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

Jonathan Cameron
2024-12-15T13:47:04Z

On Thu, 12 Dec 2024 16:06:16 +0200
Andy Shevchenko <andriy.shevchenko@intel.com> wrote:

> On Thu, Dec 12, 2024 at 10:46:04AM +0100, Matteo Martelli wrote:
> > On Mon, 2 Dec 2024 19:42:54 +0200, Andy Shevchenko <andriy.shevchenko@intel.com> wrote:
> > > On Fri, Nov 29, 2024 at 05:04:54PM +0100, Matteo Martelli wrote:
> > > > On Tue, 26 Nov 2024 17:41:47 +0000, Jonathan Cameron <jic23@kernel.org> wrote:
> > > > > On Tue, 26 Nov 2024 17:31:16 +0100
> > > > > Matteo Martelli <matteomartelli3@gmail.com> wrote:
> > > > > > On Sat, 23 Nov 2024 14:13:20 +0000, Jonathan Cameron <jic23@kernel.org> wrote:
> > > > > > > On Tue, 19 Nov 2024 14:05:31 +0200
> > > > > > > Andy Shevchenko <andriy.shevchenko@intel.com> 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 <andriy.shevchenko@intel.com> 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 <andriy.shevchenko@intel.com> 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 <matteomartelli3@gmail.com> wrote:
> > > > > > > > > > > > > > Quoting Jonathan Cameron (2024-10-31 15:31:29)
> > > > > > > > > > > > > > > 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.
> > > > > > > >
> > > > > > > > > > > > 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.
> > > > > >
> > > > > > Would this imply that also the read_info callback provided by the
> > > > > > iio_info struct should be replaced? Something like info->get_avail()
> > > > > > returning a iio_avail_cookie instead of info->read_avail(const int **vals)?
> > > > > > * If yes, that would be a big impact in the current code as all
> > > > > > iio drivers defining read_avail would need to be changed (I am not
> > > > > > against it but better consider it).
> > > > > > * If no, then I find odd that iio_info->avail_release(cookie) gets a
> > > > > > cookie that has been allocated outside the provider driver: the read
> > > > > > functions gives something to the user and its corresponing release
> > > > > > handle another type of object (even it's just a wrapper). Is this the
> > > > > > usual pattern for cookies?
> > > > >
> > > > > I think the trick here is that the provider drivers wouldn't be involved
> > > > > in the cookie handling. We might have done it differently if we
> > > > > were starting from scratch, but the legacy is a pain as normal!
> > > > > Consumers can stay the same as you have here as all the information in
> > > > > the cookie would be gathered from existing read_avail plus the parameters.
> > > > > Tricky bit is the provider_priv, but we may not even need that. If that
> > > > > becomes a useful thing we'd need a new optional get_avail_with_priv()
> > > > > or something like that. Might need to stash the channel info in the
> > > > > cookie as well.
> > > > >
> > > > > Provider wouldn't see the cookie at release either as we'd just
> > > > > pass parameters from the cookie into the release callback then free
> > > > > the cookie in the core code. Technically ownership of the cookie
> > > > > would lie with the consumer not the provider but we'd hide all that
> > > > > away from the consumer.
> > > >
> > > > I see now that your suggestion to use the cookie would only affect
> > > > consumers. What confused me is your example below where the producer's
> > > > release would use the cookie as well:
> > > >
> > > > iio_dev->info->avail_release(cookie);
> > > >
> > > > Anyway, if iio_info->read_avail and
> > > > iio_info->read_avail_release_resource are not going to be changed, how
> > > > could we address the case where the consumers simply forwards their
> > > > providers avail data back to the core for sysfs print, like as follows?
> > > >
> > > > static int dpot_dac_read_avail(struct iio_dev *indio_dev,
> > > > struct iio_chan_spec const *chan,
> > > > const int **vals, int *type, int *length,
> > > > long mask)
> > > > {
> > > > struct dpot_dac *dac = iio_priv(indio_dev);
> > > >
> > > > switch (mask) {
> > > > case IIO_CHAN_INFO_RAW:
> > > > *type = IIO_VAL_INT;
> > > > return iio_read_avail_channel_raw(dac->dpot, vals, length);
> > > >
> > > > /* NOTE: Here we could call the cookie = iio_channel_avail_get() and
> > > > fill the return arguments with the content of the
> > > > cookie, but wouldn't the cookie be lost? How to return
> > > > it to caller? */
> > > > }
> > > >
> > > > 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);
> > > > /* NOTE: Here the consumer should access the cookie to call
> > > > * iio_channel_avail_release(cookie), but how can it? */
> > > > }
> > > > ...
> > > > static const struct iio_info dpot_dac_info = {
> > > > .read_avail = dpot_dac_read_avail,
> > > > .read_avail_release_resource = dpot_dac_read_avail_release_res,
