Re: [PATCH] iio:dac:ad5064 mlock cleanup - move to a local lock.

[Jonathan Cameron <jic23@kernel.org>]

On Fri, 22 Feb 2019 13:58:13 +0200
Alexandru Ardelean <ardeleanalex@gmail.com> wrote:

> On Wed, Feb 20, 2019 at 8:24 PM <jic23@kernel.org> wrote:
> >  
> 
> Hey,
> 
> One comment inline.
> 
> > From: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> >
> > indio_dev->mlock is intended to protect state transitions in
> > the core. It's scope is tightly defined. For device specific
> > uses such as those made here, we should define a local lock
> > allowing the scope of the lock to be defined near to what it
> > is protecting.
> >
> > These mlock changes can be non obvious, but given we don't do
> > anything other than direct for DACs, these ones are easy to do.
> >
> > If anyone wants to help with this particular effort it would
> > be most welcome!
> >
> > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > ---
> >  drivers/iio/dac/ad5064.c | 15 +++++++++------
> >  1 file changed, 9 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/iio/dac/ad5064.c b/drivers/iio/dac/ad5064.c
> > index 2f98cb2a3b96..6c3ba143839b 100644
> > --- a/drivers/iio/dac/ad5064.c
> > +++ b/drivers/iio/dac/ad5064.c
> > @@ -112,6 +112,8 @@ struct ad5064_state {
> >         bool                            use_internal_vref;
> >
> >         ad5064_write_func               write;
> > +       /* Lock used to maintain consistency between cached and dev state */
> > +       struct mutex lock;
> >
> >         /*
> >          * DMA (thus cache coherency maintenance) requires the
> > @@ -248,11 +250,11 @@ static int ad5064_set_powerdown_mode(struct iio_dev *indio_dev,
> >         struct ad5064_state *st = iio_priv(indio_dev);
> >         int ret;
> >
> > -       mutex_lock(&indio_dev->mlock);
> > +       mutex_lock(&st->lock);
> >         st->pwr_down_mode[chan->channel] = mode + 1;
> >
> >         ret = ad5064_sync_powerdown_mode(st, chan);
> > -       mutex_unlock(&indio_dev->mlock);
> > +       mutex_unlock(&st->lock);
> >
> >         return ret;
> >  }
> > @@ -291,11 +293,11 @@ static ssize_t ad5064_write_dac_powerdown(struct iio_dev *indio_dev,
> >         if (ret)
> >                 return ret;
> >
> > -       mutex_lock(&indio_dev->mlock);
> > +       mutex_lock(&st->lock);
> >         st->pwr_down[chan->channel] = pwr_down;
> >
> >         ret = ad5064_sync_powerdown_mode(st, chan);
> > -       mutex_unlock(&indio_dev->mlock);
> > +       mutex_unlock(&st->lock);
> >         return ret ? ret : len;
> >  }
> >
> > @@ -349,12 +351,12 @@ static int ad5064_write_raw(struct iio_dev *indio_dev,
> >                 if (val >= (1 << chan->scan_type.realbits) || val < 0)
> >                         return -EINVAL;
> >
> > -               mutex_lock(&indio_dev->mlock);
> > +               mutex_lock(&st->lock);
> >                 ret = ad5064_write(st, AD5064_CMD_WRITE_INPUT_N_UPDATE_N,
> >                                 chan->address, val, chan->scan_type.shift);
> >                 if (ret == 0)
> >                         st->dac_cache[chan->channel] = val;
> > -               mutex_unlock(&indio_dev->mlock);
> > +               mutex_unlock(&st->lock);
> >                 break;
> >         default:
> >                 ret = -EINVAL;
> > @@ -856,6 +858,7 @@ static int ad5064_probe(struct device *dev, enum ad5064_type type,
> >                 return  -ENOMEM;
> >
> >         st = iio_priv(indio_dev);
> > +       mutex_init(&st->lock);  
> 
> An equivalent `mutex_destroy()` call would be a good idea in ad5064_remove()
> 
Ah, this one..

mutex_destroy is only useful in finding use after free if mutex debugging is
turned on.  In my view it is of dubious benefit if it's getting called only
in remove, on the basis we should be pretty sure the driver won't take
the lock if we hit that path.

The downside is that it complicates the remove path because there is no devm
version of mutex_init to automate the cleanup.

I'll look at whether it makes sense here (i.e. if there is too much
cost to adding it).

> >         dev_set_drvdata(dev, indio_dev);
> >
> >         st->chip_info = &ad5064_chip_info_tbl[type];
> > --
> > 2.20.1
> >