Re: [Outreachy kernel] [PATCH v5] staging: Use buf_lock instead of mlock and Refactor code

[Gargi Sharma]

On Sun, Mar 19, 2017 at 6:20 PM, simran singhal
<singhalsimran0@xxxxxxxxx> wrote:
> The IIO subsystem is redefining iio_dev->mlock to be used by
> the IIO core only for protecting device operating mode changes.
> ie. Changes between INDIO_DIRECT_MODE, INDIO_BUFFER_* modes.
>
> In this driver, mlock was being used to protect hardware state
> changes. Replace it with buf_lock in the devices global data.
>
> As buf_lock protects both the adis16060_spi_write() and
> adis16060_spi_read() functions and both are always called in
> pair. First write, then read. Thus, refactor the code to have
> one single function adis16060_spi_write_than_read() which is
> protected by the existing buf_lock.
>
> Signed-off-by: simran singhal <singhalsimran0@xxxxxxxxx>
> ---
>
>  v5:
>    -Rename val in adis16060_spi_write_than_read() to conf.
>    -Rename val2 in adis16060_spi_write_than_read() to val.
>    -Corrected Checkpatch issues.
>    -Removed goto from adis16060_read_raw().
>
>
>  drivers/staging/iio/gyro/adis16060_core.c | 42 ++++++++++++-------------------
>  1 file changed, 16 insertions(+), 26 deletions(-)
>
> diff --git a/drivers/staging/iio/gyro/adis16060_core.c b/drivers/staging/iio/gyro/adis16060_core.c
> index c9d46e7..0f12492 100644
> --- a/drivers/staging/iio/gyro/adis16060_core.c
> +++ b/drivers/staging/iio/gyro/adis16060_core.c
> @@ -40,25 +40,20 @@ struct adis16060_state {
>
>  static struct iio_dev *adis16060_iio_dev;
>
> -static int adis16060_spi_write(struct iio_dev *indio_dev, u8 val)
> +static int adis16060_spi_write_than_read(struct iio_dev *indio_dev,
> +                                        u8 conf, u16 *val)
>  {
>         int ret;
>         struct adis16060_state *st = iio_priv(indio_dev);
>
>         mutex_lock(&st->buf_lock);
> -       st->buf[2] = val; /* The last 8 bits clocked in are latched */
> +       st->buf[2] = conf; /* The last 8 bits clocked in are latched */
>         ret = spi_write(st->us_w, st->buf, 3);
> -       mutex_unlock(&st->buf_lock);
>
> -       return ret;
> -}
> -
> -static int adis16060_spi_read(struct iio_dev *indio_dev, u16 *val)
> -{
> -       int ret;
> -       struct adis16060_state *st = iio_priv(indio_dev);
> -
> -       mutex_lock(&st->buf_lock);
> +       if (ret < 0) {
> +               mutex_unlock(&st->buf_lock);
> +               return ret;
> +       }
>
>         ret = spi_read(st->us_r, st->buf, 3);
>
> @@ -69,8 +64,8 @@ static int adis16060_spi_read(struct iio_dev *indio_dev, u16 *val)
>          */
>         if (!ret)
>                 *val = ((st->buf[0] & 0x3) << 12) |
> -                       (st->buf[1] << 4) |
> -                       ((st->buf[2] >> 4) & 0xF);
> +                        (st->buf[1] << 4) |
> +                        ((st->buf[2] >> 4) & 0xF);
>         mutex_unlock(&st->buf_lock);
>
>         return ret;
> @@ -83,20 +78,19 @@ static int adis16060_read_raw(struct iio_dev *indio_dev,
>  {
>         u16 tval = 0;
>         int ret;
> +       struct adis16060_state *st = iio_priv(indio_dev);
>
>         switch (mask) {
>         case IIO_CHAN_INFO_RAW:
>                 /* Take the iio_dev status lock */
> -               mutex_lock(&indio_dev->mlock);
> -               ret = adis16060_spi_write(indio_dev, chan->address);
> +               mutex_lock(&st->buf_lock);
> +               ret = adis16060_spi_write_than_read(indio_dev,
> +                                                   chan->address, &tval);
>                 if (ret < 0)
> -                       goto out_unlock;
> +                       mutex_unlock(&st->buf_lock);
> +                       return ret;
>
> -               ret = adis16060_spi_read(indio_dev, &tval);
> -               if (ret < 0)
> -                       goto out_unlock;
> -
> -               mutex_unlock(&indio_dev->mlock);
> +               mutex_unlock(&st->buf_lock);
>                 *val = tval;
>                 return IIO_VAL_INT;
>         case IIO_CHAN_INFO_OFFSET:
> @@ -110,10 +104,6 @@ static int adis16060_read_raw(struct iio_dev *indio_dev,
>         }
>
>         return -EINVAL;
> -
> -out_unlock:
> -       mutex_unlock(&indio_dev->mlock);
> -       return ret;
>  }
>

Hey Simran,

I'm another Outreachy aspirant and I'm trying to work through a
similar patch in another driver. Can you please explain to me how you
are avoiding nested locks here? From what I understand, the function
adis16060_read_raw call a lock on &st->buf_lock and then you call the
function adis16060_spi_write_than_read which again tries to get hold
of the same lock. Isn't this a deadlock situation? Please let me know
if my understanding is incorrect.

Thank you!
Gargi

>  static const struct iio_info adis16060_info = {
> --
> 2.7.4
>
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