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SubjectRe: [PATCH v4] iio: adc: max1363: replace uses of mlock
On Sun, 8 Mar 2020 02:32:56 +0530
Rohit Sarkar <rohitsarkar5398@gmail.com> wrote:

> On Sat, Mar 07, 2020 at 02:19:46PM +0000, Jonathan Cameron wrote:
> > On Sat, 7 Mar 2020 13:34:51 +0530
> > Rohit Sarkar <rohitsarkar5398@gmail.com> wrote:
> >
> > > Replace usage indio_dev's mlock with either local lock or
> > > iio_device_claim_direct_mode.
> > >
> > > Signed-off-by: Rohit Sarkar <rohitsarkar5398@gmail.com>
> >
> > There is a subtlety in here (which is why this one never
> > got cleaned up before). We need to protect against:
> >
> > 1) Driver state being accessed from multiple places concurrently.
> > That will use your new lock.
> > 2) Doing actions that cannot occur if in buffered mode. The
> > claim_direct_mode stuff is for that.
> I did consider using both, the local driver lock and the claim_direct in
> some places, however I noticed that the claim_direct_mode internally uses
> the mlock, hence I didnt think it was necessary to set the local lock as
> well, as according to my understanding once a process acquires the mlock
> no other process can run the critical section before the initial process
> releases the mlock. Thus the driver state also remains consistent.

Any state changes in the driver done under the local lock can still happen.
There is also a question of 'obviousness'. The driver code should not
'care' what the internals of claim_direct_mode is doing.
That can be expected to protect against moving out of direct mode, but
not anything about 'how'.

Hence, take them both.

