[v5,2/5] iio: consumers: copy/release available info from producer to fix race

Commit Message

Matteo Martelli Oct. 21, 2024, 12:54 p.m. UTC

Consumers need to call the producer's read_avail_release_resource()
callback after reading producer's available info. To avoid a race
condition with the producer unregistration, change inkern
iio_channel_read_avail() so that it copies the available info from the
producer and immediately calls its release callback with info_exists
locked.

Also, modify the users of iio_read_avail_channel_raw() and
iio_read_avail_channel_attribute() to free the copied available buffers
after calling these functions. To let users free the copied buffer with
a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
consumer helper that is equivalent to iio_read_avail_channel_attribute()
but stores the available values in the returned variable.

Signed-off-by: Matteo Martelli <[e-mail]>
---
 drivers/iio/afe/iio-rescale.c          |  8 +++
 drivers/iio/dac/dpot-dac.c             |  8 +++
 drivers/iio/inkern.c                   | 99 ++++++++++++++++++++++++++++------
 drivers/iio/multiplexer/iio-mux.c      |  8 +++
 drivers/power/supply/ingenic-battery.c | 22 +++++---
 include/linux/iio/consumer.h           | 28 +++++++++-
 6 files changed, 147 insertions(+), 26 deletions(-)

Comments

Andy Shevchenko Oct. 30, 2024, 2:47 p.m. UTC | #1

On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:
> Consumers need to call the producer's read_avail_release_resource()
> callback after reading producer's available info. To avoid a race
> condition with the producer unregistration, change inkern
> iio_channel_read_avail() so that it copies the available info from the
> producer and immediately calls its release callback with info_exists
> locked.
> 
> Also, modify the users of iio_read_avail_channel_raw() and
> iio_read_avail_channel_attribute() to free the copied available buffers
> after calling these functions. To let users free the copied buffer with
> a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> consumer helper that is equivalent to iio_read_avail_channel_attribute()
> but stores the available values in the returned variable.

...

> +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> +					    struct iio_chan_spec const *chan,
> +					    const int *vals, long mask)
> +{
> +	kfree(vals);
> +}
> +
>  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
>  			      struct iio_chan_spec const *chan,
>  			      int val, int val2, long mask)
> @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
>  static const struct iio_info dpot_dac_info = {
>  	.read_raw = dpot_dac_read_raw,
>  	.read_avail = dpot_dac_read_avail,
> +	.read_avail_release_resource = dpot_dac_read_avail_release_res,
>  	.write_raw = dpot_dac_write_raw,
>  };

I have a problem with this approach. The issue is that we allocate
memory in one place and must clear it in another. This is not well
designed thingy in my opinion. I was thinking a bit of the solution and
at least these two comes to my mind:

1) having a special callback for .read_avail_with_copy (choose better
name) that will dump the data to the intermediate buffer and clean it
after all;

2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.

In any case it looks fragile and not scalable. I propose to drop this
and think again.

Yes, yes, I'm fully aware about the problem you are trying to solve and
agree on the report, I think this solution is not good enough.

Jonathan Cameron Oct. 30, 2024, 5:35 p.m. UTC | #2

On Wed, 30 Oct 2024 16:47:50 +0200
Andy Shevchenko <[e-mail]> wrote:

> On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:
> > Consumers need to call the producer's read_avail_release_resource()
> > callback after reading producer's available info. To avoid a race
> > condition with the producer unregistration, change inkern
> > iio_channel_read_avail() so that it copies the available info from the
> > producer and immediately calls its release callback with info_exists
> > locked.
> > 
> > Also, modify the users of iio_read_avail_channel_raw() and
> > iio_read_avail_channel_attribute() to free the copied available buffers
> > after calling these functions. To let users free the copied buffer with
> > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > but stores the available values in the returned variable.  
> 
> ...
> 
> > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > +					    struct iio_chan_spec const *chan,
> > +					    const int *vals, long mask)
> > +{
> > +	kfree(vals);
> > +}
> > +
> >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> >  			      struct iio_chan_spec const *chan,
> >  			      int val, int val2, long mask)
> > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> >  static const struct iio_info dpot_dac_info = {
> >  	.read_raw = dpot_dac_read_raw,
> >  	.read_avail = dpot_dac_read_avail,
> > +	.read_avail_release_resource = dpot_dac_read_avail_release_res,
> >  	.write_raw = dpot_dac_write_raw,
> >  };  
> 
> I have a problem with this approach. The issue is that we allocate
> memory in one place and must clear it in another. This is not well
> designed thingy in my opinion.

It is a tricky corner and we've not yet come up with a better
solution :(  I think one of the earlier versions did just always copy
and the reviews suggested that was painful given we are fixing a tiny
percentage of devices.  Hence we ended up with what is effectively
an optional copy if the provider knows the data is volatile.

So there are two 'potential' copies here and we need to be careful
to separate them for purposes of discussion.
A) Copy in provider if it has volatile available data.  In that
   case the copy is done in a call to it via read_avail, and release
   via a call to read_avail_release_resource().  So to my mind locally
   the same as any acquire / release pair.
B) Copy in the core for the case where we need the lifetime to persist.
   That is a effectively a kmemdup() call so we could call back to the
   core to release it but it would just be a kfree() wrapper.
 
(A) Only occurs in a tiny subset of drivers, most use non volatile data
    for read avail (constant, or constant after probe).
(B) Only occurs for consumer drivers that directly use the avail data.
    There are very few of those and no other sane way of solving this
    because we can't hold a lock into the provider for an unknown
    (long) time.


> I was thinking a bit of the solution and
> at least these two comes to my mind:
> 
> 1) having a special callback for .read_avail_with_copy (choose better
> name) that will dump the data to the intermediate buffer and clean it
> after all;

So we have that allocate the data in the provider and hand it to the
consumer which then frees it with kfree() in all cases?

Note that's what we do for the inkern interfaces (the ones consumer
drivers have to use), just in the core not the providers because that
corner is hard to close any other way. In this rare case we end up
potentially copying twice.

For the special cases where the buffer isn't passed on beyond
functions that are part of the IIO core, we avoid the need for that
(potentially second, probably only) copy because we can always
ensure the release call is made.  Note this is the common case
by far. It's the pretty printing done by the core to present this
data to sysfs etc, or to find out the max value for some consumer
that doesn't need the whole set.

> 
> 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.

The special handling that will need seems likely to be no more
obvious than the handling we have here.  I'm not really
sure how it would work.

> 
> In any case it looks fragile and not scalable. I propose to drop this
> and think again.
> 
> Yes, yes, I'm fully aware about the problem you are trying to solve and
> agree on the report, I think this solution is not good enough.

I'll back this out of my tree for now so the discussion can carry on.

Jonathan

Matteo Martelli Oct. 30, 2024, 6:23 p.m. UTC | #3

Quoting Andy Shevchenko (2024-10-30 15:47:50)
> On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:
> > Consumers need to call the producer's read_avail_release_resource()
> > callback after reading producer's available info. To avoid a race
> > condition with the producer unregistration, change inkern
> > iio_channel_read_avail() so that it copies the available info from the
> > producer and immediately calls its release callback with info_exists
> > locked.
> > 
> > Also, modify the users of iio_read_avail_channel_raw() and
> > iio_read_avail_channel_attribute() to free the copied available buffers
> > after calling these functions. To let users free the copied buffer with
> > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > but stores the available values in the returned variable.
> 
> ...
> 
> > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > +                                         struct iio_chan_spec const *chan,
> > +                                         const int *vals, long mask)
> > +{
> > +     kfree(vals);
> > +}
> > +
> >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> >                             struct iio_chan_spec const *chan,
> >                             int val, int val2, long mask)
> > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> >  static const struct iio_info dpot_dac_info = {
> >       .read_raw = dpot_dac_read_raw,
> >       .read_avail = dpot_dac_read_avail,
> > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> >       .write_raw = dpot_dac_write_raw,
> >  };
> 
> I have a problem with this approach. The issue is that we allocate
> memory in one place and must clear it in another. This is not well
> designed thingy in my opinion. I was thinking a bit of the solution and
> at least these two comes to my mind:
> 
> 1) having a special callback for .read_avail_with_copy (choose better
> name) that will dump the data to the intermediate buffer and clean it
> after all;
> 
> 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.

Could you elaborate more about these potential solutions? Maybe with some
usage examples?

If I get it correctly, in both cases you are suggesting to pass ownership
of the vals buffer to the caller, iio_read_channel_info_avail() in this
case, so that it would take care of freeing the buffer after calling
iio_format_after_*(). We considered this approach during an initial
discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
to let the driver keep the release control through a callback for two
reasons:

1) Apparently it's a bad pattern to pass the buffer ownership to the core,
   maybe Jonathan can elaborate why? The risk I can think of is that the driver
   could still keep the buffer copy in its private data after giving it away,
   resulting in fact in a double ownership. However I think it would be clear
   enough in this case that the copy should be handled by the caller, or maybe
   not?

2) Some driver might want to avoid allocating a new copy of a big table if
   the race does not occur (e.g. with additional checks on buffer access
   code) and thus wouldn't call a free() in the release callback.

> 
> In any case it looks fragile and not scalable. I propose to drop this
> and think again.

I see your concerns, I am open to reconsider this in case we come up with
better solution after addressing the points above.

> Yes, yes, I'm fully aware about the problem you are trying to solve and
> agree on the report, I think this solution is not good enough.
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 

[1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/

Best regards,
Matteo Martelli

Jonathan Cameron Oct. 30, 2024, 8:30 p.m. UTC | #4

On Wed, 30 Oct 2024 19:23:21 +0100
Matteo Martelli <[e-mail]> wrote:

> Quoting Andy Shevchenko (2024-10-30 15:47:50)
> > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:  
> > > Consumers need to call the producer's read_avail_release_resource()
> > > callback after reading producer's available info. To avoid a race
> > > condition with the producer unregistration, change inkern
> > > iio_channel_read_avail() so that it copies the available info from the
> > > producer and immediately calls its release callback with info_exists
> > > locked.
> > > 
> > > Also, modify the users of iio_read_avail_channel_raw() and
> > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > after calling these functions. To let users free the copied buffer with
> > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > but stores the available values in the returned variable.  
> > 
> > ...
> >   
> > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > +                                         struct iio_chan_spec const *chan,
> > > +                                         const int *vals, long mask)
> > > +{
> > > +     kfree(vals);
> > > +}
> > > +
> > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > >                             struct iio_chan_spec const *chan,
> > >                             int val, int val2, long mask)
> > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > >  static const struct iio_info dpot_dac_info = {
> > >       .read_raw = dpot_dac_read_raw,
> > >       .read_avail = dpot_dac_read_avail,
> > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > >       .write_raw = dpot_dac_write_raw,
> > >  };  
> > 
> > I have a problem with this approach. The issue is that we allocate
> > memory in one place and must clear it in another. This is not well
> > designed thingy in my opinion. I was thinking a bit of the solution and
> > at least these two comes to my mind:
> > 
> > 1) having a special callback for .read_avail_with_copy (choose better
> > name) that will dump the data to the intermediate buffer and clean it
> > after all;
> > 
> > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.  
> 
> Could you elaborate more about these potential solutions? Maybe with some
> usage examples?
> 
> If I get it correctly, in both cases you are suggesting to pass ownership
> of the vals buffer to the caller, iio_read_channel_info_avail() in this
> case, so that it would take care of freeing the buffer after calling
> iio_format_after_*(). We considered this approach during an initial
> discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> to let the driver keep the release control through a callback for two
> reasons:
> 
> 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
>    maybe Jonathan can elaborate why? The risk I can think of is that the driver
>    could still keep the buffer copy in its private data after giving it away,
>    resulting in fact in a double ownership. However I think it would be clear
>    enough in this case that the copy should be handled by the caller, or maybe
>    not?
Mostly the lack of desire to have to copy for the 95% of cases where it's
not needed and that it prevents any optimization like you mention.

Jonathan
> 
> 2) Some driver might want to avoid allocating a new copy of a big table if
>    the race does not occur (e.g. with additional checks on buffer access
>    code) and thus wouldn't call a free() in the release callback.
> 
> > 
> > In any case it looks fragile and not scalable. I propose to drop this
> > and think again.  
> 
> I see your concerns, I am open to reconsider this in case we come up with
> better solution after addressing the points above.
> 
> > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > agree on the report, I think this solution is not good enough.
> > 
> > -- 
> > With Best Regards,
> > Andy Shevchenko
> >   
> 
> [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/
> 
> Best regards,
> Matteo Martelli

Matteo Martelli Oct. 31, 2024, 11:26 a.m. UTC | #5

Quoting Jonathan Cameron (2024-10-30 21:30:50)
> On Wed, 30 Oct 2024 19:23:21 +0100
> Matteo Martelli <[e-mail]> wrote:
> 
> > Quoting Andy Shevchenko (2024-10-30 15:47:50)
> > > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:  
> > > > Consumers need to call the producer's read_avail_release_resource()
> > > > callback after reading producer's available info. To avoid a race
> > > > condition with the producer unregistration, change inkern
> > > > iio_channel_read_avail() so that it copies the available info from the
> > > > producer and immediately calls its release callback with info_exists
> > > > locked.
> > > > 
> > > > Also, modify the users of iio_read_avail_channel_raw() and
> > > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > > after calling these functions. To let users free the copied buffer with
> > > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > > but stores the available values in the returned variable.  
> > > 
> > > ...
> > >   
> > > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > > +                                         struct iio_chan_spec const *chan,
> > > > +                                         const int *vals, long mask)
> > > > +{
> > > > +     kfree(vals);
> > > > +}
> > > > +
> > > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > >                             struct iio_chan_spec const *chan,
> > > >                             int val, int val2, long mask)
> > > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > >  static const struct iio_info dpot_dac_info = {
> > > >       .read_raw = dpot_dac_read_raw,
> > > >       .read_avail = dpot_dac_read_avail,
> > > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > >       .write_raw = dpot_dac_write_raw,
> > > >  };  
> > > 
> > > I have a problem with this approach. The issue is that we allocate
> > > memory in one place and must clear it in another. This is not well
> > > designed thingy in my opinion. I was thinking a bit of the solution and
> > > at least these two comes to my mind:
> > > 
> > > 1) having a special callback for .read_avail_with_copy (choose better
> > > name) that will dump the data to the intermediate buffer and clean it
> > > after all;
> > > 
> > > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.  
> > 
> > Could you elaborate more about these potential solutions? Maybe with some
> > usage examples?
> > 
> > If I get it correctly, in both cases you are suggesting to pass ownership
> > of the vals buffer to the caller, iio_read_channel_info_avail() in this
> > case, so that it would take care of freeing the buffer after calling
> > iio_format_after_*(). We considered this approach during an initial
> > discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> > to let the driver keep the release control through a callback for two
> > reasons:
> > 
> > 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
> >    maybe Jonathan can elaborate why? The risk I can think of is that the driver
> >    could still keep the buffer copy in its private data after giving it away,
> >    resulting in fact in a double ownership. However I think it would be clear
> >    enough in this case that the copy should be handled by the caller, or maybe
> >    not?
> Mostly the lack of desire to have to copy for the 95% of cases where it's
> not needed and that it prevents any optimization like you mention.

I think the suggestion here is to add an additional .read_avail_with_copy()
without replacing the original .read_avail(), so all the current drivers that
use a constant avail list would not be affected. And I think this was the same
idea for the additional read_avail_ext() or the additional argument for the
read_avail() we were considering in [1]. So I would think that
iio_read_channel_info_avail() would do something like the following:

    if (indio_dev->info->read_avail_with_copy)
        indio_dev->info->read_avail_with_copy(vals);
    else
        indio_dev->info->read_avail(vals);

    ...
    iio_format_avail_list(vals);
    ...

    if (indio_dev->info->read_avail_with_copy)
        kfree(vals);

And the drivers would choose whether to define the read_avail or the
read_avail_with_copy.

What I was referring to is that, back then, you mentioned you would have
preferred to avoid passing ownership of the buffer around:

> That's a corner case we should think about closing. Would require an indicator
> to read_avail that the buffer it has been passed is a snapshot that it should
> free on completion of the string building.  I don't like passing ownership
> of data around like that, but it is fiddly to do anything else given
> any simple double buffering is subject to race conditions.

I guess there is some other reason other than avoiding the copy when not
necessary, since by introducing an additional function or argument or return
type, most of the unnecessary copies would already be avoided right?

Anyway any of this solutions would still prevent the potential optimizations of
point 2). It's worth mentioning that those kind of optimizations are currently
not adopted by any driver.

> 
> Jonathan
> > 
> > 2) Some driver might want to avoid allocating a new copy of a big table if
> >    the race does not occur (e.g. with additional checks on buffer access
> >    code) and thus wouldn't call a free() in the release callback.
> > 
> > > 
> > > In any case it looks fragile and not scalable. I propose to drop this
> > > and think again.  
> > 
> > I see your concerns, I am open to reconsider this in case we come up with
> > better solution after addressing the points above.
> > 
> > > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > > agree on the report, I think this solution is not good enough.
> > > 
> > > -- 
> > > With Best Regards,
> > > Andy Shevchenko
> > >   
> > 
> > [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/
> > 
> > Best regards,
> > Matteo Martelli
> 

I hope I've brought a little more clarity to the discussion by providing some
history instead of making it more confusing.

