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

[Jonathan Cameron <jic23@kernel.org>]

On 19/03/17 17:14, Gargi Sharma wrote:
> On Sun, Mar 19, 2017 at 6:20 PM, simran singhal
> <singhalsimran0@gmail.com> 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@gmail.com>
>> ---
>>
>>  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.
Well spotted. That is indeed the case.  Just goes to show how more
eyes on code is always a good thing!

The locks in read_raw itself should be dropped as we now have a single
safe function with the locks inside it being called.

Jonathan
> 
> Thank you!
> Gargi
> 
>>  static const struct iio_info adis16060_info = {
>> --
>> 2.7.4
>>
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