On Thu, 31 Oct 2024 12:26:24 +0100 Matteo Martelli <matteomartelli3@xxxxxxxxx> wrote: > Quoting Jonathan Cameron (2024-10-30 21:30:50) > > On Wed, 30 Oct 2024 19:23:21 +0100 > > Matteo Martelli <matteomartelli3@xxxxxxxxx> 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 > >