Best regards,
Matteo Martelli

Jonathan Cameron Oct. 31, 2024, 2:31 p.m. UTC | #6

On Thu, 31 Oct 2024 12:26:24 +0100
Matteo Martelli <[e-mail]> wrote:

> Quoting Jonathan Cameron (2024-10-30 21:30:50)
> > On Wed, 30 Oct 2024 19:23:21 +0100
> > Matteo Martelli <[e-mail]> wrote:
> >   
> > > Quoting Andy Shevchenko (2024-10-30 15:47:50)  
> > > > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:    
> > > > > Consumers need to call the producer's read_avail_release_resource()
> > > > > callback after reading producer's available info. To avoid a race
> > > > > condition with the producer unregistration, change inkern
> > > > > iio_channel_read_avail() so that it copies the available info from the
> > > > > producer and immediately calls its release callback with info_exists
> > > > > locked.
> > > > > 
> > > > > Also, modify the users of iio_read_avail_channel_raw() and
> > > > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > > > after calling these functions. To let users free the copied buffer with
> > > > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > > > but stores the available values in the returned variable.    
> > > > 
> > > > ...
> > > >     
> > > > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > > > +                                         struct iio_chan_spec const *chan,
> > > > > +                                         const int *vals, long mask)
> > > > > +{
> > > > > +     kfree(vals);
> > > > > +}
> > > > > +
> > > > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > >                             struct iio_chan_spec const *chan,
> > > > >                             int val, int val2, long mask)
> > > > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > >  static const struct iio_info dpot_dac_info = {
> > > > >       .read_raw = dpot_dac_read_raw,
> > > > >       .read_avail = dpot_dac_read_avail,
> > > > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > > >       .write_raw = dpot_dac_write_raw,
> > > > >  };    
> > > > 
> > > > I have a problem with this approach. The issue is that we allocate
> > > > memory in one place and must clear it in another. This is not well
> > > > designed thingy in my opinion. I was thinking a bit of the solution and
> > > > at least these two comes to my mind:
> > > > 
> > > > 1) having a special callback for .read_avail_with_copy (choose better
> > > > name) that will dump the data to the intermediate buffer and clean it
> > > > after all;
> > > > 
> > > > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.    
> > > 
> > > Could you elaborate more about these potential solutions? Maybe with some
> > > usage examples?
> > > 
> > > If I get it correctly, in both cases you are suggesting to pass ownership
> > > of the vals buffer to the caller, iio_read_channel_info_avail() in this
> > > case, so that it would take care of freeing the buffer after calling
> > > iio_format_after_*(). We considered this approach during an initial
> > > discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> > > to let the driver keep the release control through a callback for two
> > > reasons:
> > > 
> > > 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
> > >    maybe Jonathan can elaborate why? The risk I can think of is that the driver
> > >    could still keep the buffer copy in its private data after giving it away,
> > >    resulting in fact in a double ownership. However I think it would be clear
> > >    enough in this case that the copy should be handled by the caller, or maybe
> > >    not?  
> > Mostly the lack of desire to have to copy for the 95% of cases where it's
> > not needed and that it prevents any optimization like you mention.  
> 
> I think the suggestion here is to add an additional .read_avail_with_copy()
> without replacing the original .read_avail(), so all the current drivers that
> use a constant avail list would not be affected. And I think this was the same
> idea for the additional read_avail_ext() or the additional argument for the
> read_avail() we were considering in [1]. So I would think that
> iio_read_channel_info_avail() would do something like the following:
> 
>     if (indio_dev->info->read_avail_with_copy)
>         indio_dev->info->read_avail_with_copy(vals);
>     else
>         indio_dev->info->read_avail(vals);
> 
>     ...
>     iio_format_avail_list(vals);
>     ...
> 
>     if (indio_dev->info->read_avail_with_copy)
>         kfree(vals);

Ok, sure that would work, but...

I don't really see this as being much less fragile than
the existing solution + in cases that we do have where
only some available are not const we will have to copy them
all.

If anything it's more complex than making it a driver problem
to provide the release call however it wants to do it.
 

> 
> And the drivers would choose whether to define the read_avail or the
> read_avail_with_copy.
> 
> What I was referring to is that, back then, you mentioned you would have
> preferred to avoid passing ownership of the buffer around:
> 
> > That's a corner case we should think about closing. Would require an indicator
> > to read_avail that the buffer it has been passed is a snapshot that it should
> > free on completion of the string building.  I don't like passing ownership
> > of data around like that, but it is fiddly to do anything else given
> > any simple double buffering is subject to race conditions.  
> 
> I guess there is some other reason other than avoiding the copy when not
> necessary, since by introducing an additional function or argument or return
> type, most of the unnecessary copies would already be avoided right?

It's not a strong reason beyond limiting scope of clever design +
the key bit my mind is that the above is not substantially simpler and
reduces our flexibility.

> 
> Anyway any of this solutions would still prevent the potential optimizations of
> point 2). It's worth mentioning that those kind of optimizations are currently
> not adopted by any driver.

That one indeed not, but mixing dynamic and non dynamic is something
you do in your pac1921 patch.

Jonathan


> 
> > 
> > Jonathan  
> > > 
> > > 2) Some driver might want to avoid allocating a new copy of a big table if
> > >    the race does not occur (e.g. with additional checks on buffer access
> > >    code) and thus wouldn't call a free() in the release callback.
> > >   
> > > > 
> > > > In any case it looks fragile and not scalable. I propose to drop this
> > > > and think again.    
> > > 
> > > I see your concerns, I am open to reconsider this in case we come up with
> > > better solution after addressing the points above.
> > >   
> > > > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > > > agree on the report, I think this solution is not good enough.
> > > > 
> > > > -- 
> > > > With Best Regards,
> > > > Andy Shevchenko
> > > >     
> > > 
> > > [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/
> > > 
> > > Best regards,
> > > Matteo Martelli  
> >   
> 
> I hope I've brought a little more clarity to the discussion by providing some
> history instead of making it more confusing.

Sure, the code example in particular is useful.

Jonathan

> 
> Best regards,
> Matteo Martelli
> 
>

Matteo Martelli Oct. 31, 2024, 6:06 p.m. UTC | #7

Quoting Jonathan Cameron (2024-10-31 15:31:29)
> On Thu, 31 Oct 2024 12:26:24 +0100
> Matteo Martelli <[e-mail]> wrote:
> 
> > Quoting Jonathan Cameron (2024-10-30 21:30:50)
> > > On Wed, 30 Oct 2024 19:23:21 +0100
> > > Matteo Martelli <[e-mail]> wrote:
> > >   
> > > > Quoting Andy Shevchenko (2024-10-30 15:47:50)  
> > > > > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:    
> > > > > > Consumers need to call the producer's read_avail_release_resource()
> > > > > > callback after reading producer's available info. To avoid a race
> > > > > > condition with the producer unregistration, change inkern
> > > > > > iio_channel_read_avail() so that it copies the available info from the
> > > > > > producer and immediately calls its release callback with info_exists
> > > > > > locked.
> > > > > > 
> > > > > > Also, modify the users of iio_read_avail_channel_raw() and
> > > > > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > > > > after calling these functions. To let users free the copied buffer with
> > > > > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > > > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > > > > but stores the available values in the returned variable.    
> > > > > 
> > > > > ...
> > > > >     
> > > > > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > > > > +                                         struct iio_chan_spec const *chan,
> > > > > > +                                         const int *vals, long mask)
> > > > > > +{
> > > > > > +     kfree(vals);
> > > > > > +}
> > > > > > +
> > > > > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > >                             struct iio_chan_spec const *chan,
> > > > > >                             int val, int val2, long mask)
> > > > > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > >  static const struct iio_info dpot_dac_info = {
> > > > > >       .read_raw = dpot_dac_read_raw,
> > > > > >       .read_avail = dpot_dac_read_avail,
> > > > > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > > > >       .write_raw = dpot_dac_write_raw,
> > > > > >  };    
> > > > > 
> > > > > I have a problem with this approach. The issue is that we allocate
> > > > > memory in one place and must clear it in another. This is not well
> > > > > designed thingy in my opinion. I was thinking a bit of the solution and
> > > > > at least these two comes to my mind:
> > > > > 
> > > > > 1) having a special callback for .read_avail_with_copy (choose better
> > > > > name) that will dump the data to the intermediate buffer and clean it
> > > > > after all;
> > > > > 
> > > > > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.    
> > > > 
> > > > Could you elaborate more about these potential solutions? Maybe with some
> > > > usage examples?
> > > > 
> > > > If I get it correctly, in both cases you are suggesting to pass ownership
> > > > of the vals buffer to the caller, iio_read_channel_info_avail() in this
> > > > case, so that it would take care of freeing the buffer after calling
> > > > iio_format_after_*(). We considered this approach during an initial
> > > > discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> > > > to let the driver keep the release control through a callback for two
> > > > reasons:
> > > > 
> > > > 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
> > > >    maybe Jonathan can elaborate why? The risk I can think of is that the driver
> > > >    could still keep the buffer copy in its private data after giving it away,
> > > >    resulting in fact in a double ownership. However I think it would be clear
> > > >    enough in this case that the copy should be handled by the caller, or maybe
> > > >    not?  
> > > Mostly the lack of desire to have to copy for the 95% of cases where it's
> > > not needed and that it prevents any optimization like you mention.  
> > 
> > I think the suggestion here is to add an additional .read_avail_with_copy()
> > without replacing the original .read_avail(), so all the current drivers that
> > use a constant avail list would not be affected. And I think this was the same
> > idea for the additional read_avail_ext() or the additional argument for the
> > read_avail() we were considering in [1]. So I would think that
> > iio_read_channel_info_avail() would do something like the following:
> > 
> >     if (indio_dev->info->read_avail_with_copy)
> >         indio_dev->info->read_avail_with_copy(vals);
> >     else
> >         indio_dev->info->read_avail(vals);
> > 
> >     ...
> >     iio_format_avail_list(vals);
> >     ...
> > 
> >     if (indio_dev->info->read_avail_with_copy)
> >         kfree(vals);
> 
> Ok, sure that would work, but...
> 
> I don't really see this as being much less fragile than
> the existing solution + in cases that we do have where
> only some available are not const we will have to copy them
> all.
> 
> If anything it's more complex than making it a driver problem
> to provide the release call however it wants to do it.
>  
> 
> > 
> > And the drivers would choose whether to define the read_avail or the
> > read_avail_with_copy.
> > 
> > What I was referring to is that, back then, you mentioned you would have
> > preferred to avoid passing ownership of the buffer around:
> > 
> > > That's a corner case we should think about closing. Would require an indicator
> > > to read_avail that the buffer it has been passed is a snapshot that it should
> > > free on completion of the string building.  I don't like passing ownership
> > > of data around like that, but it is fiddly to do anything else given
> > > any simple double buffering is subject to race conditions.  
> > 
> > I guess there is some other reason other than avoiding the copy when not
> > necessary, since by introducing an additional function or argument or return
> > type, most of the unnecessary copies would already be avoided right?
> 
> It's not a strong reason beyond limiting scope of clever design +
> the key bit my mind is that the above is not substantially simpler and
> reduces our flexibility.
> 
> > 
> > Anyway any of this solutions would still prevent the potential optimizations of
> > point 2). It's worth mentioning that those kind of optimizations are currently
> > not adopted by any driver.
> 
> That one indeed not, but mixing dynamic and non dynamic is something
> you do in your pac1921 patch.

Good point! I didn't think about it, or more likely I forgot, that with an
additional read_avail_with_copy() used as in the example you cannot mix dynamic
and non dynamic available lists, thus those drivers that need at least one
dynamic available list would always copy all of them as they need to rely to
the read_avail_with_copy(). I guess this could be worked around with an
additional return argument for the read_avail() or an additional type like the
IIO_AVAIL_LIST_ALLOC suggested by Andy to signal the caller it needs to free
the list after use. However, I think they would introduce a more invasive
change in the current API compared to an additional optional callback, so I
agree that the current release callback is still a better option.

> 
> Jonathan
> 
> 
> > 
> > > 
> > > Jonathan  
> > > > 
> > > > 2) Some driver might want to avoid allocating a new copy of a big table if
> > > >    the race does not occur (e.g. with additional checks on buffer access
> > > >    code) and thus wouldn't call a free() in the release callback.
> > > >   
> > > > > 
> > > > > In any case it looks fragile and not scalable. I propose to drop this
> > > > > and think again.    
> > > > 
> > > > I see your concerns, I am open to reconsider this in case we come up with
> > > > better solution after addressing the points above.
> > > >   
> > > > > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > > > > agree on the report, I think this solution is not good enough.
> > > > > 
> > > > > -- 
> > > > > With Best Regards,
> > > > > Andy Shevchenko
> > > > >     
> > > > 
> > > > [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/
> > > > 
> > > > Best regards,
> > > > Matteo Martelli  
> > >   
> > 
> > I hope I've brought a little more clarity to the discussion by providing some
> > history instead of making it more confusing.
> 
> Sure, the code example in particular is useful.
> 
> Jonathan
> 
> > 
> > Best regards,
> > Matteo Martelli
> > 
> > 
> 
Best regards,
Matteo Martelli

Matteo Martelli Nov. 15, 2024, 2:25 p.m. UTC | #8

On Thu, 31 Oct 2024 19:06:32 +0100, Matteo Martelli <[e-mail]> wrote:
> Quoting Jonathan Cameron (2024-10-31 15:31:29)
> > On Thu, 31 Oct 2024 12:26:24 +0100
> > Matteo Martelli <[e-mail]> wrote:
> > 
> > > Quoting Jonathan Cameron (2024-10-30 21:30:50)
> > > > On Wed, 30 Oct 2024 19:23:21 +0100
> > > > Matteo Martelli <[e-mail]> wrote:
> > > >   
> > > > > Quoting Andy Shevchenko (2024-10-30 15:47:50)  
> > > > > > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:    
> > > > > > > Consumers need to call the producer's read_avail_release_resource()
> > > > > > > callback after reading producer's available info. To avoid a race
> > > > > > > condition with the producer unregistration, change inkern
> > > > > > > iio_channel_read_avail() so that it copies the available info from the
> > > > > > > producer and immediately calls its release callback with info_exists
> > > > > > > locked.
> > > > > > > 
> > > > > > > Also, modify the users of iio_read_avail_channel_raw() and
> > > > > > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > > > > > after calling these functions. To let users free the copied buffer with
> > > > > > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > > > > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > > > > > but stores the available values in the returned variable.    
> > > > > > 
> > > > > > ...
> > > > > >     
> > > > > > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > > > > > +                                         struct iio_chan_spec const *chan,
> > > > > > > +                                         const int *vals, long mask)
> > > > > > > +{
> > > > > > > +     kfree(vals);
> > > > > > > +}
> > > > > > > +
> > > > > > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > >                             struct iio_chan_spec const *chan,
> > > > > > >                             int val, int val2, long mask)
> > > > > > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > >  static const struct iio_info dpot_dac_info = {
> > > > > > >       .read_raw = dpot_dac_read_raw,
> > > > > > >       .read_avail = dpot_dac_read_avail,
> > > > > > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > > > > >       .write_raw = dpot_dac_write_raw,
> > > > > > >  };    
> > > > > > 
> > > > > > I have a problem with this approach. The issue is that we allocate
> > > > > > memory in one place and must clear it in another. This is not well
> > > > > > designed thingy in my opinion. I was thinking a bit of the solution and
> > > > > > at least these two comes to my mind:
> > > > > > 
> > > > > > 1) having a special callback for .read_avail_with_copy (choose better
> > > > > > name) that will dump the data to the intermediate buffer and clean it
> > > > > > after all;
> > > > > > 
> > > > > > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.    
> > > > > 
> > > > > Could you elaborate more about these potential solutions? Maybe with some
> > > > > usage examples?
> > > > > 
> > > > > If I get it correctly, in both cases you are suggesting to pass ownership
> > > > > of the vals buffer to the caller, iio_read_channel_info_avail() in this
> > > > > case, so that it would take care of freeing the buffer after calling
> > > > > iio_format_after_*(). We considered this approach during an initial
> > > > > discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> > > > > to let the driver keep the release control through a callback for two
> > > > > reasons:
> > > > > 
> > > > > 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
> > > > >    maybe Jonathan can elaborate why? The risk I can think of is that the driver
> > > > >    could still keep the buffer copy in its private data after giving it away,
> > > > >    resulting in fact in a double ownership. However I think it would be clear
> > > > >    enough in this case that the copy should be handled by the caller, or maybe
> > > > >    not?  
> > > > Mostly the lack of desire to have to copy for the 95% of cases where it's
> > > > not needed and that it prevents any optimization like you mention.  
> > > 
> > > I think the suggestion here is to add an additional .read_avail_with_copy()
> > > without replacing the original .read_avail(), so all the current drivers that
> > > use a constant avail list would not be affected. And I think this was the same
> > > idea for the additional read_avail_ext() or the additional argument for the
> > > read_avail() we were considering in [1]. So I would think that
> > > iio_read_channel_info_avail() would do something like the following:
> > > 
> > >     if (indio_dev->info->read_avail_with_copy)
> > >         indio_dev->info->read_avail_with_copy(vals);
> > >     else
> > >         indio_dev->info->read_avail(vals);
> > > 
> > >     ...
> > >     iio_format_avail_list(vals);
> > >     ...
> > > 
> > >     if (indio_dev->info->read_avail_with_copy)
> > >         kfree(vals);
> > 
> > Ok, sure that would work, but...
> > 
> > I don't really see this as being much less fragile than
> > the existing solution + in cases that we do have where
> > only some available are not const we will have to copy them
> > all.
> > 
> > If anything it's more complex than making it a driver problem
> > to provide the release call however it wants to do it.
> >  
> > 
> > > 
> > > And the drivers would choose whether to define the read_avail or the
> > > read_avail_with_copy.
> > > 
> > > What I was referring to is that, back then, you mentioned you would have
> > > preferred to avoid passing ownership of the buffer around:
> > > 
> > > > That's a corner case we should think about closing. Would require an indicator
> > > > to read_avail that the buffer it has been passed is a snapshot that it should
> > > > free on completion of the string building.  I don't like passing ownership
> > > > of data around like that, but it is fiddly to do anything else given
> > > > any simple double buffering is subject to race conditions.  
> > > 
> > > I guess there is some other reason other than avoiding the copy when not
> > > necessary, since by introducing an additional function or argument or return
> > > type, most of the unnecessary copies would already be avoided right?
> > 
> > It's not a strong reason beyond limiting scope of clever design +
> > the key bit my mind is that the above is not substantially simpler and
> > reduces our flexibility.
> > 
> > > 
> > > Anyway any of this solutions would still prevent the potential optimizations of
> > > point 2). It's worth mentioning that those kind of optimizations are currently
> > > not adopted by any driver.
> > 
> > That one indeed not, but mixing dynamic and non dynamic is something
> > you do in your pac1921 patch.
> 
> Good point! I didn't think about it, or more likely I forgot, that with an
> additional read_avail_with_copy() used as in the example you cannot mix dynamic
> and non dynamic available lists, thus those drivers that need at least one
> dynamic available list would always copy all of them as they need to rely to
> the read_avail_with_copy(). I guess this could be worked around with an
> additional return argument for the read_avail() or an additional type like the
> IIO_AVAIL_LIST_ALLOC suggested by Andy to signal the caller it needs to free
> the list after use. However, I think they would introduce a more invasive
> change in the current API compared to an additional optional callback, so I
> agree that the current release callback is still a better option.
> 
> > 
> > Jonathan
> > 
> > 
> > > 
> > > > 
> > > > Jonathan  
> > > > > 
> > > > > 2) Some driver might want to avoid allocating a new copy of a big table if
> > > > >    the race does not occur (e.g. with additional checks on buffer access
> > > > >    code) and thus wouldn't call a free() in the release callback.
> > > > >   
> > > > > > 
> > > > > > In any case it looks fragile and not scalable. I propose to drop this
> > > > > > and think again.    
> > > > > 
> > > > > I see your concerns, I am open to reconsider this in case we come up with
> > > > > better solution after addressing the points above.
> > > > >   
> > > > > > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > > > > > agree on the report, I think this solution is not good enough.
> > > > > > 
> > > > > > -- 
> > > > > > With Best Regards,
> > > > > > Andy Shevchenko
> > > > > >     
> > > > > 
> > > > > [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/
> > > > > 
> > > > > Best regards,
> > > > > Matteo Martelli  
> > > >   
> > > 
> > > I hope I've brought a little more clarity to the discussion by providing some
> > > history instead of making it more confusing.
> > 
> > Sure, the code example in particular is useful.
> > 
> > Jonathan
> > 
> > > 
> > > Best regards,
> > > Matteo Martelli
> > > 
> > > 
> > 
> Best regards,
> Matteo Martelli