> > > > };
> > > >
> > > >
> > > > >
> > > > > Whether we would use the cookie magic in the inkern code other
> > > > > than the getter itself would depend a bit on what it looks like
> > > > >
> > > > > We might need to do a global rename of read_avail to get_avail
> > > > > though to make the relationship to release_avail obvious.
> > > > >
> > > > >
> > > > > >
> > > > > > >
> > > > > > > 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;
> > > > > > > }
> > > > > > >
> > > > > >
> > > > > > I suppose that struct iio_avail_cookie and their access functions like
> > > > > > iio_avail_from_cookie would be define in iio.h as they are required for
> > > > > > producer drivers too. Correct?
> > > > >
> > > > > Initially at least I'd try just making them visible to the consumer.
> > > > >
> > > > > >
> > > > > > >
> > > > > > > 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.
> > > > > > > */
> > > > > > > }
> > > > > >
> > > > > > Do these two functions refer to inkern consumer APIs? Would
> > > > > > iio_channel_avail_get() replace the current inkern
> > > > > > iio_read_avail_channel_attribute()?
> > > > >
> > > > > Yes.
> > > > >
> > > > > >In that case I think
> > > > > > iio_channel_avail_get() would copy the cookie (and its inner avail
> > > > > > buffer) from the provider driver, or allocate a new cookie with the
> > > > > > copied avail buffer if info->read_avail() is kept unchanged, and
> > > > > > immediately call the provider info->avail_release(cookie) to do
> > > > > > copy+release with info_exist_lock locked.
> > > > >
> > > > > I don't think the provider ever explicitly deals with the cookie,
> > > > > just data read from it in the inkern code.
> > > > >
> > > > > > At that point
> > > > > > iio_channel_avail_release() would only need to call
> > > > > > kfree(iio_avail_from_cookie(cookie)) and kfree(cookie).
> > > > >
> > > > >
> > > > > Ah. I'm forgetting the issue with the provider device instance
> > > > > going away. In that case it may well have to copy the avail data
> > > > > to fill the cookie returned to the consumer driver much like we
> > > > > copy it now.+ free it. We could do something smarter with that
> > > > > cookie though to avoid a free if it's static const stuff as the
> > > > > provider module should be locked in place I think.
> > > > >
> > > > > >
> > > > > > >
> > > > > > > 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?
> > > > > >
> > > > > > I cannot answer about what readers would expect since I am quite new to
> > > > > > kernel internals and I was not aware about the cookie pattern myself.
> > > > > > However, I agree that it seems more clear than the current solution but
> > > > > > only if it's going to replace the iio_info read_avail() callback,
> > > > > > otherwise I think that only using the cookie on the release callback
> > > > > > would make it even more confusing. It is worth noting that in that case
> > > > > > all current provider drivers defining the read_avail callback should be
> > > > > > changed.
> > > > > Key I think is we really don't need to be careful with what happens in inkern.c
> > > > > (though obviously good to use infrastructure we invent for other things),
> > > > > only what is presented by consumer.h interfaces to consumer drivers.
> > > > > Those get and release the cookie.
> > > > >
> > > > > That's a much smaller set of drivers to modify.
> > > > >
> > > > > Or as Andy suggested, maybe it's just a question of naming and we need
> > > > > a get and release but otherwise don't bother with the complexity of the cookie.
> > > > > Maybe just rename read_avail to make it obvious.
> > > > >
> > > > > Right now I'm thinking the cookie wrappers around get_avail / release_avail
> > > > > to wrap up basically the parameters passed to get_avail + the output so
> > > > > that we have a neat package to pass to release_avail will end up the neatest
> > > > > solution but I may be wrong :(
> > > >
> > > > If there is a way to address my issue above I can see some benefits in
> > > > terms of clarity. In general I think it's more clear to return a struct
> > > > instead of using return arguments, it would also simplify the __free()
> > > > cleanup as we wouldn't need the additional *_retvals wrapper for that
> > > > purpose. Also, I think using an inkern helper for the release adds more
> > > > clarity to the consumer drivers even if it's just a wrapper to kfree(),
> > > > but that could be achieved without the cookie too.
> > >
> > > IIUC we need also new read_avail_alloc() which returns cookie structure,
> > > in this case the read_aval_release() will take it as a parameter. It means
> > > it will be the driver's responsibility to carry on the cookie.

