Re: [PATCH 11/16] iio: adc: at91-sama5d2_adc: add locking parameter to at91_adc_read_info_raw()

Jonathan Cameron
2022-06-11T17:49:47Z

On Thu, 9 Jun 2022 11:32:08 +0300
Claudiu Beznea <claudiu.beznea@microchip.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).

Jonathan

>
> Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.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)