Just a friendly reminder this has been sitting for a while, any news or
additional considerations?

Best regards,
Matteo Martelli

Andy Shevchenko Nov. 18, 2024, 10:21 a.m. UTC | #9

On Fri, Nov 15, 2024 at 03:25:06PM +0100, Matteo Martelli wrote:
> On Thu, 31 Oct 2024 19:06:32 +0100, Matteo Martelli <[e-mail]> wrote:
> > Quoting Jonathan Cameron (2024-10-31 15:31:29)
> > > On Thu, 31 Oct 2024 12:26:24 +0100
> > > Matteo Martelli <[e-mail]> wrote:
> > > > Quoting Jonathan Cameron (2024-10-30 21:30:50)
> > > > > On Wed, 30 Oct 2024 19:23:21 +0100
> > > > > Matteo Martelli <[e-mail]> wrote:
> > > > > > Quoting Andy Shevchenko (2024-10-30 15:47:50)  
> > > > > > > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:    

> > > > > > > > Consumers need to call the producer's read_avail_release_resource()
> > > > > > > > callback after reading producer's available info. To avoid a race
> > > > > > > > condition with the producer unregistration, change inkern
> > > > > > > > iio_channel_read_avail() so that it copies the available info from the
> > > > > > > > producer and immediately calls its release callback with info_exists
> > > > > > > > locked.
> > > > > > > > 
> > > > > > > > Also, modify the users of iio_read_avail_channel_raw() and
> > > > > > > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > > > > > > after calling these functions. To let users free the copied buffer with
> > > > > > > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > > > > > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > > > > > > but stores the available values in the returned variable.    

...

> > > > > > > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > > > > > > +                                         struct iio_chan_spec const *chan,
> > > > > > > > +                                         const int *vals, long mask)
> > > > > > > > +{
> > > > > > > > +     kfree(vals);
> > > > > > > > +}
> > > > > > > > +
> > > > > > > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > >                             struct iio_chan_spec const *chan,
> > > > > > > >                             int val, int val2, long mask)
> > > > > > > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > >  static const struct iio_info dpot_dac_info = {
> > > > > > > >       .read_raw = dpot_dac_read_raw,
> > > > > > > >       .read_avail = dpot_dac_read_avail,
> > > > > > > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > > > > > >       .write_raw = dpot_dac_write_raw,
> > > > > > > >  };    
> > > > > > > 
> > > > > > > I have a problem with this approach. The issue is that we allocate
> > > > > > > memory in one place and must clear it in another. This is not well
> > > > > > > designed thingy in my opinion. I was thinking a bit of the solution and
> > > > > > > at least these two comes to my mind:
> > > > > > > 
> > > > > > > 1) having a special callback for .read_avail_with_copy (choose better
> > > > > > > name) that will dump the data to the intermediate buffer and clean it
> > > > > > > after all;
> > > > > > > 
> > > > > > > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.    
> > > > > > 
> > > > > > Could you elaborate more about these potential solutions? Maybe with some
> > > > > > usage examples?
> > > > > > 
> > > > > > If I get it correctly, in both cases you are suggesting to pass ownership
> > > > > > of the vals buffer to the caller, iio_read_channel_info_avail() in this
> > > > > > case, so that it would take care of freeing the buffer after calling
> > > > > > iio_format_after_*(). We considered this approach during an initial
> > > > > > discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> > > > > > to let the driver keep the release control through a callback for two
> > > > > > reasons:
> > > > > > 
> > > > > > 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
> > > > > >    maybe Jonathan can elaborate why? The risk I can think of is that the driver
> > > > > >    could still keep the buffer copy in its private data after giving it away,
> > > > > >    resulting in fact in a double ownership. However I think it would be clear
> > > > > >    enough in this case that the copy should be handled by the caller, or maybe
> > > > > >    not?  
> > > > > Mostly the lack of desire to have to copy for the 95% of cases where it's
> > > > > not needed and that it prevents any optimization like you mention.  
> > > > 
> > > > I think the suggestion here is to add an additional .read_avail_with_copy()
> > > > without replacing the original .read_avail(), so all the current drivers that
> > > > use a constant avail list would not be affected.

Yes.

> > > > And I think this was the same
> > > > idea for the additional read_avail_ext() or the additional argument for the
> > > > read_avail() we were considering in [1]. So I would think that
> > > > iio_read_channel_info_avail() would do something like the following:
> > > > 
> > > >     if (indio_dev->info->read_avail_with_copy)
> > > >         indio_dev->info->read_avail_with_copy(vals);
> > > >     else
> > > >         indio_dev->info->read_avail(vals);
> > > > 
> > > >     ...
> > > >     iio_format_avail_list(vals);
> > > >     ...
> > > > 
> > > >     if (indio_dev->info->read_avail_with_copy)
> > > >         kfree(vals);

Right. At least that's what I see can be done with the existing users.

> > > Ok, sure that would work, but...
> > > 
> > > I don't really see this as being much less fragile than
> > > the existing solution + in cases that we do have where
> > > only some available are not const we will have to copy them
> > > all.
> > > 
> > > If anything it's more complex than making it a driver problem
> > > to provide the release call however it wants to do it.

...but make a driver to allocate what's needed as well then.

> > > > And the drivers would choose whether to define the read_avail or the
> > > > read_avail_with_copy.

Either way drivers should know what to do with a data supplied to read_aval().
In one case we assume the [simple] workflow in the core, in the other we all
rely on the driver. Current approach makes a mix of these two. And that's what
I don't like.

> > > > What I was referring to is that, back then, you mentioned you would have
> > > > preferred to avoid passing ownership of the buffer around:
> > > > 
> > > > > That's a corner case we should think about closing. Would require an indicator
> > > > > to read_avail that the buffer it has been passed is a snapshot that it should
> > > > > free on completion of the string building.  I don't like passing ownership
> > > > > of data around like that, but it is fiddly to do anything else given
> > > > > any simple double buffering is subject to race conditions.  
> > > > 
> > > > I guess there is some other reason other than avoiding the copy when not
> > > > necessary, since by introducing an additional function or argument or return
> > > > type, most of the unnecessary copies would already be avoided right?
> > > 
> > > It's not a strong reason beyond limiting scope of clever design +
> > > the key bit my mind is that the above is not substantially simpler and
> > > reduces our flexibility.
> > > 
> > > > Anyway any of this solutions would still prevent the potential optimizations of
> > > > point 2). It's worth mentioning that those kind of optimizations are currently
> > > > not adopted by any driver.
> > > 
> > > That one indeed not, but mixing dynamic and non dynamic is something
> > > you do in your pac1921 patch.
> > 
> > Good point! I didn't think about it, or more likely I forgot, that with an
> > additional read_avail_with_copy() used as in the example you cannot mix dynamic
> > and non dynamic available lists, thus those drivers that need at least one
> > dynamic available list would always copy all of them as they need to rely to
> > the read_avail_with_copy(). I guess this could be worked around with an
> > additional return argument for the read_avail() or an additional type like the
> > IIO_AVAIL_LIST_ALLOC suggested by Andy to signal the caller it needs to free
> > the list after use. However, I think they would introduce a more invasive
> > change in the current API compared to an additional optional callback,

It even sounds originally that it should be more invasive, so I don't think it's
a problem here.

> > so I agree that the current release callback is still a better option.

I disagree on this as I pointed above why.

> > > > > > 2) Some driver might want to avoid allocating a new copy of a big table if
> > > > > >    the race does not occur (e.g. with additional checks on buffer access
> > > > > >    code) and thus wouldn't call a free() in the release callback.
> > > > > >   
> > > > > > > In any case it looks fragile and not scalable. I propose to drop this
> > > > > > > and think again.    
> > > > > > 
> > > > > > I see your concerns, I am open to reconsider this in case we come up with
> > > > > > better solution after addressing the points above.
> > > > > >   
> > > > > > > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > > > > > > agree on the report, I think this solution is not good enough.
> > > > > > 
> > > > > > [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/
> > > > 
> > > > I hope I've brought a little more clarity to the discussion by providing some
> > > > history instead of making it more confusing.
> > > 
> > > Sure, the code example in particular is useful.
> 
> Just a friendly reminder this has been sitting for a while, any news or
> additional considerations?

Moving the allocation control to the drivers will satisfy me as well, however
it makes even more duplication of the code, but at least it will be cleaner
design-wise in my opinion.

In any case the last word is on Jonathan.

Matteo Martelli Nov. 18, 2024, 2:45 p.m. UTC | #10

On Mon, 18 Nov 2024 12:21:44 +0200, Andy Shevchenko <[e-mail]> wrote:
> On Fri, Nov 15, 2024 at 03:25:06PM +0100, Matteo Martelli wrote:
> > On Thu, 31 Oct 2024 19:06:32 +0100, Matteo Martelli <[e-mail]> wrote:
> > > Quoting Jonathan Cameron (2024-10-31 15:31:29)
> > > > On Thu, 31 Oct 2024 12:26:24 +0100
> > > > Matteo Martelli <[e-mail]> wrote:
> > > > > Quoting Jonathan Cameron (2024-10-30 21:30:50)
> > > > > > On Wed, 30 Oct 2024 19:23:21 +0100
> > > > > > Matteo Martelli <[e-mail]> wrote:
> > > > > > > Quoting Andy Shevchenko (2024-10-30 15:47:50)  
> > > > > > > > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:    
> 
> > > > > > > > > Consumers need to call the producer's read_avail_release_resource()
> > > > > > > > > callback after reading producer's available info. To avoid a race
> > > > > > > > > condition with the producer unregistration, change inkern
> > > > > > > > > iio_channel_read_avail() so that it copies the available info from the
> > > > > > > > > producer and immediately calls its release callback with info_exists
> > > > > > > > > locked.
> > > > > > > > > 
> > > > > > > > > Also, modify the users of iio_read_avail_channel_raw() and
> > > > > > > > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > > > > > > > after calling these functions. To let users free the copied buffer with
> > > > > > > > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > > > > > > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > > > > > > > but stores the available values in the returned variable.    
> 
> ...
> 
> > > > > > > > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > > > > > > > +                                         struct iio_chan_spec const *chan,
> > > > > > > > > +                                         const int *vals, long mask)
> > > > > > > > > +{
> > > > > > > > > +     kfree(vals);
> > > > > > > > > +}
> > > > > > > > > +
> > > > > > > > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > > >                             struct iio_chan_spec const *chan,
> > > > > > > > >                             int val, int val2, long mask)
> > > > > > > > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > > >  static const struct iio_info dpot_dac_info = {
> > > > > > > > >       .read_raw = dpot_dac_read_raw,
> > > > > > > > >       .read_avail = dpot_dac_read_avail,
> > > > > > > > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > > > > > > >       .write_raw = dpot_dac_write_raw,
> > > > > > > > >  };    
> > > > > > > > 
> > > > > > > > I have a problem with this approach. The issue is that we allocate
> > > > > > > > memory in one place and must clear it in another. This is not well
> > > > > > > > designed thingy in my opinion. I was thinking a bit of the solution and
> > > > > > > > at least these two comes to my mind:
> > > > > > > > 
> > > > > > > > 1) having a special callback for .read_avail_with_copy (choose better
> > > > > > > > name) that will dump the data to the intermediate buffer and clean it
> > > > > > > > after all;
> > > > > > > > 
> > > > > > > > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.    
> > > > > > > 
> > > > > > > Could you elaborate more about these potential solutions? Maybe with some
> > > > > > > usage examples?
> > > > > > > 
> > > > > > > If I get it correctly, in both cases you are suggesting to pass ownership
> > > > > > > of the vals buffer to the caller, iio_read_channel_info_avail() in this
> > > > > > > case, so that it would take care of freeing the buffer after calling
> > > > > > > iio_format_after_*(). We considered this approach during an initial
> > > > > > > discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> > > > > > > to let the driver keep the release control through a callback for two
> > > > > > > reasons:
> > > > > > > 
> > > > > > > 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
> > > > > > >    maybe Jonathan can elaborate why? The risk I can think of is that the driver
> > > > > > >    could still keep the buffer copy in its private data after giving it away,
> > > > > > >    resulting in fact in a double ownership. However I think it would be clear
> > > > > > >    enough in this case that the copy should be handled by the caller, or maybe
> > > > > > >    not?  
> > > > > > Mostly the lack of desire to have to copy for the 95% of cases where it's
> > > > > > not needed and that it prevents any optimization like you mention.  
> > > > > 
> > > > > I think the suggestion here is to add an additional .read_avail_with_copy()
> > > > > without replacing the original .read_avail(), so all the current drivers that
> > > > > use a constant avail list would not be affected.
> 
> Yes.
> 
> > > > > And I think this was the same
> > > > > idea for the additional read_avail_ext() or the additional argument for the
> > > > > read_avail() we were considering in [1]. So I would think that
> > > > > iio_read_channel_info_avail() would do something like the following:
> > > > > 
> > > > >     if (indio_dev->info->read_avail_with_copy)
> > > > >         indio_dev->info->read_avail_with_copy(vals);
> > > > >     else
> > > > >         indio_dev->info->read_avail(vals);
> > > > > 
> > > > >     ...
> > > > >     iio_format_avail_list(vals);
> > > > >     ...
> > > > > 
> > > > >     if (indio_dev->info->read_avail_with_copy)
> > > > >         kfree(vals);
> 
> Right. At least that's what I see can be done with the existing users.
> 
> > > > Ok, sure that would work, but...
> > > > 
> > > > I don't really see this as being much less fragile than
> > > > the existing solution + in cases that we do have where
> > > > only some available are not const we will have to copy them
> > > > all.
> > > > 
> > > > If anything it's more complex than making it a driver problem
> > > > to provide the release call however it wants to do it.
> 
> ...but make a driver to allocate what's needed as well then.
> 
> > > > > And the drivers would choose whether to define the read_avail or the
> > > > > read_avail_with_copy.
> 
> Either way drivers should know what to do with a data supplied to read_aval().
> In one case we assume the [simple] workflow in the core, in the other we all
> rely on the driver. Current approach makes a mix of these two. And that's what
> I don't like.
> 

If I understand your concern correctly, you are referring to the inkern
iio_channel_read_avail() that makes the allocation for the consumer's
buffer copy and you are suggesting that such copy should be done by the
consumer driver code itself, this to be consistent with the producer
drivers which directly handle the allocation of the copy.