>
> > 3) Prevent us enabling monitor mode when in buffered mode, or
> > buffered mode when in monitor mode (because it's really
> > fiddly to support both given the odd way this hardware
> > works and I never bothered). That requires making sure
> > direct mode is claimed before trying to enable the monitor
> > mode and also that we don't enable buffered mode. Now interestingly
> > there doesn't actually seem to be anything stopping buffered mode
> > if monitor mode is on. Probably a bug, but I'd be nervous about
> > fixing that without test hardware.
> >
> > Ignoring that last subtle bit, we sometimes for 1 and 2 need to take
> > both locks. We can't be in buffered mode and we need to edit
> > local state.
> >
> > Jonathan
> >
> > > ---
> >
> > That's better :) I should have looked at v4 before v3 I guess.
> >
> Forgot to add the changelog in v3 :) . Should have mentioned that
> explicitly, my bad!
> > > Changelog v3 -> v4
> > > * Fix indentation
> > >
> > > Changelog v2 -> v3
> > > * use iio_device_claim_direct when switching modes
> > > * replace mlock usage in max1363_write_event_config
> > >
> > > Changelog v1 -> v2
> > > * Fix indentation
> > >
> > > drivers/iio/adc/max1363.c | 24 +++++++++++++++---------
> > > 1 file changed, 15 insertions(+), 9 deletions(-)
> > >
> > > diff --git a/drivers/iio/adc/max1363.c b/drivers/iio/adc/max1363.c
> > > index 5c2cc61b666e..a1550c0b4c0a 100644
> > > --- a/drivers/iio/adc/max1363.c
> > > +++ b/drivers/iio/adc/max1363.c
> > > @@ -150,6 +150,7 @@ struct max1363_chip_info {
> > > * @current_mode: the scan mode of this chip
> > > * @requestedmask: a valid requested set of channels
> > > * @reg: supply regulator
> > > + * @lock lock to ensure state is consistent
> > > * @monitor_on: whether monitor mode is enabled
> > > * @monitor_speed: parameter corresponding to device monitor speed setting
> > > * @mask_high: bitmask for enabled high thresholds
> > > @@ -169,6 +170,7 @@ struct max1363_state {
> > > const struct max1363_mode *current_mode;
> > > u32 requestedmask;
> > > struct regulator *reg;
> > > + struct mutex lock;
> > >
> > > /* Using monitor modes and buffer at the same time is
> > > currently not supported */
> > > @@ -364,7 +366,9 @@ static int max1363_read_single_chan(struct iio_dev *indio_dev,
> > > struct max1363_state *st = iio_priv(indio_dev);
> > > struct i2c_client *client = st->client;
> > >
> > > - mutex_lock(&indio_dev->mlock);
> > > + ret = iio_device_claim_direct_mode(indio_dev);
> >
> > So this protects against change of state from polled to buffered mode.
> > Great. However, we also use state in here which should be protected
> > by the local lock. The solution is to take that local lock as well.
> >
> > > + if (ret < 0)
> > > + goto error_ret;
> > > /*
> > > * If monitor mode is enabled, the method for reading a single
> > > * channel will have to be rather different and has not yet
> > > @@ -372,7 +376,7 @@ static int max1363_read_single_chan(struct iio_dev *indio_dev,
> > > *
> > > * Also, cannot read directly if buffered capture enabled.
> > > */
> > > - if (st->monitor_on || iio_buffer_enabled(indio_dev)) {
> > > + if (st->monitor_on) {
> > > ret = -EBUSY;
> > > goto error_ret;
> > > }
> > > @@ -404,8 +408,9 @@ static int max1363_read_single_chan(struct iio_dev *indio_dev,
> > > data = rxbuf[0];
> > > }
> > > *val = data;
> > > +
> > > error_ret:
> > > - mutex_unlock(&indio_dev->mlock);
> > > + iio_device_release_direct_mode(indio_dev);
> > > return ret;
> > >
> > > }
> > > @@ -705,9 +710,9 @@ static ssize_t max1363_monitor_store_freq(struct device *dev,
> > > if (!found)
> > > return -EINVAL;
> > >
> > > - mutex_lock(&indio_dev->mlock);
> > > + mutex_lock(&st->lock);
> > > st->monitor_speed = i;
> > > - mutex_unlock(&indio_dev->mlock);
> > > + mutex_unlock(&st->lock);
> > >
> > > return 0;
> > > }
> > > @@ -810,12 +815,12 @@ static int max1363_read_event_config(struct iio_dev *indio_dev,
> > > int val;
> > > int number = chan->channel;
> > >
> > > - mutex_lock(&indio_dev->mlock);
> > > + mutex_lock(&st->lock);
> > > if (dir == IIO_EV_DIR_FALLING)
> > > val = (1 << number) & st->mask_low;
> > > else
> > > val = (1 << number) & st->mask_high;
> > > - mutex_unlock(&indio_dev->mlock);
> > > + mutex_unlock(&st->lock);
> > >
> > > return val;
> > > }
> > > @@ -962,7 +967,7 @@ static int max1363_write_event_config(struct iio_dev *indio_dev,
> > > u16 unifiedmask;
> > > int number = chan->channel;
> > >
> > > - mutex_lock(&indio_dev->mlock);
> > > + iio_device_claim_direct_mode(indio_dev);
> >
> > So the question is whether we are stopping this changing because
> > we are in buffered mode or because it's local state...
> >
> > > unifiedmask = st->mask_low | st->mask_high;
> > > if (dir == IIO_EV_DIR_FALLING) {
> > >
> > > @@ -989,7 +994,7 @@ static int max1363_write_event_config(struct iio_dev *indio_dev,
> > >
> > > max1363_monitor_mode_update(st, !!(st->mask_high | st->mask_low));
> > > error_ret:
> > > - mutex_unlock(&indio_dev->mlock);
> > > + iio_device_release_direct_mode(indio_dev);
> > >
> > > return ret;
> > > }
> > > @@ -1587,6 +1592,7 @@ static int max1363_probe(struct i2c_client *client,
> > >
> > > st = iio_priv(indio_dev);
> > >
> > > + mutex_init(&st->lock);
> > > st->reg = devm_regulator_get(&client->dev, "vcc");
> > > if (IS_ERR(st->reg)) {
> > > ret = PTR_ERR(st->reg);
> >

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