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

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Date:   Mon, 13 Mar 2017 14:34:50 +0100
From:   Lars-Peter Clausen <lars@...afoo.de>
To:     SIMRAN SINGHAL <singhalsimran0@...il.com>
Cc:     Michael Hennerich <Michael.Hennerich@...log.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Pete Meerwald-Stadler <pmeerw@...erw.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-iio@...r.kernel.org, devel@...verdev.osuosl.org,
        linux-kernel@...r.kernel.org,
        outreachy-kernel <outreachy-kernel@...glegroups.com>
Subject: Re: [PATCH] staging: iio: ade7753: replace mlock with driver private
 lock

On 03/13/2017 01:33 PM, SIMRAN SINGHAL wrote:
> On Mon, Mar 13, 2017 at 5:30 PM, Lars-Peter Clausen <lars@...afoo.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@...il.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.
>>
>> 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.
>>
> 
> Are you trying to say that their is no need of introducing "lock",
> I can using "buf_lock" only.

Yes, there should be no need for two locks. But you need to slightly
refactor the code to avoid taking the same lock nested.

> 
> Thanks!
> 
>>>   **/
>>>  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;
>>>  }
>>>
>>