One thing to notice is that the inkern iio_channel_read_avail() does
together producer->read_avail() + copy + producer->read_avail_release()
with info_exists locked. Also, the consumer driver would need to know
the avail buffer size to allocate the buffer copy prior the
iio_channel_read_avail() call, but such size is unknown before calling
the actual producer's read_avail(). This would mean calling the
producer's read_avail() and read_avail_release() callbacks separately
without the lock held, with the risk of a memleak if the producer is
unregistered between those calls.

> > > > > What I was referring to is that, back then, you mentioned you would have
> > > > > preferred to avoid passing ownership of the buffer around:
> > > > > 
> > > > > > That's a corner case we should think about closing. Would require an indicator
> > > > > > to read_avail that the buffer it has been passed is a snapshot that it should
> > > > > > free on completion of the string building.  I don't like passing ownership
> > > > > > of data around like that, but it is fiddly to do anything else given
> > > > > > any simple double buffering is subject to race conditions.  
> > > > > 
> > > > > I guess there is some other reason other than avoiding the copy when not
> > > > > necessary, since by introducing an additional function or argument or return
> > > > > type, most of the unnecessary copies would already be avoided right?
> > > > 
> > > > It's not a strong reason beyond limiting scope of clever design +
> > > > the key bit my mind is that the above is not substantially simpler and
> > > > reduces our flexibility.
> > > > 
> > > > > Anyway any of this solutions would still prevent the potential optimizations of
> > > > > point 2). It's worth mentioning that those kind of optimizations are currently
> > > > > not adopted by any driver.
> > > > 
> > > > That one indeed not, but mixing dynamic and non dynamic is something
> > > > you do in your pac1921 patch.
> > > 
> > > Good point! I didn't think about it, or more likely I forgot, that with an
> > > additional read_avail_with_copy() used as in the example you cannot mix dynamic
> > > and non dynamic available lists, thus those drivers that need at least one
> > > dynamic available list would always copy all of them as they need to rely to
> > > the read_avail_with_copy(). I guess this could be worked around with an
> > > additional return argument for the read_avail() or an additional type like the
> > > IIO_AVAIL_LIST_ALLOC suggested by Andy to signal the caller it needs to free
> > > the list after use. However, I think they would introduce a more invasive
> > > change in the current API compared to an additional optional callback,
> 
> It even sounds originally that it should be more invasive, so I don't think it's
> a problem here.
> 

In the hope it helps the discussion let me provide examples for the
additional two options we have other than the current
read_avail_release_resource() (fix-1) and the read_avail_with_copy()
(fix-2) already shown above:

fix-3) iio_read_channel_info_avail():
{
    ...
    bool release_avail = false;

    ret = indio_dev->info->read_avail(vals, ..., &release_avail);

    ...
    ret = iio_format_avail_list(vals, ...);
    ...

    if (release_avail)
        kfree(vals);

    return ret;
}


fix-4) iio_read_channel_info_avail():
{
    ...
    indio_dev->info->read_avail(vals, ...);

    if (ret < 0)
            return ret;
    switch (ret) {
    case IIO_AVAIL_LIST_ALLOC:
            ret = iio_format_avail_list(buf, vals, type, length);
            kfree(vals);
            return ret;
    case IIO_AVAIL_LIST:
            return iio_format_avail_list(buf, vals, type, length);
    case IIO_AVAIL_RANGE:
            return iio_format_avail_range(buf, vals, type);
    default:
            return -EINVAL;
    }

}

> > > so I agree that the current release callback is still a better option.
> 
> I disagree on this as I pointed above why.
> 
> > > > > > > 2) Some driver might want to avoid allocating a new copy of a big table if
> > > > > > >    the race does not occur (e.g. with additional checks on buffer access
> > > > > > >    code) and thus wouldn't call a free() in the release callback.
> > > > > > >   
> > > > > > > > In any case it looks fragile and not scalable. I propose to drop this
> > > > > > > > and think again.    
> > > > > > > 
> > > > > > > I see your concerns, I am open to reconsider this in case we come up with
> > > > > > > better solution after addressing the points above.
> > > > > > >   
> > > > > > > > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > > > > > > > agree on the report, I think this solution is not good enough.
> > > > > > > 
> > > > > > > [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/
> > > > > 
> > > > > I hope I've brought a little more clarity to the discussion by providing some
> > > > > history instead of making it more confusing.
> > > > 
> > > > Sure, the code example in particular is useful.
> > 
> > Just a friendly reminder this has been sitting for a while, any news or
> > additional considerations?
> 
> Moving the allocation control to the drivers will satisfy me as well, however
> it makes even more duplication of the code, but at least it will be cleaner
> design-wise in my opinion.

Would it work with the constraints on the info_exists lock mentioned
above?

> 
> In any case the last word is on Jonathan.
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
> 

Best regards,
Matteo Martelli

Andy Shevchenko Nov. 18, 2024, 4:05 p.m. UTC | #11

On Mon, Nov 18, 2024 at 03:45:25PM +0100, Matteo Martelli wrote:
> On Mon, 18 Nov 2024 12:21:44 +0200, Andy Shevchenko <[e-mail]> wrote:
> > On Fri, Nov 15, 2024 at 03:25:06PM +0100, Matteo Martelli wrote:
> > > On Thu, 31 Oct 2024 19:06:32 +0100, Matteo Martelli <[e-mail]> wrote:
> > > > Quoting Jonathan Cameron (2024-10-31 15:31:29)
> > > > > On Thu, 31 Oct 2024 12:26:24 +0100
> > > > > Matteo Martelli <[e-mail]> wrote:
> > > > > > Quoting Jonathan Cameron (2024-10-30 21:30:50)
> > > > > > > On Wed, 30 Oct 2024 19:23:21 +0100
> > > > > > > Matteo Martelli <[e-mail]> wrote:
> > > > > > > > Quoting Andy Shevchenko (2024-10-30 15:47:50)  
> > > > > > > > > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:    
> > 
> > > > > > > > > > Consumers need to call the producer's read_avail_release_resource()
> > > > > > > > > > callback after reading producer's available info. To avoid a race
> > > > > > > > > > condition with the producer unregistration, change inkern
> > > > > > > > > > iio_channel_read_avail() so that it copies the available info from the
> > > > > > > > > > producer and immediately calls its release callback with info_exists
> > > > > > > > > > locked.
> > > > > > > > > > 
> > > > > > > > > > Also, modify the users of iio_read_avail_channel_raw() and
> > > > > > > > > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > > > > > > > > after calling these functions. To let users free the copied buffer with
> > > > > > > > > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > > > > > > > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > > > > > > > > but stores the available values in the returned variable.    

...

> > > > > > > > > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > > > > > > > > +                                         struct iio_chan_spec const *chan,
> > > > > > > > > > +                                         const int *vals, long mask)
> > > > > > > > > > +{
> > > > > > > > > > +     kfree(vals);
> > > > > > > > > > +}
> > > > > > > > > > +
> > > > > > > > > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > > > >                             struct iio_chan_spec const *chan,
> > > > > > > > > >                             int val, int val2, long mask)
> > > > > > > > > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > > > >  static const struct iio_info dpot_dac_info = {
> > > > > > > > > >       .read_raw = dpot_dac_read_raw,
> > > > > > > > > >       .read_avail = dpot_dac_read_avail,
> > > > > > > > > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > > > > > > > >       .write_raw = dpot_dac_write_raw,
> > > > > > > > > >  };    
> > > > > > > > > 
> > > > > > > > > I have a problem with this approach. The issue is that we allocate
> > > > > > > > > memory in one place and must clear it in another. This is not well
> > > > > > > > > designed thingy in my opinion. I was thinking a bit of the solution and
> > > > > > > > > at least these two comes to my mind:
> > > > > > > > > 
> > > > > > > > > 1) having a special callback for .read_avail_with_copy (choose better
> > > > > > > > > name) that will dump the data to the intermediate buffer and clean it
> > > > > > > > > after all;
> > > > > > > > > 
> > > > > > > > > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.    
> > > > > > > > 
> > > > > > > > Could you elaborate more about these potential solutions? Maybe with some
> > > > > > > > usage examples?
> > > > > > > > 
> > > > > > > > If I get it correctly, in both cases you are suggesting to pass ownership
> > > > > > > > of the vals buffer to the caller, iio_read_channel_info_avail() in this
> > > > > > > > case, so that it would take care of freeing the buffer after calling
> > > > > > > > iio_format_after_*(). We considered this approach during an initial
> > > > > > > > discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> > > > > > > > to let the driver keep the release control through a callback for two
> > > > > > > > reasons:
> > > > > > > > 
> > > > > > > > 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
> > > > > > > >    maybe Jonathan can elaborate why? The risk I can think of is that the driver
> > > > > > > >    could still keep the buffer copy in its private data after giving it away,
> > > > > > > >    resulting in fact in a double ownership. However I think it would be clear
> > > > > > > >    enough in this case that the copy should be handled by the caller, or maybe
> > > > > > > >    not?  
> > > > > > > Mostly the lack of desire to have to copy for the 95% of cases where it's
> > > > > > > not needed and that it prevents any optimization like you mention.  
> > > > > > 
> > > > > > I think the suggestion here is to add an additional .read_avail_with_copy()
> > > > > > without replacing the original .read_avail(), so all the current drivers that
> > > > > > use a constant avail list would not be affected.
> > 
> > Yes.
> > 
> > > > > > And I think this was the same
> > > > > > idea for the additional read_avail_ext() or the additional argument for the
> > > > > > read_avail() we were considering in [1]. So I would think that
> > > > > > iio_read_channel_info_avail() would do something like the following:
> > > > > > 
> > > > > >     if (indio_dev->info->read_avail_with_copy)
> > > > > >         indio_dev->info->read_avail_with_copy(vals);
> > > > > >     else
> > > > > >         indio_dev->info->read_avail(vals);
> > > > > > 
> > > > > >     ...
> > > > > >     iio_format_avail_list(vals);
> > > > > >     ...
> > > > > > 
> > > > > >     if (indio_dev->info->read_avail_with_copy)
> > > > > >         kfree(vals);
> > 
> > Right. At least that's what I see can be done with the existing users.
> > 
> > > > > Ok, sure that would work, but...
> > > > > 
> > > > > I don't really see this as being much less fragile than
> > > > > the existing solution + in cases that we do have where
> > > > > only some available are not const we will have to copy them
> > > > > all.
> > > > > 
> > > > > If anything it's more complex than making it a driver problem
> > > > > to provide the release call however it wants to do it.
> > 
> > ...but make a driver to allocate what's needed as well then.
> > 
> > > > > > And the drivers would choose whether to define the read_avail or the
> > > > > > read_avail_with_copy.
> > 
> > Either way drivers should know what to do with a data supplied to read_aval().
> > In one case we assume the [simple] workflow in the core, in the other we all
> > rely on the driver. Current approach makes a mix of these two. And that's what
> > I don't like.
> 
> If I understand your concern correctly, you are referring to the inkern
> iio_channel_read_avail() that makes the allocation for the consumer's
> buffer copy and you are suggesting that such copy should be done by the
> consumer driver code itself, this to be consistent with the producer
> drivers which directly handle the allocation of the copy.

One of the options, yes.

> One thing to notice is that the inkern iio_channel_read_avail() does
> together producer->read_avail() + copy + producer->read_avail_release()
> with info_exists locked. Also, the consumer driver would need to know
> the avail buffer size to allocate the buffer copy prior the
> iio_channel_read_avail() call, but such size is unknown before calling
> the actual producer's read_avail(). This would mean calling the
> producer's read_avail() and read_avail_release() callbacks separately
> without the lock held, with the risk of a memleak if the producer is
> unregistered between those calls.

Thanks for explaining this, but it even more makes me think that the design
is broken and your approach is rather a hack. So, what's the problem to
make IIO core to take care of the allocating and cleaning then without driver
being involved? Yes, this might require a hint from the driver on what to copy
if we want to avoid copying everything.

> > > > > > What I was referring to is that, back then, you mentioned you would have
> > > > > > preferred to avoid passing ownership of the buffer around:
> > > > > > 
> > > > > > > That's a corner case we should think about closing. Would require an indicator
> > > > > > > to read_avail that the buffer it has been passed is a snapshot that it should
> > > > > > > free on completion of the string building.  I don't like passing ownership
> > > > > > > of data around like that, but it is fiddly to do anything else given
> > > > > > > any simple double buffering is subject to race conditions.  
> > > > > > 
> > > > > > I guess there is some other reason other than avoiding the copy when not
> > > > > > necessary, since by introducing an additional function or argument or return
> > > > > > type, most of the unnecessary copies would already be avoided right?
> > > > > 
> > > > > It's not a strong reason beyond limiting scope of clever design +
> > > > > the key bit my mind is that the above is not substantially simpler and
> > > > > reduces our flexibility.
> > > > > 
> > > > > > Anyway any of this solutions would still prevent the potential optimizations of
> > > > > > point 2). It's worth mentioning that those kind of optimizations are currently
> > > > > > not adopted by any driver.
> > > > > 
> > > > > That one indeed not, but mixing dynamic and non dynamic is something
> > > > > you do in your pac1921 patch.
> > > > 
> > > > Good point! I didn't think about it, or more likely I forgot, that with an
> > > > additional read_avail_with_copy() used as in the example you cannot mix dynamic
> > > > and non dynamic available lists, thus those drivers that need at least one
> > > > dynamic available list would always copy all of them as they need to rely to
> > > > the read_avail_with_copy(). I guess this could be worked around with an
> > > > additional return argument for the read_avail() or an additional type like the
> > > > IIO_AVAIL_LIST_ALLOC suggested by Andy to signal the caller it needs to free
> > > > the list after use. However, I think they would introduce a more invasive
> > > > change in the current API compared to an additional optional callback,
> > 
> > It even sounds originally that it should be more invasive, so I don't think it's
> > a problem here.
> 
> In the hope it helps the discussion let me provide examples for the
> additional two options we have other than the current
> read_avail_release_resource() (fix-1) and the read_avail_with_copy()
> (fix-2) already shown above:

Thanks!

> fix-3) iio_read_channel_info_avail():
> {
>     ...
>     bool release_avail = false;
> 
>     ret = indio_dev->info->read_avail(vals, ..., &release_avail);
> 
>     ...
>     ret = iio_format_avail_list(vals, ...);
>     ...
> 
>     if (release_avail)
>         kfree(vals);
> 
>     return ret;
> }
> 
> 
> fix-4) iio_read_channel_info_avail():
> {
>     ...
>     indio_dev->info->read_avail(vals, ...);
> 
>     if (ret < 0)
>             return ret;
>     switch (ret) {
>     case IIO_AVAIL_LIST_ALLOC:
>             ret = iio_format_avail_list(buf, vals, type, length);
>             kfree(vals);
>             return ret;
>     case IIO_AVAIL_LIST:
>             return iio_format_avail_list(buf, vals, type, length);
>     case IIO_AVAIL_RANGE:
>             return iio_format_avail_range(buf, vals, type);
>     default:
>             return -EINVAL;
>     }
> 
> }
> 
> > > > so I agree that the current release callback is still a better option.
> > 
> > I disagree on this as I pointed above why.
> > 
> > > > > > > > 2) Some driver might want to avoid allocating a new copy of a big table if
> > > > > > > >    the race does not occur (e.g. with additional checks on buffer access
> > > > > > > >    code) and thus wouldn't call a free() in the release callback.
> > > > > > > >   
> > > > > > > > > In any case it looks fragile and not scalable. I propose to drop this
> > > > > > > > > and think again.    
> > > > > > > > 
> > > > > > > > I see your concerns, I am open to reconsider this in case we come up with
> > > > > > > > better solution after addressing the points above.
> > > > > > > >   
> > > > > > > > > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > > > > > > > > agree on the report, I think this solution is not good enough.
> > > > > > > > 
> > > > > > > > [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/
> > > > > > 
> > > > > > I hope I've brought a little more clarity to the discussion by providing some
> > > > > > history instead of making it more confusing.
> > > > > 
> > > > > Sure, the code example in particular is useful.
> > > 
> > > Just a friendly reminder this has been sitting for a while, any news or
> > > additional considerations?
> > 
> > Moving the allocation control to the drivers will satisfy me as well, however
> > it makes even more duplication of the code, but at least it will be cleaner
> > design-wise in my opinion.
> 
> Would it work with the constraints on the info_exists lock mentioned
> above?

None of the given examples (fix-N) provides a lock, so I have no clue how it's
involved here. May be you can elaborate more?

> > In any case the last word is on Jonathan.

