linux-kernel - Re: [Outreachy kernel] [PATCH v4] staging: iio: ade7753: Replace mlock with driver private lock

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Message-ID: <20170328183732.GA3808@d830.WORKGROUP>
Date:   Tue, 28 Mar 2017 11:37:33 -0700
From:   Alison Schofield <amsfield22@...il.com>
To:     SIMRAN SINGHAL <singhalsimran0@...il.com>
Cc:     Lars-Peter Clausen <lars@...afoo.de>,
        Michael Hennerich <Michael.Hennerich@...log.com>,
        Jonathan Cameron <jic23@...nel.org>,
        Hartmut Knaack <knaack.h@....de>,
        Peter Meerwald-Stadler <pmeerw@...erw.net>,
        Greg KH <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: [Outreachy kernel] [PATCH v4] staging: iio: ade7753: Replace
 mlock with driver private lock

On Tue, Mar 28, 2017 at 10:55:17PM +0530, SIMRAN SINGHAL wrote:
> On Fri, Mar 24, 2017 at 12:51 AM, Alison Schofield <amsfield22@...il.com> wrote:
> > On Fri, Mar 24, 2017 at 12:05:20AM +0530, 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.
> >
> > Hi Simran,
> >
> > Please post all revision histories below the --- not just the most
> > recent.
> >
> Sorry, will not repeat this.
> 
> > Does this lock enforce the needed "atomicity" in the write_frequency
> > function? I read Jonathans comment on a previous revision about
> > "ensuring the spi bus frequency and sampling frequency of the device
> > are changed in an atomic fashion"
> >
> 
> By introducing another lock I am protecting read_modify_write and
> in this way also protecting the designated register that we are about
> to write.

I see it protecting this path from being re-entered.  My uncertainty
is about other paths to read/write.

> 
> > Is it possible for another spi bus transaction (read or write) to
> > occur between the read and write in write_frequency?
> >
> 
> Gargi has also come up with a solution.
> https://groups.google.com/forum/#!topic/outreachy-kernel/kzE9CrI5Bd8
> 
> Should I do like her as her's also seem correct or go ahead with this.

My suggestion would be to wait for feedback on Gargi's patch. 
(See the Outreachy log about creating similar solutions.)

We will not be able to close on this set of patches during the
Outreachy application window.  You can continue to push for closure
beyond the March 30th date as your time allows :)

Thanks,
alisons

> 
> > alisons
> >>
> >> Signed-off-by: simran singhal <singhalsimran0@...il.com>
> >> ---
> >>
> >>  v4:
> >>    -Add mutex_init
> >>
> >>  drivers/staging/iio/meter/ade7753.c | 7 +++++--
> >>  1 file changed, 5 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/staging/iio/meter/ade7753.c b/drivers/staging/iio/meter/ade7753.c
> >> index b71fbd3..30aebaf 100644
> >> --- a/drivers/staging/iio/meter/ade7753.c
> >> +++ b/drivers/staging/iio/meter/ade7753.c
> >> @@ -80,11 +80,13 @@
> >>   * @us:         actual spi_device
> >>   * @tx:         transmit buffer
> >>   * @rx:         receive buffer
> >> + * @lock:       protect sensor state
> >>   * @buf_lock:       mutex to protect tx and rx
> >>   **/
> >>  struct ade7753_state {
> >>       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];
> >>  };
> >> @@ -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;
> >>  }
> >> @@ -581,6 +583,7 @@ static int ade7753_probe(struct spi_device *spi)
> >>       st = iio_priv(indio_dev);
> >>       st->us = spi;
> >>       mutex_init(&st->buf_lock);
> >> +     mutex_init(&st->lock);
> >>
> >>       indio_dev->name = spi->dev.driver->name;
> >>       indio_dev->dev.parent = &spi->dev;
> >> --
> >> 2.7.4
> >>
> >> --
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