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

From: SIMRAN SINGHAL
Date: Sun Mar 19 2017 - 17:15:38 EST

On Mon, Mar 20, 2017 at 2:13 AM, Jonathan Cameron <jic23@xxxxxxxxxx> wrote:
> On 19/03/17 17:14, Gargi Sharma wrote:
>> 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.
> Well spotted. That is indeed the case. Just goes to show how more
> eyes on code is always a good thing!
>

Jonathan, I have already sent the version 6 of this patch in which I
have dropped the
locks in the function adis16060_spi_write_than_read and keep the locks
of function
read_raw as it is.

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

I keep the locks inside read_raw as it is because it will be more
safe, if we see in terms of
security. If I am wrong here, please correct me.

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