Matteo Martelli Nov. 19, 2024, 11:25 a.m. UTC | #12

On Mon, 18 Nov 2024 18:05:35 +0200, Andy Shevchenko <[e-mail]> wrote:
> On Mon, Nov 18, 2024 at 03:45:25PM +0100, Matteo Martelli wrote:
> > On Mon, 18 Nov 2024 12:21:44 +0200, Andy Shevchenko <[e-mail]> wrote:
> > > On Fri, Nov 15, 2024 at 03:25:06PM +0100, Matteo Martelli wrote:
> > > > On Thu, 31 Oct 2024 19:06:32 +0100, Matteo Martelli <[e-mail]> wrote:
> > > > > Quoting Jonathan Cameron (2024-10-31 15:31:29)
> > > > > > On Thu, 31 Oct 2024 12:26:24 +0100
> > > > > > Matteo Martelli <[e-mail]> wrote:
> > > > > > > Quoting Jonathan Cameron (2024-10-30 21:30:50)
> > > > > > > > On Wed, 30 Oct 2024 19:23:21 +0100
> > > > > > > > Matteo Martelli <[e-mail]> wrote:
> > > > > > > > > Quoting Andy Shevchenko (2024-10-30 15:47:50)  
> > > > > > > > > > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:    
> > > 
> > > > > > > > > > > Consumers need to call the producer's read_avail_release_resource()
> > > > > > > > > > > callback after reading producer's available info. To avoid a race
> > > > > > > > > > > condition with the producer unregistration, change inkern
> > > > > > > > > > > iio_channel_read_avail() so that it copies the available info from the
> > > > > > > > > > > producer and immediately calls its release callback with info_exists
> > > > > > > > > > > locked.
> > > > > > > > > > > 
> > > > > > > > > > > Also, modify the users of iio_read_avail_channel_raw() and
> > > > > > > > > > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > > > > > > > > > after calling these functions. To let users free the copied buffer with
> > > > > > > > > > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > > > > > > > > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > > > > > > > > > but stores the available values in the returned variable.    
> 
> ...
> 
> > > > > > > > > > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > > > > > > > > > +                                         struct iio_chan_spec const *chan,
> > > > > > > > > > > +                                         const int *vals, long mask)
> > > > > > > > > > > +{
> > > > > > > > > > > +     kfree(vals);
> > > > > > > > > > > +}
> > > > > > > > > > > +
> > > > > > > > > > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > > > > >                             struct iio_chan_spec const *chan,
> > > > > > > > > > >                             int val, int val2, long mask)
> > > > > > > > > > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > > > > >  static const struct iio_info dpot_dac_info = {
> > > > > > > > > > >       .read_raw = dpot_dac_read_raw,
> > > > > > > > > > >       .read_avail = dpot_dac_read_avail,
> > > > > > > > > > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > > > > > > > > >       .write_raw = dpot_dac_write_raw,
> > > > > > > > > > >  };    
> > > > > > > > > > 
> > > > > > > > > > I have a problem with this approach. The issue is that we allocate
> > > > > > > > > > memory in one place and must clear it in another. This is not well
> > > > > > > > > > designed thingy in my opinion. I was thinking a bit of the solution and
> > > > > > > > > > at least these two comes to my mind:
> > > > > > > > > > 
> > > > > > > > > > 1) having a special callback for .read_avail_with_copy (choose better
> > > > > > > > > > name) that will dump the data to the intermediate buffer and clean it
> > > > > > > > > > after all;
> > > > > > > > > > 
> > > > > > > > > > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.    
> > > > > > > > > 
> > > > > > > > > Could you elaborate more about these potential solutions? Maybe with some
> > > > > > > > > usage examples?
> > > > > > > > > 
> > > > > > > > > If I get it correctly, in both cases you are suggesting to pass ownership
> > > > > > > > > of the vals buffer to the caller, iio_read_channel_info_avail() in this
> > > > > > > > > case, so that it would take care of freeing the buffer after calling
> > > > > > > > > iio_format_after_*(). We considered this approach during an initial
> > > > > > > > > discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> > > > > > > > > to let the driver keep the release control through a callback for two
> > > > > > > > > reasons:
> > > > > > > > > 
> > > > > > > > > 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
> > > > > > > > >    maybe Jonathan can elaborate why? The risk I can think of is that the driver
> > > > > > > > >    could still keep the buffer copy in its private data after giving it away,
> > > > > > > > >    resulting in fact in a double ownership. However I think it would be clear
> > > > > > > > >    enough in this case that the copy should be handled by the caller, or maybe
> > > > > > > > >    not?  
> > > > > > > > Mostly the lack of desire to have to copy for the 95% of cases where it's
> > > > > > > > not needed and that it prevents any optimization like you mention.  
> > > > > > > 
> > > > > > > I think the suggestion here is to add an additional .read_avail_with_copy()
> > > > > > > without replacing the original .read_avail(), so all the current drivers that
> > > > > > > use a constant avail list would not be affected.
> > > 
> > > Yes.
> > > 
> > > > > > > And I think this was the same
> > > > > > > idea for the additional read_avail_ext() or the additional argument for the
> > > > > > > read_avail() we were considering in [1]. So I would think that
> > > > > > > iio_read_channel_info_avail() would do something like the following:
> > > > > > > 
> > > > > > >     if (indio_dev->info->read_avail_with_copy)
> > > > > > >         indio_dev->info->read_avail_with_copy(vals);
> > > > > > >     else
> > > > > > >         indio_dev->info->read_avail(vals);
> > > > > > > 
> > > > > > >     ...
> > > > > > >     iio_format_avail_list(vals);
> > > > > > >     ...
> > > > > > > 
> > > > > > >     if (indio_dev->info->read_avail_with_copy)
> > > > > > >         kfree(vals);
> > > 
> > > Right. At least that's what I see can be done with the existing users.
> > > 
> > > > > > Ok, sure that would work, but...
> > > > > > 
> > > > > > I don't really see this as being much less fragile than
> > > > > > the existing solution + in cases that we do have where
> > > > > > only some available are not const we will have to copy them
> > > > > > all.
> > > > > > 
> > > > > > If anything it's more complex than making it a driver problem
> > > > > > to provide the release call however it wants to do it.
> > > 
> > > ...but make a driver to allocate what's needed as well then.
> > > 
> > > > > > > And the drivers would choose whether to define the read_avail or the
> > > > > > > read_avail_with_copy.
> > > 
> > > Either way drivers should know what to do with a data supplied to read_aval().
> > > In one case we assume the [simple] workflow in the core, in the other we all
> > > rely on the driver. Current approach makes a mix of these two. And that's what
> > > I don't like.
> > 
> > If I understand your concern correctly, you are referring to the inkern
> > iio_channel_read_avail() that makes the allocation for the consumer's
> > buffer copy and you are suggesting that such copy should be done by the
> > consumer driver code itself, this to be consistent with the producer
> > drivers which directly handle the allocation of the copy.
> 
> One of the options, yes.
> 
> > One thing to notice is that the inkern iio_channel_read_avail() does
> > together producer->read_avail() + copy + producer->read_avail_release()
> > with info_exists locked. Also, the consumer driver would need to know
> > the avail buffer size to allocate the buffer copy prior the
> > iio_channel_read_avail() call, but such size is unknown before calling
> > the actual producer's read_avail(). This would mean calling the
> > producer's read_avail() and read_avail_release() callbacks separately
> > without the lock held, with the risk of a memleak if the producer is
> > unregistered between those calls.
> 
> Thanks for explaining this, but it even more makes me think that the design
> is broken and your approach is rather a hack. So, what's the problem to
> make IIO core to take care of the allocating and cleaning then without driver
> being involved? Yes, this might require a hint from the driver on what to copy
> if we want to avoid copying everything.
> 

I am not particularly against it, other than the concerns that have
emerged during this (and previous) discussion. Let me summarize them:

fix-1) the current one. Your concerns are:
    * for consumers the copy allocation is taken care by the inkern API
      but the release is handled by the consumer driver code, making it
      a fragile design.
    * consumers and producers manage the allocation differently, the
      first handles it via the inkern API, the second one in the
      producer driver code, making it inconsistent.

fix-2) adding a read_avail_with_copy(): a driver with both const avail
lists and mutable avail lists would always return a copy for all of
them, including the const ones. Example above.

fix-3) adding a release_avail return param to read_avail(): this would
require a change to all the drivers using it. Also it
looks to me an unusual pattern, are there other similar patterns around
the codebase? Example below.

fix-4) adding a new enum variant to the avail type like
IIO_AVAIL_LIST_ALLOC: to me this looks hacky as it mixes the logic type
of the data structure and how it is handled in memory. I think the
latter should better fit in a different field, however this modification
would have little impact in the current code. Example below.

So far these alternatives only consider moving the release of the copy
buffer in the IIO core but not its allocation. You also suggest to make
the IIO core take care of the copy allocation. The problem I see with
this is that if the copy is handled outside the driver it could take
place concurrently with the modification of the original buffer since it
would not be locked by driver private mutex, thus making the copy
useless. This might be worked around by adding an additional optional
callback (e.g. read_avail_will_copy/read_avail_is_mutable) to just take
the size and check if a copy will be provided, so maybe something like:

fix-5) iio_read_channel_info_avail():
{
    ...
    int *vals;
    bool copy = false;
    if (indio_dev->info->read_avail_will_copy) {
        copy = indio_dev->info->read_avail_will_copy(..., &length, ...);
        if (copy) {
            vals = kcalloc(length, sizeof(int), GFP_KERNEL);
        }
    }

    indio_dev->info->read_avail(&vals, ...);

    if (ret < 0)
            return ret;
    switch (ret) {
    case IIO_AVAIL_LIST:
            ret = iio_format_avail_list(buf, vals, type, length);
    case IIO_AVAIL_RANGE:
            ret = iio_format_avail_range(buf, vals, type);
    default:
            ret = -EINVAL;
    }

    if (copy)
        kfree(vals);
}

If I am not missing anything this could work and maybe it could also
avoid the double copy on the consumers but would require all of them to
wrap the read_avail_will_copy(). Also, I find it quite a weird pattern
that in some cases vals would be an input buffer to be filled and in
other cases it would be a return argument pointing to the const buffer
stored inside the driver. At least I wouldn't say it's more robust than
the current fix-1.

All these alternatives also prevents some potential optimization already
mentioned before. Reporting it again as it is now lost in the mess below:
    Some driver might want to avoid allocating a new copy of a big table if
    the race does not occur (e.g. with additional checks on buffer access
    code) and thus wouldn't call a free() in the release callback.

In the end I don't find any of the above alternatives to provide an
obvious better solution.

> > > > > > > What I was referring to is that, back then, you mentioned you would have
> > > > > > > preferred to avoid passing ownership of the buffer around:
> > > > > > > 
> > > > > > > > That's a corner case we should think about closing. Would require an indicator
> > > > > > > > to read_avail that the buffer it has been passed is a snapshot that it should
> > > > > > > > free on completion of the string building.  I don't like passing ownership
> > > > > > > > of data around like that, but it is fiddly to do anything else given
> > > > > > > > any simple double buffering is subject to race conditions.  
> > > > > > > 
> > > > > > > I guess there is some other reason other than avoiding the copy when not
> > > > > > > necessary, since by introducing an additional function or argument or return
> > > > > > > type, most of the unnecessary copies would already be avoided right?
> > > > > > 
> > > > > > It's not a strong reason beyond limiting scope of clever design +
> > > > > > the key bit my mind is that the above is not substantially simpler and
> > > > > > reduces our flexibility.
> > > > > > 
> > > > > > > Anyway any of this solutions would still prevent the potential optimizations of
> > > > > > > point 2). It's worth mentioning that those kind of optimizations are currently
> > > > > > > not adopted by any driver.
> > > > > > 
> > > > > > That one indeed not, but mixing dynamic and non dynamic is something
> > > > > > you do in your pac1921 patch.
> > > > > 
> > > > > Good point! I didn't think about it, or more likely I forgot, that with an
> > > > > additional read_avail_with_copy() used as in the example you cannot mix dynamic
> > > > > and non dynamic available lists, thus those drivers that need at least one
> > > > > dynamic available list would always copy all of them as they need to rely to
> > > > > the read_avail_with_copy(). I guess this could be worked around with an
> > > > > additional return argument for the read_avail() or an additional type like the
> > > > > IIO_AVAIL_LIST_ALLOC suggested by Andy to signal the caller it needs to free
> > > > > the list after use. However, I think they would introduce a more invasive
> > > > > change in the current API compared to an additional optional callback,
> > > 
> > > It even sounds originally that it should be more invasive, so I don't think it's
> > > a problem here.
> > 
> > In the hope it helps the discussion let me provide examples for the
> > additional two options we have other than the current
> > read_avail_release_resource() (fix-1) and the read_avail_with_copy()
> > (fix-2) already shown above:
> 
> Thanks!
> 
> > fix-3) iio_read_channel_info_avail():
> > {
> >     ...
> >     bool release_avail = false;
> > 
> >     ret = indio_dev->info->read_avail(vals, ..., &release_avail);
> > 
> >     ...
> >     ret = iio_format_avail_list(vals, ...);
> >     ...
> > 
> >     if (release_avail)
> >         kfree(vals);
> > 
> >     return ret;
> > }
> > 
> > 
> > fix-4) iio_read_channel_info_avail():
> > {
> >     ...
> >     indio_dev->info->read_avail(vals, ...);
> > 
> >     if (ret < 0)
> >             return ret;
> >     switch (ret) {
> >     case IIO_AVAIL_LIST_ALLOC:
> >             ret = iio_format_avail_list(buf, vals, type, length);
> >             kfree(vals);
> >             return ret;
> >     case IIO_AVAIL_LIST:
> >             return iio_format_avail_list(buf, vals, type, length);
> >     case IIO_AVAIL_RANGE:
> >             return iio_format_avail_range(buf, vals, type);
> >     default:
> >             return -EINVAL;
> >     }
> > 
> > }
> > 
> > > > > so I agree that the current release callback is still a better option.
> > > 
> > > I disagree on this as I pointed above why.
> > > 
> > > > > > > > > 2) Some driver might want to avoid allocating a new copy of a big table if
> > > > > > > > >    the race does not occur (e.g. with additional checks on buffer access
> > > > > > > > >    code) and thus wouldn't call a free() in the release callback.
> > > > > > > > >   
> > > > > > > > > > In any case it looks fragile and not scalable. I propose to drop this
> > > > > > > > > > and think again.    
> > > > > > > > > 
> > > > > > > > > I see your concerns, I am open to reconsider this in case we come up with
> > > > > > > > > better solution after addressing the points above.
> > > > > > > > >   
> > > > > > > > > > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > > > > > > > > > agree on the report, I think this solution is not good enough.
> > > > > > > > > 
> > > > > > > > > [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/
> > > > > > > 
> > > > > > > I hope I've brought a little more clarity to the discussion by providing some
> > > > > > > history instead of making it more confusing.
> > > > > > 
> > > > > > Sure, the code example in particular is useful.
> > > > 
> > > > Just a friendly reminder this has been sitting for a while, any news or
> > > > additional considerations?
> > > 
> > > Moving the allocation control to the drivers will satisfy me as well, however
> > > it makes even more duplication of the code, but at least it will be cleaner
> > > design-wise in my opinion.
> > 
> > Would it work with the constraints on the info_exists lock mentioned
> > above?
> 
> None of the given examples (fix-N) provides a lock, so I have no clue how it's
> involved here. May be you can elaborate more?
> 

I thought that with "Moving the allocation control to the drivers" you
were referring to the option (not included among fix-N) to move the
allocation of the consumer copy from the inkern iio_channel_read_avail()
to the consumer drivers themselves. You elaborated this point above
where I answered with the concerns about the info_exists lock that
should be addressed.

> > > In any case the last word is on Jonathan.
> 
> -- 
> With Best Regards,
> Andy Shevchenko
> 
> 

Best regards,
Matteo Martelli

Andy Shevchenko Nov. 19, 2024, 12:05 p.m. UTC | #13

On Tue, Nov 19, 2024 at 12:25:18PM +0100, Matteo Martelli wrote:
> On Mon, 18 Nov 2024 18:05:35 +0200, Andy Shevchenko <[e-mail]> wrote:
> > On Mon, Nov 18, 2024 at 03:45:25PM +0100, Matteo Martelli wrote:
> > > On Mon, 18 Nov 2024 12:21:44 +0200, Andy Shevchenko <[e-mail]> wrote:
> > > > On Fri, Nov 15, 2024 at 03:25:06PM +0100, Matteo Martelli wrote:
> > > > > On Thu, 31 Oct 2024 19:06:32 +0100, Matteo Martelli <[e-mail]> wrote:
> > > > > > Quoting Jonathan Cameron (2024-10-31 15:31:29)
> > > > > > > On Thu, 31 Oct 2024 12:26:24 +0100
> > > > > > > Matteo Martelli <[e-mail]> wrote:
> > > > > > > > Quoting Jonathan Cameron (2024-10-30 21:30:50)
> > > > > > > > > On Wed, 30 Oct 2024 19:23:21 +0100
> > > > > > > > > Matteo Martelli <[e-mail]> wrote:
> > > > > > > > > > Quoting Andy Shevchenko (2024-10-30 15:47:50)  
> > > > > > > > > > > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:    

> > > > > > > > > > > > Consumers need to call the producer's read_avail_release_resource()
> > > > > > > > > > > > callback after reading producer's available info. To avoid a race
> > > > > > > > > > > > condition with the producer unregistration, change inkern
> > > > > > > > > > > > iio_channel_read_avail() so that it copies the available info from the
> > > > > > > > > > > > producer and immediately calls its release callback with info_exists
> > > > > > > > > > > > locked.
> > > > > > > > > > > > 
> > > > > > > > > > > > Also, modify the users of iio_read_avail_channel_raw() and
> > > > > > > > > > > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > > > > > > > > > > after calling these functions. To let users free the copied buffer with
> > > > > > > > > > > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > > > > > > > > > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > > > > > > > > > > but stores the available values in the returned variable.    

