[Outreachy kernel] Re: [PATCH] staging: iio: ade7753: replace mlock with driver private lock

[Jonathan Cameron]

On 17/03/17 09:32, Gargi Sharma wrote:
> On Mon, Mar 13, 2017 at 5:30 PM, Lars-Peter Clausen <lars at metafoo.de> wrote:
>>
>> On 03/12/2017 02:32 PM, simran singhal 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 a lock in the devices global data.
>>>
>>> Fix some coding style issues related to white space also.
>>>
>>> Signed-off-by: simran singhal <singhalsimran0 at gmail.com>
>>> ---
>>>  drivers/staging/iio/meter/ade7753.c | 14 ++++++++------
>>>  1 file changed, 8 insertions(+), 6 deletions(-)
>>>
>>> diff --git a/drivers/staging/iio/meter/ade7753.c b/drivers/staging/iio/meter/ade7753.c
>>> index dfd8b71..ca99d82 100644
>>> --- a/drivers/staging/iio/meter/ade7753.c
>>> +++ b/drivers/staging/iio/meter/ade7753.c
>>> @@ -81,12 +81,14 @@
>>>   * @tx:         transmit buffer
>>>   * @rx:         receive buffer
>>>   * @buf_lock:       mutex to protect tx and rx
>>> + * @lock:    protect sensor state
>>
>> It might make sense to reuse the existing lock which currently protects the
>> read/write functions. You can do this by introducing a variant of
>> ade7753_spi_{read,write}_reg_16() that does not take a lock and use these to
>> implement the read-modify-write cycle in a protected section.
> 
> There are other read/write functions for example,
> ade7753_spi_{read/write}_reg_8 that use the mutex as well. Should a
> variant of these functions be introduced as well? Also, how does one
> go about implementing RMW inside a protected section.
Hmm. Simran has also been progressing with patches for this.

You raise a good question. There are other read/modify/write sequences in
the driver.  They don't have the same issue with potentially deadlocking
against the buf lock as they are all using the spi subsystems provisions
for small write/read cycles where buffer protection is handled internally.

So let us address the cases in turn:

static int ade7753_reset(struct device *dev)
{
	u16 val;
	int ret;

	ret = ade7753_spi_read_reg_16(dev, ADE7753_MODE, &val);
	if (ret)
		return ret;

	val |= BIT(6); /* Software Chip Reset */

	return ade7753_spi_write_reg_16(dev, ADE7753_MODE, val);
}
This is only called in the device initialization.  At that point
we should be fine in assuming no parallel calls.  Crucial point
is it is before the call to iio_device_register which exposes
the userspace interfaces.

static int ade7753_set_irq(struct device *dev, bool enable)
{
	int ret;
	u8 irqen;

	ret = ade7753_spi_read_reg_8(dev, ADE7753_IRQEN, &irqen);
	if (ret)
		goto error_ret;

	if (enable)
		irqen |= BIT(3); /* Enables an interrupt when a data is
				  * present in the waveform register
				  */
	else
		irqen &= ~BIT(3);

	ret = ade7753_spi_write_reg_8(dev, ADE7753_IRQEN, irqen);

error_ret:
	return ret;
}

This one is actually safe because it is the only function that
modifies that particular register.

/* Power down the device */
static int ade7753_stop_device(struct device *dev)
{
	u16 val;
	int ret;

	ret = ade7753_spi_read_reg_16(dev, ADE7753_MODE, &val);
	if (ret)
		return ret;

	val |= BIT(4);  /* AD converters can be turned off */

	return ade7753_spi_write_reg_16(dev, ADE7753_MODE, val);
}

Only called in remove (after userspace interfaces have been
removed by the iio_device_unregister call so also should not
be running concurrently with much else.

So I think all the other cases are safe.  Perhaps it would have
been better to have had a lock around them, purely to make
the code more resilient against future changes though.  
Probably a job to do as part of a larger scale pile of work
on that driver rather than as a one off patch.

Jonathan







> 
> 
>>
>> Looking through the driver there seem to be other places as well that do
>> read-modify-write that should be protected by a lock, but currently are not.
>> This might be a good task.
> 
> Am I right in understanding that we want to introduce mutex lock for
> writes in other drivers as well?
> 
> Thanks,
> Gargi
>>
>>>   **/
>>>  struct ade7753_state {
>>> -         struct spi_device   *us;
>>> -                 struct mutex        buf_lock;
>>> -                         u8          tx[ADE7753_MAX_TX] ____cacheline_aligned;
>>> -                                 u8          rx[ADE7753_MAX_RX];
>>> +     struct spi_device   *us;
>>> +     struct mutex        buf_lock;
>>> +     struct mutex        lock;       /* protect sensor state */
>>> +     u8          tx[ADE7753_MAX_TX] ____cacheline_aligned;
>>> +     u8          rx[ADE7753_MAX_RX];
>>>  };
>>>
>>>  static int ade7753_spi_write_reg_8(struct device *dev,
>>> @@ -484,7 +486,7 @@ static ssize_t ade7753_write_frequency(struct device *dev,
>>>       if (!val)
>>>               return -EINVAL;
>>>
>>> -     mutex_lock(&indio_dev->mlock);
>>> +     mutex_lock(&st->lock);
>>>
>>>       t = 27900 / val;
>>>       if (t > 0)
>>> @@ -505,7 +507,7 @@ static ssize_t ade7753_write_frequency(struct device *dev,
>>>       ret = ade7753_spi_write_reg_16(dev, ADE7753_MODE, reg);
>>>
>>>  out:
>>> -     mutex_unlock(&indio_dev->mlock);
>>> +     mutex_unlock(&st->lock);
>>>
>>>       return ret ? ret : len;
>>>  }
>>>
>>
>> --
> --
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