That's a no because of the cost in driver rework as any driver
could have it's read_avail_alloc() call called if they provide
a read_avail() today. We could provide a default function but that's
effectively the same as just doing the handling the IIO core and leaving
the provider drivers alone. I don't mind this approach if it makes it
look more standard but I'm not seeing the advantage so far.

> >
> > I think this would be in contrast of Jonathan's suggestion to keep the
> > cookie pattern out of the provider drivers scope.
>
> In my understanding of cookies is that it's an opaque pointer / object,
> so it is fine to pass it through the caller / user. We do cookies, e.g.,
> in DMA engine subsystem and it's fine, the actual cookie's data is kept
> out of the scope of the provider drivers.
>
> Did I miss anything?

If we were starting from scratch, then pushing it all the way through would
maybe be fine (with a bunch of helpers to avoid the pain for the really simple
(and way more frequent) cases.

The problem here is a big miss balance. Lots of drivers c. 100 from a
quick grep, provide the relevant available callbacks because they are
mostly just used by the IIO core to provide data for sysfs functions. We could
fix all those drivers up but in general I don't want to complicate
that path (which doesn't have these lifetimes or ownership issues) or
the vast majority of drivers that don't have this lifetime issue because
the data they are providing is const.

So to keep this simple, I think we can have cookies that only the consumer
side (which is way smaller, just a few drivers) + the IIO core have to
deal with.

That also means they aren't exposed for the inkern calls that just grab
one number from an array (we have min and max) and hence can manage
the lifetime themselves.

We only need them in the tiny fraction of cases where the array of
data itself is passed to the consumer. Therefore, if we create
the cookies in the interface layer it all looks like the ideal situation
for those consumers (nice opaque object + a few accessor calls) and
keeps the complexity out of the provider drivers. If later we need
to provide alternative paths into the providers for when they do need
to more directly manage the cookie (i.e. squirt more data in it for
some reason) that is fine, because it only affects the providers
that need it - not all of them.

So in conclusion, I'm thinking we either:
1) Push the cookie allocator to a default function that is used in all provider
drivers but in theory can be overridden - maybe it is worth overriding
for those cases that were modified in this set. I'm not sure we gain
anything but I haven't looked closely.

2) For now just do the cookie allocator in the core with the option later
to push it to provider drivers where they want to do something different
from the provider.

Perhaps have a play with doing option 1 and see how bad it is? I think
it only makes sense if we end up with no special casing in the core
and that means every iio_info structure with a read_avail needs the
new callback set as well + runtime checks that we have done that.
(whereas option 2 is basically just have the 'special' case code in the
core).

Jonathan