...

> > > > > > > > > > > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > > > > > > > > > > +                                         struct iio_chan_spec const *chan,
> > > > > > > > > > > > +                                         const int *vals, long mask)
> > > > > > > > > > > > +{
> > > > > > > > > > > > +     kfree(vals);
> > > > > > > > > > > > +}
> > > > > > > > > > > > +
> > > > > > > > > > > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > > > > > >                             struct iio_chan_spec const *chan,
> > > > > > > > > > > >                             int val, int val2, long mask)
> > > > > > > > > > > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > > > > > >  static const struct iio_info dpot_dac_info = {
> > > > > > > > > > > >       .read_raw = dpot_dac_read_raw,
> > > > > > > > > > > >       .read_avail = dpot_dac_read_avail,
> > > > > > > > > > > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > > > > > > > > > >       .write_raw = dpot_dac_write_raw,
> > > > > > > > > > > >  };    
> > > > > > > > > > > 
> > > > > > > > > > > I have a problem with this approach. The issue is that we allocate
> > > > > > > > > > > memory in one place and must clear it in another. This is not well
> > > > > > > > > > > designed thingy in my opinion. I was thinking a bit of the solution and
> > > > > > > > > > > at least these two comes to my mind:
> > > > > > > > > > > 
> > > > > > > > > > > 1) having a special callback for .read_avail_with_copy (choose better
> > > > > > > > > > > name) that will dump the data to the intermediate buffer and clean it
> > > > > > > > > > > after all;
> > > > > > > > > > > 
> > > > > > > > > > > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.    
> > > > > > > > > > 
> > > > > > > > > > Could you elaborate more about these potential solutions? Maybe with some
> > > > > > > > > > usage examples?
> > > > > > > > > > 
> > > > > > > > > > If I get it correctly, in both cases you are suggesting to pass ownership
> > > > > > > > > > of the vals buffer to the caller, iio_read_channel_info_avail() in this
> > > > > > > > > > case, so that it would take care of freeing the buffer after calling
> > > > > > > > > > iio_format_after_*(). We considered this approach during an initial
> > > > > > > > > > discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> > > > > > > > > > to let the driver keep the release control through a callback for two
> > > > > > > > > > reasons:
> > > > > > > > > > 
> > > > > > > > > > 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
> > > > > > > > > >    maybe Jonathan can elaborate why? The risk I can think of is that the driver
> > > > > > > > > >    could still keep the buffer copy in its private data after giving it away,
> > > > > > > > > >    resulting in fact in a double ownership. However I think it would be clear
> > > > > > > > > >    enough in this case that the copy should be handled by the caller, or maybe
> > > > > > > > > >    not?  
> > > > > > > > > Mostly the lack of desire to have to copy for the 95% of cases where it's
> > > > > > > > > not needed and that it prevents any optimization like you mention.  
> > > > > > > > 
> > > > > > > > I think the suggestion here is to add an additional .read_avail_with_copy()
> > > > > > > > without replacing the original .read_avail(), so all the current drivers that
> > > > > > > > use a constant avail list would not be affected.

> > > > Yes.

> > > > > > > > And I think this was the same
> > > > > > > > idea for the additional read_avail_ext() or the additional argument for the
> > > > > > > > read_avail() we were considering in [1]. So I would think that
> > > > > > > > iio_read_channel_info_avail() would do something like the following:
> > > > > > > > 
> > > > > > > >     if (indio_dev->info->read_avail_with_copy)
> > > > > > > >         indio_dev->info->read_avail_with_copy(vals);
> > > > > > > >     else
> > > > > > > >         indio_dev->info->read_avail(vals);
> > > > > > > > 
> > > > > > > >     ...
> > > > > > > >     iio_format_avail_list(vals);
> > > > > > > >     ...
> > > > > > > > 
> > > > > > > >     if (indio_dev->info->read_avail_with_copy)
> > > > > > > >         kfree(vals);
> > > > 
> > > > Right. At least that's what I see can be done with the existing users.
> > > > 
> > > > > > > Ok, sure that would work, but...
> > > > > > > 
> > > > > > > I don't really see this as being much less fragile than
> > > > > > > the existing solution + in cases that we do have where
> > > > > > > only some available are not const we will have to copy them
> > > > > > > all.
> > > > > > > 
> > > > > > > If anything it's more complex than making it a driver problem
> > > > > > > to provide the release call however it wants to do it.
> > > > 
> > > > ...but make a driver to allocate what's needed as well then.
> > > > 
> > > > > > > > And the drivers would choose whether to define the read_avail or the
> > > > > > > > read_avail_with_copy.
> > > > 
> > > > Either way drivers should know what to do with a data supplied to read_aval().
> > > > In one case we assume the [simple] workflow in the core, in the other we all
> > > > rely on the driver. Current approach makes a mix of these two. And that's what
> > > > I don't like.
> > > 
> > > If I understand your concern correctly, you are referring to the inkern
> > > iio_channel_read_avail() that makes the allocation for the consumer's
> > > buffer copy and you are suggesting that such copy should be done by the
> > > consumer driver code itself, this to be consistent with the producer
> > > drivers which directly handle the allocation of the copy.
> > 
> > One of the options, yes.
> > 
> > > One thing to notice is that the inkern iio_channel_read_avail() does
> > > together producer->read_avail() + copy + producer->read_avail_release()
> > > with info_exists locked. Also, the consumer driver would need to know
> > > the avail buffer size to allocate the buffer copy prior the
> > > iio_channel_read_avail() call, but such size is unknown before calling
> > > the actual producer's read_avail(). This would mean calling the
> > > producer's read_avail() and read_avail_release() callbacks separately
> > > without the lock held, with the risk of a memleak if the producer is
> > > unregistered between those calls.
> > 
> > Thanks for explaining this, but it even more makes me think that the design
> > is broken and your approach is rather a hack. So, what's the problem to
> > make IIO core to take care of the allocating and cleaning then without driver
> > being involved? Yes, this might require a hint from the driver on what to copy
> > if we want to avoid copying everything.
> 
> I am not particularly against it, other than the concerns that have
> emerged during this (and previous) discussion. Let me summarize them:

Thank you for a very good summary and fix-N proposals. I think I have nothing
to add and we should wait for Jonathan to finally choose (or propose a fix-N+1)
here.

> fix-1) the current one. Your concerns are:
>     * for consumers the copy allocation is taken care by the inkern API
>       but the release is handled by the consumer driver code, making it
>       a fragile design.
>     * consumers and producers manage the allocation differently, the
>       first handles it via the inkern API, the second one in the
>       producer driver code, making it inconsistent.
> 
> fix-2) adding a read_avail_with_copy(): a driver with both const avail
> lists and mutable avail lists would always return a copy for all of
> them, including the const ones. Example above.
> 
> fix-3) adding a release_avail return param to read_avail(): this would
> require a change to all the drivers using it. Also it
> looks to me an unusual pattern, are there other similar patterns around
> the codebase? Example below.
> 
> fix-4) adding a new enum variant to the avail type like
> IIO_AVAIL_LIST_ALLOC: to me this looks hacky as it mixes the logic type
> of the data structure and how it is handled in memory. I think the
> latter should better fit in a different field, however this modification
> would have little impact in the current code. Example below.
> 
> So far these alternatives only consider moving the release of the copy
> buffer in the IIO core but not its allocation. You also suggest to make
> the IIO core take care of the copy allocation. The problem I see with
> this is that if the copy is handled outside the driver it could take
> place concurrently with the modification of the original buffer since it
> would not be locked by driver private mutex, thus making the copy
> useless. This might be worked around by adding an additional optional
> callback (e.g. read_avail_will_copy/read_avail_is_mutable) to just take
> the size and check if a copy will be provided, so maybe something like:
> 
> fix-5) iio_read_channel_info_avail():
> {
>     ...
>     int *vals;
>     bool copy = false;
>     if (indio_dev->info->read_avail_will_copy) {
>         copy = indio_dev->info->read_avail_will_copy(..., &length, ...);
>         if (copy) {
>             vals = kcalloc(length, sizeof(int), GFP_KERNEL);
>         }
>     }
> 
>     indio_dev->info->read_avail(&vals, ...);
> 
>     if (ret < 0)
>             return ret;
>     switch (ret) {
>     case IIO_AVAIL_LIST:
>             ret = iio_format_avail_list(buf, vals, type, length);
>     case IIO_AVAIL_RANGE:
>             ret = iio_format_avail_range(buf, vals, type);
>     default:
>             ret = -EINVAL;
>     }
> 
>     if (copy)
>         kfree(vals);
> }
> 
> If I am not missing anything this could work and maybe it could also
> avoid the double copy on the consumers but would require all of them to
> wrap the read_avail_will_copy(). Also, I find it quite a weird pattern
> that in some cases vals would be an input buffer to be filled and in
> other cases it would be a return argument pointing to the const buffer
> stored inside the driver. At least I wouldn't say it's more robust than
> the current fix-1.
> 
> All these alternatives also prevents some potential optimization already
> mentioned before. Reporting it again as it is now lost in the mess below:
>     Some driver might want to avoid allocating a new copy of a big table if
>     the race does not occur (e.g. with additional checks on buffer access
>     code) and thus wouldn't call a free() in the release callback.
> 
> In the end I don't find any of the above alternatives to provide an
> obvious better solution.
> 
> > > > > > > > What I was referring to is that, back then, you mentioned you would have
> > > > > > > > preferred to avoid passing ownership of the buffer around:
> > > > > > > > 
> > > > > > > > > That's a corner case we should think about closing. Would require an indicator
> > > > > > > > > to read_avail that the buffer it has been passed is a snapshot that it should
> > > > > > > > > free on completion of the string building.  I don't like passing ownership
> > > > > > > > > of data around like that, but it is fiddly to do anything else given
> > > > > > > > > any simple double buffering is subject to race conditions.  
> > > > > > > > 
> > > > > > > > I guess there is some other reason other than avoiding the copy when not
> > > > > > > > necessary, since by introducing an additional function or argument or return
> > > > > > > > type, most of the unnecessary copies would already be avoided right?
> > > > > > > 
> > > > > > > It's not a strong reason beyond limiting scope of clever design +
> > > > > > > the key bit my mind is that the above is not substantially simpler and
> > > > > > > reduces our flexibility.
> > > > > > > 
> > > > > > > > Anyway any of this solutions would still prevent the potential optimizations of
> > > > > > > > point 2). It's worth mentioning that those kind of optimizations are currently
> > > > > > > > not adopted by any driver.
> > > > > > > 
> > > > > > > That one indeed not, but mixing dynamic and non dynamic is something
> > > > > > > you do in your pac1921 patch.
> > > > > > 
> > > > > > Good point! I didn't think about it, or more likely I forgot, that with an
> > > > > > additional read_avail_with_copy() used as in the example you cannot mix dynamic
> > > > > > and non dynamic available lists, thus those drivers that need at least one
> > > > > > dynamic available list would always copy all of them as they need to rely to
> > > > > > the read_avail_with_copy(). I guess this could be worked around with an
> > > > > > additional return argument for the read_avail() or an additional type like the
> > > > > > IIO_AVAIL_LIST_ALLOC suggested by Andy to signal the caller it needs to free
> > > > > > the list after use. However, I think they would introduce a more invasive
> > > > > > change in the current API compared to an additional optional callback,
> > > > 
> > > > It even sounds originally that it should be more invasive, so I don't think it's
> > > > a problem here.
> > > 
> > > In the hope it helps the discussion let me provide examples for the
> > > additional two options we have other than the current
> > > read_avail_release_resource() (fix-1) and the read_avail_with_copy()
> > > (fix-2) already shown above:
> > 
> > Thanks!
> > 
> > > fix-3) iio_read_channel_info_avail():
> > > {
> > >     ...
> > >     bool release_avail = false;
> > > 
> > >     ret = indio_dev->info->read_avail(vals, ..., &release_avail);
> > > 
> > >     ...
> > >     ret = iio_format_avail_list(vals, ...);
> > >     ...
> > > 
> > >     if (release_avail)
> > >         kfree(vals);
> > > 
> > >     return ret;
> > > }
> > > 
> > > 
> > > fix-4) iio_read_channel_info_avail():
> > > {
> > >     ...
> > >     indio_dev->info->read_avail(vals, ...);
> > > 
> > >     if (ret < 0)
> > >             return ret;
> > >     switch (ret) {
> > >     case IIO_AVAIL_LIST_ALLOC:
> > >             ret = iio_format_avail_list(buf, vals, type, length);
> > >             kfree(vals);
> > >             return ret;
> > >     case IIO_AVAIL_LIST:
> > >             return iio_format_avail_list(buf, vals, type, length);
> > >     case IIO_AVAIL_RANGE:
> > >             return iio_format_avail_range(buf, vals, type);
> > >     default:
> > >             return -EINVAL;
> > >     }
> > > }
> > > 
> > > > > > so I agree that the current release callback is still a better option.
> > > > 
> > > > I disagree on this as I pointed above why.
> > > > 
> > > > > > > > > > 2) Some driver might want to avoid allocating a new copy of a big table if
> > > > > > > > > >    the race does not occur (e.g. with additional checks on buffer access
> > > > > > > > > >    code) and thus wouldn't call a free() in the release callback.
> > > > > > > > > >   
> > > > > > > > > > > In any case it looks fragile and not scalable. I propose to drop this
> > > > > > > > > > > and think again.    
> > > > > > > > > > 
> > > > > > > > > > I see your concerns, I am open to reconsider this in case we come up with
> > > > > > > > > > better solution after addressing the points above.
> > > > > > > > > >   
> > > > > > > > > > > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > > > > > > > > > > agree on the report, I think this solution is not good enough.
> > > > > > > > > > 
> > > > > > > > > > [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/
> > > > > > > > 
> > > > > > > > I hope I've brought a little more clarity to the discussion by providing some
> > > > > > > > history instead of making it more confusing.
> > > > > > > 
> > > > > > > Sure, the code example in particular is useful.
> > > > > 
> > > > > Just a friendly reminder this has been sitting for a while, any news or
> > > > > additional considerations?
> > > > 
> > > > Moving the allocation control to the drivers will satisfy me as well, however
> > > > it makes even more duplication of the code, but at least it will be cleaner
> > > > design-wise in my opinion.
> > > 
> > > Would it work with the constraints on the info_exists lock mentioned
> > > above?
> > 
> > None of the given examples (fix-N) provides a lock, so I have no clue how it's
> > involved here. May be you can elaborate more?
> 
> I thought that with "Moving the allocation control to the drivers" you
> were referring to the option (not included among fix-N) to move the
> allocation of the consumer copy from the inkern iio_channel_read_avail()
> to the consumer drivers themselves. You elaborated this point above
> where I answered with the concerns about the info_exists lock that
> should be addressed.
> 
> > > > In any case the last word is on Jonathan.

