Re: [PATCH 14/15] iio: health: max30102: do not use internal iio_dev lock

["Nuno Sá" <noname.nuno@gmail.com>]

On Sun, 2022-10-02 at 12:08 +0100, Jonathan Cameron wrote:
> On Fri, 30 Sep 2022 12:04:39 +0200
> Nuno Sá <noname.nuno@gmail.com> wrote:
> 
> > On Sat, 2022-09-24 at 16:54 +0100, Jonathan Cameron wrote:
> > > On Tue, 20 Sep 2022 13:28:20 +0200
> > > Nuno Sá <nuno.sa@analog.com> wrote:
> > >   
> > > > The pattern used in this device does not quite fit in the
> > > > iio_device_claim_direct_mode() typical usage. In this case,
> > > > iio_buffer_enabled() was being used not to prevent the raw
> > > > access
> > > > but to decide whether or not the device needs to be powered on
> > > > before.
> > > > If buffering, then the device is already on. To guarantee the
> > > > same
> > > > behavior, a combination of locks is being used:
> > > > 
> > > > 1. Use iio_device_claim_direct_mode() to check if direct mode
> > > > can
> > > > be
> > > > claimed and if we can, then we keep it until the reading is
> > > > done
> > > > (which
> > > > also means the device will be powered on and off);
> > > > 2. If already buffering, we need to make sure that buffering is
> > > > not
> > > > disabled (and hence, powering off the device) while doing a raw
> > > > read. For
> > > > that, we can make use of the local lock that already exists.
> > > > 
> > > > Signed-off-by: Nuno Sá <nuno.sa@analog.com>  
> > > Obviously same dance in here as the related previous patch. So
> > > same
> > > solution
> > > needs adopting.  I just thought I'd reply to make sure we didn't
> > > forget to
> > > cover them both :)
> > > 
> > >   
> > Hi Jonathan,
> > 
> > So I was working on v2 in the morning and went with your
> > iio_device_claim_buffer_mode() approach... And bah, well it works
> > like
> > a charm in the previous patch, it fails in this one:
> > 
> > -               mutex_lock(&indio_dev->mlock);
> > -               if (!iio_buffer_enabled(indio_dev))
> > +               if (iio_device_claim_buffer_mode(indio_dev)) {
> >                         ret = max30102_get_temp(data, val, true);
> > -               else
> > +               } else {
> >                         ret = max30102_get_temp(data, val, false);
> > -               mutex_unlock(&indio_dev->mlock);
> > -               if (ret)
> > +                       iio_device_release_buffer_mode(indio_dev);
> > +               }
> > +               if(ret)
> >                         return ret;
> > 
> > 
> > Note that if we are not in buffered mode we won't get mlock and
> > call
> > max30102_get_temp(data, val, true) without any lock. While it's
> > very
> > unlikely for someone, in the meantime, to enable the buffer and
> > then
> > disable it, it's still racy and possible (at least in theory).
> 
> Ah. That's indeed tedious. I'd close the race by claiming direct mode
> for the else branch.  If that fails, pah, just fail the call with a
> suitable
> error return (-EAGAIN probably). 
> Or put a retry look around the whole thing to make it even less
> likely
> we'll hit the gap in the locking.
> 

Hmm I did thought about that but it looked very "dirty"... Anyways, I
can do it for v2 just so we have a look on how it looks like.

- Nuno Sá