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

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Date:   Sat, 11 Jun 2022 18:58:51 +0100
From:   Jonathan Cameron <jic23@...nel.org>
To:     Claudiu Beznea <claudiu.beznea@...rochip.com>
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 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).

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)