Jonathan Cameron Nov. 23, 2024, 2:13 p.m. UTC | #14

On Tue, 19 Nov 2024 14:05:31 +0200
Andy Shevchenko <[e-mail]> wrote:

> On Tue, Nov 19, 2024 at 12:25:18PM +0100, Matteo Martelli wrote:
> > On Mon, 18 Nov 2024 18:05:35 +0200, Andy Shevchenko <[e-mail]> wrote:  
> > > On Mon, Nov 18, 2024 at 03:45:25PM +0100, Matteo Martelli wrote:  
> > > > On Mon, 18 Nov 2024 12:21:44 +0200, Andy Shevchenko <[e-mail]> wrote:  
> > > > > On Fri, Nov 15, 2024 at 03:25:06PM +0100, Matteo Martelli wrote:  
> > > > > > On Thu, 31 Oct 2024 19:06:32 +0100, Matteo Martelli <[e-mail]> wrote:  
> > > > > > > Quoting Jonathan Cameron (2024-10-31 15:31:29)  
> > > > > > > > On Thu, 31 Oct 2024 12:26:24 +0100
> > > > > > > > Matteo Martelli <[e-mail]> wrote:  
> > > > > > > > > Quoting Jonathan Cameron (2024-10-30 21:30:50)  
> > > > > > > > > > On Wed, 30 Oct 2024 19:23:21 +0100
> > > > > > > > > > Matteo Martelli <[e-mail]> wrote:  
> > > > > > > > > > > Quoting Andy Shevchenko (2024-10-30 15:47:50)    
> > > > > > > > > > > > On Mon, Oct 21, 2024 at 02:54:15PM +0200, Matteo Martelli wrote:      
> 
> > > > > > > > > > > > > Consumers need to call the producer's read_avail_release_resource()
> > > > > > > > > > > > > callback after reading producer's available info. To avoid a race
> > > > > > > > > > > > > condition with the producer unregistration, change inkern
> > > > > > > > > > > > > iio_channel_read_avail() so that it copies the available info from the
> > > > > > > > > > > > > producer and immediately calls its release callback with info_exists
> > > > > > > > > > > > > locked.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > Also, modify the users of iio_read_avail_channel_raw() and
> > > > > > > > > > > > > iio_read_avail_channel_attribute() to free the copied available buffers
> > > > > > > > > > > > > after calling these functions. To let users free the copied buffer with
> > > > > > > > > > > > > a cleanup pattern, also add a iio_read_avail_channel_attr_retvals()
> > > > > > > > > > > > > consumer helper that is equivalent to iio_read_avail_channel_attribute()
> > > > > > > > > > > > > but stores the available values in the returned variable.      
> 
> ...
> 
> > > > > > > > > > > > > +static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
> > > > > > > > > > > > > +                                         struct iio_chan_spec const *chan,
> > > > > > > > > > > > > +                                         const int *vals, long mask)
> > > > > > > > > > > > > +{
> > > > > > > > > > > > > +     kfree(vals);
> > > > > > > > > > > > > +}
> > > > > > > > > > > > > +
> > > > > > > > > > > > >  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > > > > > > >                             struct iio_chan_spec const *chan,
> > > > > > > > > > > > >                             int val, int val2, long mask)
> > > > > > > > > > > > > @@ -125,6 +132,7 @@ static int dpot_dac_write_raw(struct iio_dev *indio_dev,
> > > > > > > > > > > > >  static const struct iio_info dpot_dac_info = {
> > > > > > > > > > > > >       .read_raw = dpot_dac_read_raw,
> > > > > > > > > > > > >       .read_avail = dpot_dac_read_avail,
> > > > > > > > > > > > > +     .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > > > > > > > > > > >       .write_raw = dpot_dac_write_raw,
> > > > > > > > > > > > >  };      
> > > > > > > > > > > > 
> > > > > > > > > > > > I have a problem with this approach. The issue is that we allocate
> > > > > > > > > > > > memory in one place and must clear it in another. This is not well
> > > > > > > > > > > > designed thingy in my opinion. I was thinking a bit of the solution and
> > > > > > > > > > > > at least these two comes to my mind:
> > > > > > > > > > > > 
> > > > > > > > > > > > 1) having a special callback for .read_avail_with_copy (choose better
> > > > > > > > > > > > name) that will dump the data to the intermediate buffer and clean it
> > > > > > > > > > > > after all;
> > > > > > > > > > > > 
> > > > > > > > > > > > 2) introduce a new type (or bit there), like IIO_AVAIL_LIST_ALLOC.      
> > > > > > > > > > > 
> > > > > > > > > > > Could you elaborate more about these potential solutions? Maybe with some
> > > > > > > > > > > usage examples?
> > > > > > > > > > > 
> > > > > > > > > > > If I get it correctly, in both cases you are suggesting to pass ownership
> > > > > > > > > > > of the vals buffer to the caller, iio_read_channel_info_avail() in this
> > > > > > > > > > > case, so that it would take care of freeing the buffer after calling
> > > > > > > > > > > iio_format_after_*(). We considered this approach during an initial
> > > > > > > > > > > discussion with Jonathan (see read_avail_ext() in [1]), where he suggested
> > > > > > > > > > > to let the driver keep the release control through a callback for two
> > > > > > > > > > > reasons:
> > > > > > > > > > > 
> > > > > > > > > > > 1) Apparently it's a bad pattern to pass the buffer ownership to the core,
> > > > > > > > > > >    maybe Jonathan can elaborate why? The risk I can think of is that the driver
> > > > > > > > > > >    could still keep the buffer copy in its private data after giving it away,
> > > > > > > > > > >    resulting in fact in a double ownership. However I think it would be clear
> > > > > > > > > > >    enough in this case that the copy should be handled by the caller, or maybe
> > > > > > > > > > >    not?    
> > > > > > > > > > Mostly the lack of desire to have to copy for the 95% of cases where it's
> > > > > > > > > > not needed and that it prevents any optimization like you mention.    
> > > > > > > > > 
> > > > > > > > > I think the suggestion here is to add an additional .read_avail_with_copy()
> > > > > > > > > without replacing the original .read_avail(), so all the current drivers that
> > > > > > > > > use a constant avail list would not be affected.  
> 
> > > > > Yes.  
> 
> > > > > > > > > And I think this was the same
> > > > > > > > > idea for the additional read_avail_ext() or the additional argument for the
> > > > > > > > > read_avail() we were considering in [1]. So I would think that
> > > > > > > > > iio_read_channel_info_avail() would do something like the following:
> > > > > > > > > 
> > > > > > > > >     if (indio_dev->info->read_avail_with_copy)
> > > > > > > > >         indio_dev->info->read_avail_with_copy(vals);
> > > > > > > > >     else
> > > > > > > > >         indio_dev->info->read_avail(vals);
> > > > > > > > > 
> > > > > > > > >     ...
> > > > > > > > >     iio_format_avail_list(vals);
> > > > > > > > >     ...
> > > > > > > > > 
> > > > > > > > >     if (indio_dev->info->read_avail_with_copy)
> > > > > > > > >         kfree(vals);  
> > > > > 
> > > > > Right. At least that's what I see can be done with the existing users.
> > > > >   
> > > > > > > > Ok, sure that would work, but...
> > > > > > > > 
> > > > > > > > I don't really see this as being much less fragile than
> > > > > > > > the existing solution + in cases that we do have where
> > > > > > > > only some available are not const we will have to copy them
> > > > > > > > all.
> > > > > > > > 
> > > > > > > > If anything it's more complex than making it a driver problem
> > > > > > > > to provide the release call however it wants to do it.  
> > > > > 
> > > > > ...but make a driver to allocate what's needed as well then.
> > > > >   
> > > > > > > > > And the drivers would choose whether to define the read_avail or the
> > > > > > > > > read_avail_with_copy.  
> > > > > 
> > > > > Either way drivers should know what to do with a data supplied to read_aval().
> > > > > In one case we assume the [simple] workflow in the core, in the other we all
> > > > > rely on the driver. Current approach makes a mix of these two. And that's what
> > > > > I don't like.  
> > > > 
> > > > If I understand your concern correctly, you are referring to the inkern
> > > > iio_channel_read_avail() that makes the allocation for the consumer's
> > > > buffer copy and you are suggesting that such copy should be done by the
> > > > consumer driver code itself, this to be consistent with the producer
> > > > drivers which directly handle the allocation of the copy.  
> > > 
> > > One of the options, yes.
> > >   
> > > > One thing to notice is that the inkern iio_channel_read_avail() does
> > > > together producer->read_avail() + copy + producer->read_avail_release()
> > > > with info_exists locked. Also, the consumer driver would need to know
> > > > the avail buffer size to allocate the buffer copy prior the
> > > > iio_channel_read_avail() call, but such size is unknown before calling
> > > > the actual producer's read_avail(). This would mean calling the
> > > > producer's read_avail() and read_avail_release() callbacks separately
> > > > without the lock held, with the risk of a memleak if the producer is
> > > > unregistered between those calls.  
> > > 
> > > Thanks for explaining this, but it even more makes me think that the design
> > > is broken and your approach is rather a hack. So, what's the problem to
> > > make IIO core to take care of the allocating and cleaning then without driver
> > > being involved? Yes, this might require a hint from the driver on what to copy
> > > if we want to avoid copying everything.  
> > 
> > I am not particularly against it, other than the concerns that have
> > emerged during this (and previous) discussion. Let me summarize them:  
> 
> Thank you for a very good summary and fix-N proposals. I think I have nothing
> to add and we should wait for Jonathan to finally choose (or propose a fix-N+1)
> here.
Agreed. This is very useful enumeration of various options with plenty
of details!

One absolute key thing to note here is we should not care at all what
inkern does for it's handling internally of the available lists.  The big
ABI question is all about consumers drivers directly using the resulting list of
available values.  The use in the IIO core and the inkern helpers should
naturally follow.


> 
> > fix-1) the current one. Your concerns are:
> >     * for consumers the copy allocation is taken care by the inkern API
> >       but the release is handled by the consumer driver code, making it
> >       a fragile design.

So this was something I'm not sure I agree with.  There are plenty
of get / release patterns out there. This is just another one of those
but perhaps it doesn't 'smell' enough like that.

Perhaps think of it as

int *iio_channel_avail_get()
void iio_channel_avail_release()

We could perhaps make it look more standard using a cookie rather than
reconstructing the equivalent data at the release call.

struct iio_avail_cookie {
	const int *avail;
	void *provider_priv;
// see later for a maybe...
	struct iio_dev *indio_dev;
};

const int *iio_avail_from_cookie(struct iio_avail_cookie *cookie)
{
	return cookie->avail;
}

struct iio_avail_cookie *iio_channel_avail_get(struct iio_dev, struct iio_chan_spec)
{
	allocate a cookie and fill it in.
}

and code would always explicitly release after it is done with the cookie.

Something like

void iio_channel_avail_release(struct iio_dev *iio_dev, struct iio_avail_cookie *cookie)
// could even move the iio_dev pointer into the cookie, so it becomes
// iio_channel_avail_release(struct iio_avail_cookie *cookie) and suitable for __free magic.
{
	if (iio_dev->info->avail_release)
		iio_dev->info->avail_release(cookie);
	kfree(cookie);
	/*
	 * Could add optimizations around cookie handling to avoid alloc + free in most cases
	 * or use an object pool.
	 */
}

The current proposal just avoid the need for a cookie as for all known cases so far
provider_priv could == the channel requested.


> >     * consumers and producers manage the allocation differently, the
> >       first handles it via the inkern API, the second one in the
> >       producer driver code, making it inconsistent.

The inkern API changes are mostly an attempt to reduce boiler plate. The only
case we really should be worrying about to my mind is the consumer wanting
to access the full available list. 

> > 
> > fix-2) adding a read_avail_with_copy(): a driver with both const avail
> > lists and mutable avail lists would always return a copy for all of
> > them, including the const ones. Example above.

Hmm. So this could work but with the firm rule that a provider must never
provide both options and a core check on drivers to enforce that probe.
Any existing consumers must be modified to try both paths
(read_avail_with_copy then read_avail) to avoid regressions.

For future code, if we miss a case that doesn't do this then the upshot
is that the call will fail and the consumer needs fixing but at least
it is not a regression because it will never have worked for that
particular consumer + producer pair.  Not too horrible, but I'm not
really seeing it as better than option 1.

> > 
> > fix-3) adding a release_avail return param to read_avail(): this would
> > require a change to all the drivers using it. Also it
> > looks to me an unusual pattern, are there other similar patterns around
> > the codebase? Example below.

No advantage that I can see vs an explicit get / release where the
release may do nothing if there was no allocation.

> > 
> > fix-4) adding a new enum variant to the avail type like
> > IIO_AVAIL_LIST_ALLOC: to me this looks hacky as it mixes the logic type
> > of the data structure and how it is handled in memory. I think the
> > latter should better fit in a different field, however this modification
> > would have little impact in the current code. Example below.

This one I really don't like. Needs non obvious / subtle handling in the
consumer drivers.

> > 
> > So far these alternatives only consider moving the release of the copy
> > buffer in the IIO core but not its allocation.

I'm confused.  Moving it in, or out of the core?  What does this mean
for a consumer driver after the avail list?

> You also suggest to make
> > the IIO core take care of the copy allocation. The problem I see with
> > this is that if the copy is handled outside the driver it could take
> > place concurrently with the modification of the original buffer since it
> > would not be locked by driver private mutex, thus making the copy
> > useless. This might be worked around by adding an additional optional
> > callback (e.g. read_avail_will_copy/read_avail_is_mutable) to just take
> > the size and check if a copy will be provided, so maybe something like:
> > 
> > fix-5) iio_read_channel_info_avail():

This is picking on the wrong code for this discussion.  Use
iio_read_avail_channel_attribute() for example because that's the one
where ABI matters.  Anything within the IIO core is just a question of
'niceness' it isn't important like a function called by a consumer driver.

Code of a consumer driver will be similar to this however.  A few things
would be needed to make this pattern work.
 
> > {
> >     ...
> >     int *vals;
> >     bool copy = false;
> >     if (indio_dev->info->read_avail_will_copy) {
> >         copy = indio_dev->info->read_avail_will_copy(..., &length, ...);

return length as 0 can reasonably mean we don't need to allocate.
That value must be the maximum possible size that can ever be needed, not the
current one.

> >         if (copy) {
> >             vals = kcalloc(length, sizeof(int), GFP_KERNEL);
> >         }
> >     }
> > 
> >     indio_dev->info->read_avail(&vals, ...);

For iio_read_avail_channel_attribute it will a little fiddlier but end result
is the same but done under the exist lock. If the device went away before this
call then we will get an error, otherwise this will fill vals and provide
the right length. 

> > 
> >     if (ret < 0)
> >             return ret;
> >     switch (ret) {
> >     case IIO_AVAIL_LIST:
> >             ret = iio_format_avail_list(buf, vals, type, length);
> >     case IIO_AVAIL_RANGE:
> >             ret = iio_format_avail_range(buf, vals, type);
> >     default:
> >             ret = -EINVAL;
> >     }
> > 
> >     if (copy)
> >         kfree(vals);
> > }
> > 
> > If I am not missing anything this could work and maybe it could also
> > avoid the double copy on the consumers but would require all of them to
> > wrap the read_avail_will_copy(). Also, I find it quite a weird pattern
> > that in some cases vals would be an input buffer to be filled and in
> > other cases it would be a return argument pointing to the const buffer
> > stored inside the driver. At least I wouldn't say it's more robust than
> > the current fix-1.
Agreed. It works, but I'm not seeing the advantage and the multiple use
of the vals parameter is too subtle to be maintainable.

> > 
> > All these alternatives also prevents some potential optimization already
> > mentioned before. Reporting it again as it is now lost in the mess below:
> >     Some driver might want to avoid allocating a new copy of a big table if
> >     the race does not occur (e.g. with additional checks on buffer access
> >     code) and thus wouldn't call a free() in the release callback.
> > 
> > In the end I don't find any of the above alternatives to provide an
> > obvious better solution.

Agreed.  My only question vs just taking the existing solution is whether
it makes sense to use a more explicit struct iio_avail_cookie
to hold all the info that we pass to release.  I don't particularly like
that we'll end up allocating that cookie structure but it would make it more
like a typical get / release and perhaps closer to what readers would
expect to see?

What do you think?

