rochip.com>, <alexandre.belloni@...tlin.com>,
        <robh+dt@...nel.org>, <krzk+dt@...nel.org>,
        <ludovic.desroches@...el.com>, <linux-iio@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        <devicetree@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 11/16] iio: adc: at91-sama5d2_adc: add locking parameter
 to at91_adc_read_info_raw()

On 11.06.2022 20:58, Jonathan Cameron wrote:
> EXTERNAL EMAIL: Do not click links or open attachments unless you know the content is safe
> 
> On Thu, 9 Jun 2022 11:32:08 +0300
> Claudiu Beznea <claudiu.beznea@...rochip.com> wrote:
> 
>> Add a parameter to at91_adc_read_info_raw() to specify if st->lock mutex
>> need to be acquired. This prepares for the addition of temperature sensor
>> code which will re-use at91_adc_read_info_raw() function to read 2 voltages
>> for determining the real temperature.
> 
> This looks like a potential lock dependency issue.
> iio_device_claim_direct_mode() takes an internal iio lock, and
> you then take st->lock.
> 
> If you are going to invert that locking order in another path
> you have a deadlock.
> 
> So rethink this. If you want to reuse the code you'll need to factor
> it out to a separate function that takes none of the locks then
> take all locks needed in each call path (in the same order).

OK, I'll check it.

> 
> Jonathan
> 
> 
>>
>> Signed-off-by: Claudiu Beznea <claudiu.beznea@...rochip.com>
>> ---
>>  drivers/iio/adc/at91-sama5d2_adc.c | 15 ++++++++++-----
>>  1 file changed, 10 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/iio/adc/at91-sama5d2_adc.c b/drivers/iio/adc/at91-sama5d2_adc.c
>> index 1283bcf4e682..8f8fef42de84 100644
>> --- a/drivers/iio/adc/at91-sama5d2_adc.c
>> +++ b/drivers/iio/adc/at91-sama5d2_adc.c
>> @@ -1583,7 +1583,8 @@ static irqreturn_t at91_adc_interrupt(int irq, void *private)
>>  }
>>
>>  static int at91_adc_read_info_raw(struct iio_dev *indio_dev,
>> -                               struct iio_chan_spec const *chan, int *val)
>> +                               struct iio_chan_spec const *chan, int *val,
>> +                               bool lock)
>>  {
>>       struct at91_adc_state *st = iio_priv(indio_dev);
>>       int (*fn)(struct at91_adc_state *, int, u16 *) = NULL;
>> @@ -1602,13 +1603,15 @@ static int at91_adc_read_info_raw(struct iio_dev *indio_dev,
>>       ret = iio_device_claim_direct_mode(indio_dev);
>>       if (ret)
>>               return ret;
>> -     mutex_lock(&st->lock);
>> +     if (lock)
>> +             mutex_lock(&st->lock);
>>
>>       if (fn) {
>>               ret = fn(st, chan->channel, &tmp_val);
>>               *val = tmp_val;
>>               ret = at91_adc_adjust_val_osr(st, val);
>> -             mutex_unlock(&st->lock);
>> +             if (lock)
>> +                     mutex_unlock(&st->lock);
>>               iio_device_release_direct_mode(indio_dev);
>>
>>               return ret;
>> @@ -1644,7 +1647,8 @@ static int at91_adc_read_info_raw(struct iio_dev *indio_dev,
>>       /* Needed to ACK the DRDY interruption */
>>       at91_adc_readl(st, LCDR);
>>
>> -     mutex_unlock(&st->lock);
>> +     if (lock)
>> +             mutex_unlock(&st->lock);
>>
>>       iio_device_release_direct_mode(indio_dev);
>>       return ret;
>> @@ -1658,7 +1662,8 @@ static int at91_adc_read_raw(struct iio_dev *indio_dev,
>>
>>       switch (mask) {
>>       case IIO_CHAN_INFO_RAW:
>> -             return at91_adc_read_info_raw(indio_dev, chan, val);
>> +             return at91_adc_read_info_raw(indio_dev, chan, val, true);
>> +
>>       case IIO_CHAN_INFO_SCALE:
>>               *val = st->vref_uv / 1000;
>>               if (chan->differential)
>