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

Jonathan Cameron
2024-10-31T14:31:35Z

On Thu, 31 Oct 2024 12:26:24 +0100
Matteo Martelli <matteomartelli3@gmail.com> wrote:

> Quoting Jonathan Cameron (2024-10-30 21:30:50)
> > On Wed, 30 Oct 2024 19:23:21 +0100
> > Matteo Martelli <matteomartelli3@gmail.com> 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
>
>