Jonathan

> >   
> > > > > > > > > What I was referring to is that, back then, you mentioned you would have
> > > > > > > > > preferred to avoid passing ownership of the buffer around:
> > > > > > > > >   
> > > > > > > > > > That's a corner case we should think about closing. Would require an indicator
> > > > > > > > > > to read_avail that the buffer it has been passed is a snapshot that it should
> > > > > > > > > > free on completion of the string building.  I don't like passing ownership
> > > > > > > > > > of data around like that, but it is fiddly to do anything else given
> > > > > > > > > > any simple double buffering is subject to race conditions.    
> > > > > > > > > 
> > > > > > > > > I guess there is some other reason other than avoiding the copy when not
> > > > > > > > > necessary, since by introducing an additional function or argument or return
> > > > > > > > > type, most of the unnecessary copies would already be avoided right?  
> > > > > > > > 
> > > > > > > > It's not a strong reason beyond limiting scope of clever design +
> > > > > > > > the key bit my mind is that the above is not substantially simpler and
> > > > > > > > reduces our flexibility.
> > > > > > > >   
> > > > > > > > > Anyway any of this solutions would still prevent the potential optimizations of
> > > > > > > > > point 2). It's worth mentioning that those kind of optimizations are currently
> > > > > > > > > not adopted by any driver.  
> > > > > > > > 
> > > > > > > > That one indeed not, but mixing dynamic and non dynamic is something
> > > > > > > > you do in your pac1921 patch.  
> > > > > > > 
> > > > > > > Good point! I didn't think about it, or more likely I forgot, that with an
> > > > > > > additional read_avail_with_copy() used as in the example you cannot mix dynamic
> > > > > > > and non dynamic available lists, thus those drivers that need at least one
> > > > > > > dynamic available list would always copy all of them as they need to rely to
> > > > > > > the read_avail_with_copy(). I guess this could be worked around with an
> > > > > > > additional return argument for the read_avail() or an additional type like the
> > > > > > > IIO_AVAIL_LIST_ALLOC suggested by Andy to signal the caller it needs to free
> > > > > > > the list after use. However, I think they would introduce a more invasive
> > > > > > > change in the current API compared to an additional optional callback,  
> > > > > 
> > > > > It even sounds originally that it should be more invasive, so I don't think it's
> > > > > a problem here.  
> > > > 
> > > > In the hope it helps the discussion let me provide examples for the
> > > > additional two options we have other than the current
> > > > read_avail_release_resource() (fix-1) and the read_avail_with_copy()
> > > > (fix-2) already shown above:  
> > > 
> > > Thanks!
> > >   
> > > > fix-3) iio_read_channel_info_avail():
> > > > {
> > > >     ...
> > > >     bool release_avail = false;
> > > > 
> > > >     ret = indio_dev->info->read_avail(vals, ..., &release_avail);
> > > > 
> > > >     ...
> > > >     ret = iio_format_avail_list(vals, ...);
> > > >     ...
> > > > 
> > > >     if (release_avail)
> > > >         kfree(vals);
> > > > 
> > > >     return ret;
> > > > }
> > > > 
> > > > 
> > > > fix-4) iio_read_channel_info_avail():
> > > > {
> > > >     ...
> > > >     indio_dev->info->read_avail(vals, ...);
> > > > 
> > > >     if (ret < 0)
> > > >             return ret;
> > > >     switch (ret) {
> > > >     case IIO_AVAIL_LIST_ALLOC:
> > > >             ret = iio_format_avail_list(buf, vals, type, length);
> > > >             kfree(vals);
> > > >             return ret;
> > > >     case IIO_AVAIL_LIST:
> > > >             return iio_format_avail_list(buf, vals, type, length);
> > > >     case IIO_AVAIL_RANGE:
> > > >             return iio_format_avail_range(buf, vals, type);
> > > >     default:
> > > >             return -EINVAL;
> > > >     }
> > > > }
> > > >   
> > > > > > > so I agree that the current release callback is still a better option.  
> > > > > 
> > > > > I disagree on this as I pointed above why.
> > > > >   
> > > > > > > > > > > 2) Some driver might want to avoid allocating a new copy of a big table if
> > > > > > > > > > >    the race does not occur (e.g. with additional checks on buffer access
> > > > > > > > > > >    code) and thus wouldn't call a free() in the release callback.
> > > > > > > > > > >     
> > > > > > > > > > > > In any case it looks fragile and not scalable. I propose to drop this
> > > > > > > > > > > > and think again.      
> > > > > > > > > > > 
> > > > > > > > > > > I see your concerns, I am open to reconsider this in case we come up with
> > > > > > > > > > > better solution after addressing the points above.
> > > > > > > > > > >     
> > > > > > > > > > > > Yes, yes, I'm fully aware about the problem you are trying to solve and
> > > > > > > > > > > > agree on the report, I think this solution is not good enough.  
> > > > > > > > > > > 
> > > > > > > > > > > [1]: https://lore.kernel.org/linux-iio/20240729211100.0d602d6e@jic23-huawei/  
> > > > > > > > > 
> > > > > > > > > I hope I've brought a little more clarity to the discussion by providing some
> > > > > > > > > history instead of making it more confusing.  
> > > > > > > > 
> > > > > > > > Sure, the code example in particular is useful.  
> > > > > > 
> > > > > > Just a friendly reminder this has been sitting for a while, any news or
> > > > > > additional considerations?  
> > > > > 
> > > > > Moving the allocation control to the drivers will satisfy me as well, however
> > > > > it makes even more duplication of the code, but at least it will be cleaner
> > > > > design-wise in my opinion.  
> > > > 
> > > > Would it work with the constraints on the info_exists lock mentioned
> > > > above?  
> > > 
> > > None of the given examples (fix-N) provides a lock, so I have no clue how it's
> > > involved here. May be you can elaborate more?  
> > 
> > I thought that with "Moving the allocation control to the drivers" you
> > were referring to the option (not included among fix-N) to move the
> > allocation of the consumer copy from the inkern iio_channel_read_avail()
> > to the consumer drivers themselves. You elaborated this point above
> > where I answered with the concerns about the info_exists lock that
> > should be addressed.
> >   
> > > > > In any case the last word is on Jonathan.  
>

Patch

diff --git a/drivers/iio/afe/iio-rescale.c b/drivers/iio/afe/iio-rescale.c
index 56e5913ab82d1c045c9ca27012008a4495502cbf..78bb86c291706748b4072a484532ad20c415ff9f 100644
--- a/drivers/iio/afe/iio-rescale.c
+++ b/drivers/iio/afe/iio-rescale.c
@@ -249,9 +249,17 @@  static int rescale_read_avail(struct iio_dev *indio_dev,
 	}
 }
 
+static void rescale_read_avail_release_res(struct iio_dev *indio_dev,
+					   struct iio_chan_spec const *chan,
+					   const int *vals, long mask)
+{
+	kfree(vals);
+}
+
 static const struct iio_info rescale_info = {
 	.read_raw = rescale_read_raw,
 	.read_avail = rescale_read_avail,
+	.read_avail_release_resource = rescale_read_avail_release_res,
 };
 
 static ssize_t rescale_read_ext_info(struct iio_dev *indio_dev,
diff --git a/drivers/iio/dac/dpot-dac.c b/drivers/iio/dac/dpot-dac.c
index f36f10bfb6be7863a56b911b5f58671ef530c977..43d68e17fc3a5fca59fad6ccf818eeadfecdb8c1 100644
--- a/drivers/iio/dac/dpot-dac.c
+++ b/drivers/iio/dac/dpot-dac.c
@@ -108,6 +108,13 @@  static int dpot_dac_read_avail(struct iio_dev *indio_dev,
 	return -EINVAL;
 }
 
+static void dpot_dac_read_avail_release_res(struct iio_dev *indio_dev,
+					    struct iio_chan_spec const *chan,
+					    const int *vals, long mask)
+{
+	kfree(vals);
+}
+
 static int dpot_dac_write_raw(struct iio_dev *indio_dev,
 			      struct iio_chan_spec const *chan,
 			      int val, int val2, long mask)
@@ -125,6 +132,7 @@  static int dpot_dac_write_raw(struct iio_dev *indio_dev,
 static const struct iio_info dpot_dac_info = {
 	.read_raw = dpot_dac_read_raw,
 	.read_avail = dpot_dac_read_avail,
+	.read_avail_release_resource = dpot_dac_read_avail_release_res,
 	.write_raw = dpot_dac_write_raw,
 };
 
diff --git a/drivers/iio/inkern.c b/drivers/iio/inkern.c
index 7f325b3ed08fae6674245312cf8f57bb151006c0..7b87d1c57d6fd1258d254192835aa6cb9355f859 100644
--- a/drivers/iio/inkern.c
+++ b/drivers/iio/inkern.c
@@ -760,12 +760,56 @@  static int iio_channel_read_avail(struct iio_channel *chan,
 	if (!iio_channel_has_available(chan->channel, info))
 		return -EINVAL;
 
-	if (iio_info->read_avail)
-		return iio_info->read_avail(chan->indio_dev, chan->channel,
-					    vals, type, length, info);
+	if (iio_info->read_avail) {
+		const int *vals_tmp;
+		int ret;
+
+		ret = iio_info->read_avail(chan->indio_dev, chan->channel,
+					   &vals_tmp, type, length, info);
+		if (ret < 0)
+			return ret;
+
+		/*
+		 * Copy the producer's avail buffer with lock_exists locked to
+		 * avoid possible race with producer unregistration.
+		 */
+		*vals = kmemdup_array(vals_tmp, *length, sizeof(int), GFP_KERNEL);
+		if (!*vals)
+			return -ENOMEM;
+
+		if (iio_info->read_avail_release_resource)
+			iio_info->read_avail_release_resource(
+				chan->indio_dev, chan->channel, vals_tmp, info);
+
+		return ret;
+	}
 	return -EINVAL;
 }
 
+/*
+ * iio_channel_read_avail_retvals() is equivalent to iio_channel_read_avail()
+ * but stores the pointer to the buffer of available values in the returned
+ * variable. Since such buffer must be freed after use, this function lets the
+ * user declare a cleanup local variable, e.g.:
+ * const int *vals = __free(kfree) = iio_channel_read_avail_retvals(...);
+ */
+static const int *iio_channel_read_avail_retvals(struct iio_channel *chan,
+						 int *type, int *length,
+						 int *avail_type,
+						 enum iio_chan_info_enum info)
+{
+	const int *vals;
+	int ret;
+
+	ret = iio_channel_read_avail(chan, &vals, type, length, info);
+	if (ret < 0)
+		return ERR_PTR(ret);
+
+	*avail_type = ret;
+
+	return vals;
+}
+
 int iio_read_avail_channel_attribute(struct iio_channel *chan,
 				     const int **vals, int *type, int *length,
 				     enum iio_chan_info_enum attribute)
@@ -780,6 +824,25 @@  int iio_read_avail_channel_attribute(struct iio_channel *chan,
 }
 EXPORT_SYMBOL_GPL(iio_read_avail_channel_attribute);
 
+const int *
+iio_read_avail_channel_attr_retvals(struct iio_channel *chan, int *type,
+				    int *length, int *avail_type,
+				    enum iio_chan_info_enum attribute)
+{
+	const int *vals;
+	int ret;
+
+	ret = iio_read_avail_channel_attribute(chan, &vals, type, length,
+					       attribute);
+	if (ret < 0)
+		return ERR_PTR(ret);
+
+	*avail_type = ret;
+
+	return vals;
+}
+EXPORT_SYMBOL_GPL(iio_read_avail_channel_attr_retvals);
+
 int iio_read_avail_channel_raw(struct iio_channel *chan,
 			       const int **vals, int *length)
 {
@@ -789,9 +852,11 @@  int iio_read_avail_channel_raw(struct iio_channel *chan,
 	ret = iio_read_avail_channel_attribute(chan, vals, &type, length,
 					       IIO_CHAN_INFO_RAW);
 
-	if (ret >= 0 && type != IIO_VAL_INT)
+	if (ret >= 0 && type != IIO_VAL_INT) {
 		/* raw values are assumed to be IIO_VAL_INT */
+		kfree(*vals);
 		ret = -EINVAL;
+	}
 
 	return ret;
 }
@@ -801,15 +866,16 @@  static int iio_channel_read_max(struct iio_channel *chan,
 				int *val, int *val2, int *type,
 				enum iio_chan_info_enum info)
 {
-	const int *vals;
 	int length;
-	int ret;
+	int avail_type;
 
-	ret = iio_channel_read_avail(chan, &vals, type, &length, info);
-	if (ret < 0)
-		return ret;
+	const int *vals __free(kfree) =
+		iio_channel_read_avail_retvals(chan, type, &length,
+					       &avail_type, info);
+	if (IS_ERR(vals))
+		return PTR_ERR(vals);
 
-	switch (ret) {
+	switch (avail_type) {
 	case IIO_AVAIL_RANGE:
 		switch (*type) {
 		case IIO_VAL_INT:
@@ -857,15 +923,16 @@  static int iio_channel_read_min(struct iio_channel *chan,
 				int *val, int *val2, int *type,
 				enum iio_chan_info_enum info)
 {
-	const int *vals;
 	int length;
-	int ret;
+	int avail_type;
 
-	ret = iio_channel_read_avail(chan, &vals, type, &length, info);
-	if (ret < 0)
-		return ret;
+	const int *vals __free(kfree) =
+		iio_channel_read_avail_retvals(chan, type, &length,
+					       &avail_type, info);
+	if (IS_ERR(vals))
+		return PTR_ERR(vals);
 
-	switch (ret) {
+	switch (avail_type) {
 	case IIO_AVAIL_RANGE:
 		switch (*type) {
 		case IIO_VAL_INT:
diff --git a/drivers/iio/multiplexer/iio-mux.c b/drivers/iio/multiplexer/iio-mux.c
index 2953403bef53bbe47a97a8ab1c475ed88d7f86d2..31345437784b01c5d6f8ea70263f4c2574388e7a 100644
--- a/drivers/iio/multiplexer/iio-mux.c
+++ b/drivers/iio/multiplexer/iio-mux.c
@@ -142,6 +142,13 @@  static int mux_read_avail(struct iio_dev *indio_dev,
 	return ret;
 }
 
+static void mux_read_avail_release_res(struct iio_dev *indio_dev,
+				       struct iio_chan_spec const *chan,
+				       const int *vals, long mask)
+{
+	kfree(vals);
+}
+
 static int mux_write_raw(struct iio_dev *indio_dev,
 			 struct iio_chan_spec const *chan,
 			 int val, int val2, long mask)
@@ -171,6 +178,7 @@  static int mux_write_raw(struct iio_dev *indio_dev,
 static const struct iio_info mux_info = {
 	.read_raw = mux_read_raw,
 	.read_avail = mux_read_avail,
+	.read_avail_release_resource = mux_read_avail_release_res,
 	.write_raw = mux_write_raw,
 };
 
diff --git a/drivers/power/supply/ingenic-battery.c b/drivers/power/supply/ingenic-battery.c
index 0a40f425c27723ccec49985b8b5e14a737b6a7eb..5d82c799ba5af3302bfcdfadb5f57adf6b12b353 100644
--- a/drivers/power/supply/ingenic-battery.c
+++ b/drivers/power/supply/ingenic-battery.c
@@ -6,12 +6,14 @@ 
  * based on drivers/power/supply/jz4740-battery.c
  */
 
+#include <linux/cleanup.h>
 #include <linux/iio/consumer.h>
 #include <linux/module.h>
 #include <linux/of.h>
 #include <linux/platform_device.h>
 #include <linux/power_supply.h>
 #include <linux/property.h>
+#include <linux/slab.h>
 
 struct ingenic_battery {
 	struct device *dev;
@@ -62,8 +64,8 @@  static int ingenic_battery_get_property(struct power_supply *psy,
  */
 static int ingenic_battery_set_scale(struct ingenic_battery *bat)
 {
-	const int *scale_raw;
-	int scale_len, scale_type, best_idx = -1, best_mV, max_raw, i, ret;
+	int scale_len, scale_type, scale_avail_type;
+	int best_idx = -1, best_mV, max_raw, i, ret;
 	u64 max_mV;
 
 	ret = iio_read_max_channel_raw(bat->channel, &max_raw);
@@ -72,14 +74,18 @@  static int ingenic_battery_set_scale(struct ingenic_battery *bat)
 		return ret;
 	}
 
-	ret = iio_read_avail_channel_attribute(bat->channel, &scale_raw,
-					       &scale_type, &scale_len,
-					       IIO_CHAN_INFO_SCALE);
-	if (ret < 0) {
+	const int *scale_raw __free(kfree) =
+		iio_read_avail_channel_attr_retvals(bat->channel,
+						    &scale_type,
+						    &scale_len,
+						    &scale_avail_type,
+						    IIO_CHAN_INFO_SCALE);
+	if (IS_ERR(scale_raw)) {
 		dev_err(bat->dev, "Unable to read channel avail scale\n");
-		return ret;
+		return PTR_ERR(scale_raw);
 	}
-	if (ret != IIO_AVAIL_LIST || scale_type != IIO_VAL_FRACTIONAL_LOG2)
+	if (scale_avail_type != IIO_AVAIL_LIST ||
+	    scale_type != IIO_VAL_FRACTIONAL_LOG2)
 		return -EINVAL;
 
 	max_mV = bat->info->voltage_max_design_uv / 1000;
diff --git a/include/linux/iio/consumer.h b/include/linux/iio/consumer.h
index 333d1d8ccb37f387fe531577ac5e0bfc7f752cec..188cc64609bd1fd8e0522e69f0b65a73b9b1606d 100644
--- a/include/linux/iio/consumer.h
+++ b/include/linux/iio/consumer.h
@@ -316,7 +316,7 @@  int iio_read_min_channel_raw(struct iio_channel *chan, int *val);
 /**
  * iio_read_avail_channel_raw() - read available raw values from a given channel
  * @chan:		The channel being queried.
- * @vals:		Available values read back.
+ * @vals:		Available values read back. Must be freed after use.
  * @length:		Number of entries in vals.
  *
  * Returns an error code, IIO_AVAIL_RANGE or IIO_AVAIL_LIST.
@@ -334,7 +334,7 @@  int iio_read_avail_channel_raw(struct iio_channel *chan,
 /**
  * iio_read_avail_channel_attribute() - read available channel attribute values
  * @chan:		The channel being queried.
- * @vals:		Available values read back.
+ * @vals:		Available values read back. Must be freed after use.
  * @type:		Type of values read back.
  * @length:		Number of entries in vals.
  * @attribute:		info attribute to be read back.
@@ -345,6 +345,30 @@  int iio_read_avail_channel_attribute(struct iio_channel *chan,
 				     const int **vals, int *type, int *length,
 				     enum iio_chan_info_enum attribute);
 
+
+/**
+ * iio_read_avail_channel_attr_retvals() - read available channel attr values
+ * @chan:		The channel being queried.
+ * @type:		Type of values read back.
+ * @length:		Number of entries in vals.
+ * @avail_type:		Available type of values read back
+ *			(IIO_AVAIL_RANGE or IIO_AVAIL_LIST).
+ * @attribute:		info attribute to be read back.
+ *
+ * This function is equivalent to iio_read_avail_channel_attribute() but stores
+ * the pointer to the buffer of available values in the returned variable.
+ * Since such buffer must be freed after use, this function lets the user
+ * declare a cleanup local variable, e.g.:
+ * const int *vals = __free(kfree) = iio_read_avail_channel_attr_retvals(...);
+ *
+ * Returns a pointer to negative errno on error otherwise returns the available
+ * values read back that must be freed after use.
+ */
+const int *
+iio_read_avail_channel_attr_retvals(struct iio_channel *chan, int *type,
+				    int *length, int *avail_type,
+				    enum iio_chan_info_enum attribute);
+
 /**
  * iio_get_channel_type() - get the type of a channel
  * @channel:		The channel being queried.