On Mon, 18 Feb 2019 14:35:51 -0500
Sven Van Asbroeck <thesven73@xxxxxxxxx> wrote:
> Hi Jonathan,
>
> Thanks again for your clear and extensive feedback !
>
> On Mon, Feb 18, 2019 at 10:16 AM Jonathan Cameron
> <jonathan.cameron@xxxxxxxxxx> wrote:
> >
> > I suspect that would break lots of devices if it happened, but
> > fair enough that explicit might be good. One option would be
> > to document clearly in regmap the requirement that bulk read is ordered.
> >
>
> Yes, it would be interesting to hear the regmap people's opinion on ordering.
> In the mean time, we can make this explicit.
> Re-reading the thread, I can also see that Peter Meerwald-Stadler was first
> to spot this race condition.
>
> > What we need to guarantee is:
> >
> > 1) If the sensor reads on an occasion where the threshold is passed, we do not miss the event
> > The event is the threshold being passed, not the existence of the reading, or how many
> > readings etc.
> >
> > 2) A data read will result in a value. There is no guarantee that it will match with the
> > event. All manner of delays could result in new data having occurred before that read.
> >
>
> My feedback was based on two incorrect assumptions:
> a. the interrupt fires whenever new PS/ALS values become available (wrong)
> b. there are strict consistency guarantees between the THRESH event, and what
> userspace will read out (also wrong)
>
> Taking that into account, I am 100% in agreement with your other comments.
> Thank you so much for the explanation!
>
> There is one exception, though:
>
> > > +static int ap3216c_write_event_config(struct iio_dev *indio_dev,
> > > + const struct iio_chan_spec *chan,
> > > + enum iio_event_type type,
> > > + enum iio_event_direction dir, int state)
> > > +{
> > > + struct ap3216c_data *data = iio_priv(indio_dev);
> > > +
> > > + switch (chan->type) {
> > > + case IIO_LIGHT:
> > > + data->als_thresh_en = state;
> > > + return 0;
> > > +
> > > + case IIO_PROXIMITY:
> > > + data->prox_thresh_en = state;
> > > + return 0;
> > > +
> > > + default:
> > > + return -EINVAL;
> > > + }
> > > +static irqreturn_t ap3216c_event_handler(int irq, void *p)
> > > +{
> > > + if ((status & AP3216C_INT_STATUS_PS_MASK) && data->prox_thresh_en)
> > > + iio_push_event(...);
> > > +
> > >
> > > I think this may not work as intended. One thread (userspace) writes
> > > a variable, another thread (threaded irq handler) checks it. but there
> > > is no explicit or implicit memory barrier. So when userspace activates
> > > thresholding, it may take a long time for the handler to 'see' it !
> >
> > Yes. But if userspace took a while to get around to writing this value,
> > it would also take longer... It's not time critical exactly when you
> > enable the event. One can create cases where someone might
> > care, but they are pretty obscure.
> >
>
> Are you sure? I suspect that it's perfectly possible for the threaded irq
> handler not to 'see' the store to (als|prox)_thresh_en for a _very_ long time.
That is a serious - "in theory" circumstance. The moment we hit any path at
all that results in a memory barrier it will see it. Here its not critical
so we can wait. In this case this is triggered by a userspace write.
Looks to me like that happens (I haven't checked that thoroughly) via
kernfs_fops_write which takes a mutex - so we have a barrier.
There are of course cases where multiple concurrent in kernel actions need
to be protected and need a memory barrier, but this doesn't look like one
of those to me.
>
> AFAIK only a memory barrier will guarantee that the handler 'sees' the store
> right away. A lock will do - it issues an implicit memory barrier.
>
> Most drivers use a lock to guarantee visibility. There are a few drivers that
> resort to explicit barriers to make a flag visible from one thread to another.
That's misleading. Most drivers use a lock to protect state against concurrent
inconsistent writes. They don't take a lock because of it's memory barrier.
I have no objection to seeing one here as it's easier to know it is correct,
and the scope of lock can be nice and apparent.
>
> E.g. search for mb() or wmb() in:
> drivers/input/keyboard/matrix_keypad.c
> drivers/input/misc/cm109.c
> drivers/input/misc/yealink.c
