Re: [RFC PATCH 1/8] iio: locking: introduce __cleanup() based direct mode claiming infrastructure

["Nuno Sá" <noname.nuno@gmail.com>]

On Mon, 2023-10-23 at 15:34 +0100, Jonathan Cameron wrote:
> On Mon, 23 Oct 2023 13:51:04 +0200
> Nuno Sá <noname.nuno@gmail.com> wrote:
> 
> > On Mon, 2023-10-23 at 10:53 +0100, Jonathan Cameron wrote:
> > > On Mon, 23 Oct 2023 10:55:56 +0200
> > > Nuno Sá <noname.nuno@gmail.com> wrote:
> > >   
> > > > On Sun, 2023-10-22 at 16:10 -0500, David Lechner wrote:  
> > > > > On Sun, Oct 22, 2023 at 10:47 AM Jonathan Cameron <jic23@kernel.org>
> > > > > wrote:    
> > > > > > 
> > > > > > From: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > > > > > 
> > > > > > Allows use of:
> > > > > > 
> > > > > >         CLASS(iio_claim_direct, claimed_dev)(indio_dev);
> > > > > >         if (IS_ERR(claimed_dev))
> > > > > >                 return PTR_ERR(claimed_dev);
> > > > > > 
> > > > > >         st = iio_priv(claimed_dev);
> > > > > > 
> > > > > > to automatically call iio_device_release_direct_mode() based on
> > > > > > scope.
> > > > > > Typically seen in combination with local device specific locks which
> > > > > > are already have automated cleanup options via guard(mutex)(&st-
> > > > > > >lock)
> > > > > > and scoped_guard().  Using both together allows most error handling
> > > > > > to
> > > > > > be automated.
> > > > > > 
> > > > > > Note that whilst this pattern results in a struct iio_dev
> > > > > > *claimed_dev
> > > > > > that can be used, it is not necessary to do so as long as that
> > > > > > pointer
> > > > > > has been checked for errors as in the example.
> > > > > > 
> > > > > > Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
> > > > > > ---
> > > > > >  drivers/iio/industrialio-core.c |  4 ++++
> > > > > >  include/linux/iio/iio.h         | 25 +++++++++++++++++++++++++
> > > > > >  2 files changed, 29 insertions(+)
> > > > > > 
> > > > > > diff --git a/drivers/iio/industrialio-core.c
> > > > > > b/drivers/iio/industrialio-
> > > > > > core.c
> > > > > > index c77745b594bd..93bfad105eb5 100644
> > > > > > --- a/drivers/iio/industrialio-core.c
> > > > > > +++ b/drivers/iio/industrialio-core.c
> > > > > > @@ -2065,6 +2065,10 @@
> > > > > > EXPORT_SYMBOL_GPL(iio_device_claim_direct_mode);
> > > > > >   */
> > > > > >  void iio_device_release_direct_mode(struct iio_dev *indio_dev)
> > > > > >  {
> > > > > > +       /* Auto cleanup can result in this being called with an
> > > > > > ERR_PTR
> > > > > > */
> > > > > > +       if (IS_ERR(indio_dev))
> > > > > > +               return;
> > > > > > +
> > > > > >         mutex_unlock(&to_iio_dev_opaque(indio_dev)->mlock);
> > > > > >  }
> > > > > >  EXPORT_SYMBOL_GPL(iio_device_release_direct_mode);
> > > > > > diff --git a/include/linux/iio/iio.h b/include/linux/iio/iio.h
> > > > > > index d0ce3b71106a..11c42170fda1 100644
> > > > > > --- a/include/linux/iio/iio.h
> > > > > > +++ b/include/linux/iio/iio.h
> > > > > > @@ -9,6 +9,7 @@
> > > > > > 
> > > > > >  #include <linux/device.h>
> > > > > >  #include <linux/cdev.h>
> > > > > > +#include <linux/cleanup.h>
> > > > > >  #include <linux/slab.h>
> > > > > >  #include <linux/iio/types.h>
> > > > > >  /* IIO TODO LIST */
> > > > > > @@ -644,6 +645,30 @@ int __devm_iio_device_register(struct device
> > > > > > *dev,
> > > > > > struct iio_dev *indio_dev,
> > > > > >  int iio_push_event(struct iio_dev *indio_dev, u64 ev_code, s64
> > > > > > timestamp);
> > > > > >  int iio_device_claim_direct_mode(struct iio_dev *indio_dev);
> > > > > >  void iio_device_release_direct_mode(struct iio_dev *indio_dev);
> > > > > > +/*
> > > > > > + * Auto cleanup version of iio_device_claim_direct_mode,
> > > > > > + *
> > > > > > + *     CLASS(iio_claim_direct, claimed_dev)(indio_dev);
> > > > > > + *     if (IS_ERR(claimed_dev))
> > > > > > + *             return PTR_ERR(claimed_dev);
> > > > > > + *
> > > > > > + *     st = iio_priv(claimed_dev);
> > > > > > + *     ....
> > > > > > + */
> > > > > > +DEFINE_CLASS(iio_claim_direct, struct iio_dev *,
> > > > > > +            iio_device_release_direct_mode(_T),
> > > > > > +            ({
> > > > > > +                       struct iio_dev *dev;
> > > > > > +                       int d = iio_device_claim_direct_mode(_T);
> > > > > > +
> > > > > > +                       if (d < 0)
> > > > > > +                               dev = ERR_PTR(d);
> > > > > > +                       else
> > > > > > +                               dev = _T;
> > > > > > +                       dev;
> > > > > > +            }),
> > > > > > +            struct iio_dev *_T);
> > > > > > +
> > > > > >  int iio_device_claim_buffer_mode(struct iio_dev *indio_dev);
> > > > > >  void iio_device_release_buffer_mode(struct iio_dev *indio_dev);
> > > > > > 
> > > > > > --
> > > > > > 2.42.0
> > > > > >     
> > > > > 
> > > > > What is the benefit of exposing `claimed_dev` rather than just the int
> > > > > return value? It seems like it just makes more noise in the error
> > > > > check.
> > > > >     
> > > > 
> > > > I don't really have a very strong opinion on this but what I really
> > > > don't
> > > > like
> > > > much is the pattern:
> > > > 
> > > > CLASS(type, ret), where the return value is an argument of the macro...
> > > > It
> > > > would
> > > > be nice if we could just make it like:
> > > > 
> > > > ret = guard(type)(...); //or any other variation of the guard() macro
> > > > if (ret) 
> > > > 	return ret;
> > > > 
> > > > the above could also be an error pointer or even have one variation of
> > > > each.
> > > > but
> > > > yeah, that likely means changing the cleanup.h file and that might be
> > > > out of
> > > > scope for Jonathan's patch series. 
> > > >   
> > > 
> > > I fully agree it's ugly and a little unintuitive but I don't see a way an
> > > "lvalue"
> > > can work work cleanly (due to magic types under the hood) and I suspect we
> > > will
> > > have to get used to this pattern.
> > >   
> > 
> > Yeah, given the games being played with the constructor and the _lock
> > definition
> > so we return the variable we want to "release" I agree it would be hard to
> > have
> > anything clean and likely even harder to read (more than it is already :)).
> > 
> > However, I think users of the cleanup.h stuff could build on top of it...
> > For
> > instance, in our case we could have something like:
> > 
> > #define IIO_CLAIM_DIRECT(dev) 
> > 	int __ret = 0;
> > 	CLASS(iio_claim_direct, claimed_dev)(dev);
> > 	if ((IS_ERR(claimed_dev))
> > 		__ret = PTR_ERR(claimed_dev);
> > 	__ret
> 
> Maybe, but we'll have to deal with people perpetually trying to brackets
> around
> the complex macro... 
> 
> 

Not sure what you mean here... you mean dealing with people coming up with funny
new macros around CLASS(). In IIO, this is very specific and If I'm not missing
anything the obvious, the above macro with give the same usage as 
iio_device_claim_direct_mode() but without caring about release() - so not sure
people could be that creative :).

Anyways, as I started to say in my first reply, I don't feel strong about this
at all, so feel free to add:

Reviewed-by: Nuno Sa <nuno.sa@analog.com>