Re: [PATCH v2 5/7] iio: inkern: copy/release available info from producer

[Matteo Martelli]

Quoting Nuno Sá (2024-10-08 14:37:22)
> On Tue, 2024-10-08 at 10:03 +0200, Matteo Martelli wrote:
> > Quoting Nuno Sá (2024-10-08 09:29:14)
> > > On Tue, 2024-10-08 at 08:47 +0200, Matteo Martelli wrote:
> > > > Quoting Nuno Sá (2024-10-07 17:15:13)
> > > > > On Mon, 2024-10-07 at 10:37 +0200, Matteo Martelli wrote:
> > > > > > Consumers need to call the read_avail_release_resource after reading
> > > > > > the
> > > > > > available info. To call the release with info_exists locked, copy the
> > > > > > available info from the producer and immediately call its release
> > > > > > callback. With this change, users of iio_read_avail_channel_raw() and
> > > > > > iio_read_avail_channel_attribute() must free the copied avail info
> > > > > > after
> > > > > > calling them.
> > > > > > 
> > > > > > Signed-off-by: Matteo Martelli <matteomartelli3@xxxxxxxxx>
> > > > > > ---
> > > > > >  drivers/iio/inkern.c         | 64 +++++++++++++++++++++++++++++++++--
> > > > > > ----
> > > > > > -----
> > > > > >  include/linux/iio/consumer.h |  4 +--
> > > > > >  2 files changed, 50 insertions(+), 18 deletions(-)
> > > > > > 
> > > > > > diff --git a/drivers/iio/inkern.c b/drivers/iio/inkern.c
> > > > > > index
> > > > > > 7f325b3ed08fae6674245312cf8f57bb151006c0..cc65ef79451e5aa2cea447e16800
> > > > > > 7a44
> > > > > > 7ffc0d91
> > > > > > 100644
> > > > > > --- a/drivers/iio/inkern.c
> > > > > > +++ b/drivers/iio/inkern.c
> > > > > > @@ -760,9 +760,25 @@ 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;
> > > > > > +
> > > > > > +             *vals = kmemdup_array(vals_tmp, *length, sizeof(int),
> > > > > > GFP_KERNEL);
> > > > > > +             if (!*vals)
> > > > > > +                     return -ENOMEM;
> > > > > > +
> > > > > 
> > > > > Not a big deal but I would likely prefer to avoid yet another copy. If
> > > > > I'm
> > > > > understanding things correctly, I would rather create an inkern wrapper
> > > > > API
> > > > > like 
> > > > > iio_channel_read_avail_release_resource() - maybe something with a
> > > > > smaller
> > > > > name :).
> > > > > Hence, the lifetime of the data would be only controlled by the producer
> > > > > of
> > > > > it. It
> > > > > would also produce a smaller diff (I think). I just find it a bit
> > > > > confusing
> > > > > that we
> > > > > duplicate the data in here and the producer also duplicates it on the -
> > > > > > read_avail()
> > > > > call. Another advantage I see is that often the available data is indeed
> > > > > const in
> > > > > which case no kmemdup_array() is needed at all.
> > > > 
> > > > 
> > > > If I understand correctly your suggestion you would leave the inkern
> > > > iio_channel_read_avail() untouched, then add a new inkern wrapper,
> > > > something
> > > > like iio_channel_read_avail_release_resource(), that would call the
> > > > producer's
> > > > read_avail_release_resource(). The consumer would invoke this new wrapper
> > > > in
> > > > its
> > > > own read_avail_release_resource() avoiding the additional copy. The call
> > > > stack
> > > > would look something like the following:
> > > > 
> > > > iio_read_channel_info_avail() {
> > > >     consumer->read_avail() {
> > > >         iio_read_avail_channel_raw() {
> > > >             iio_channel_read_avail() {
> > > >                 producer->read_avail() {
> > > >                     kmemdup_array();
> > > >                 }
> > > >             }
> > > >         }
> > > >     }
> > > > 
> > > >     iio_format_list();
> > > > 
> > > >     consumer->read_avail_release_resource() {
> > > >         iio_read_avail_channel_release_resource() {
> > > >             producer->read_avail_release_resource() {
> > > >                 kfree();
> > > >             }
> > > >         }
> > > >     }
> > > > }
> > > 
> > > Yeah, exactly what came to mind...
> > > 
> > > > 
> > > > 
> > > > I was going with the simpler solution you described, but my concern with
> > > > it
> > > > was
> > > > that the info_exists_lock mutex would be unlocked between a
> > > > iio_channel_read_avail()
> > > > call and its corresponding iio_channel_read_avail_release_resource() call.
> > > > To my understanding, this could potentially allow for the device to be
> > > > unregistered between the two calls and result in a memleak of the avail
> > > > buffer
> > > > allocated by the producer.
> > > > 
> > > > However, I have been trying to reproduce a similar case by adding a delay
> > > > between the consumer->read_avail() and the
> > > > consumer->read_avail_release_resources(), and by unbinding the driver
> > > > during
> > > > that delay, thus with the info_exists_lock mutex unlocked. In this case
> > > > the
> > > > driver is not unregistered until the iio_read_channel_info_avail()
> > > > function
> > > > completes, likely because of some other lock on the sysfs file after the
> > > > call
> > > > of
> > > > cdev_device_del() in iio_device_unregister().
> > > > 
> > > 
> > > Yes, you need to have some sync point at the kernfs level otherwise we could
> > > always be handling a sysfs attr while the device is being removed under our
> > > feet. But I'm not sure what you're trying to do... IIUC, the problem might
> > > come
> > > if have:
> > > 
> > > consumer->read_avail_channel_attribute()
> > >         producer->info_lock()
> > >         producer->read_avail()
> > >                 producer->kmalloc()
> > > 
> > > ...
> > > // producer unbound
> > > ...
> > > consumer->read_avail_release()
> > >         return -ENODEV;
> > > 
> > > // producer->kmalloc() never get's freed...
> > > 
> > > The above is your problem right? And I think it should be a valid one since
> > > between ->read_avail_channel_attribute() and read_avail_release() there's
> > > nothing preventing the producer from being unregistered...
> > 
> > Yes, that's the problem.
> > 
> > > 
> > > If I'm not missing nothing one solution would be for the producer to do
> > > devm_kmalloc() and devm_kfree() on read_avail() and release_resources() but
> > > at
> > > that point I'm not sure it's better than what you have since it's odd enough
> > > for
> > > being missed in reviews...
> > 
> > I honestly didn't think of this and it would in fact prevent the
> > additional copy. But I agree that it could be missed in new drivers,
> > maybe a comment in the iio_info read_avail_release_resource() callback
> > declaration would help?
> > > 
> At this point I would say whatever you or Jonathan prefer :)
> 

I run some quick tests with this approach and haven't found any issue so
far. I would personally switch to this approach as it would be much
simpler and easier to understand, and since the avail lists are const
for most of the current drivers I would not expect many new drivers
needing a dynamic available list. However, I will wait Jonathan feedback
first.

About the release wrapper name: even though "release_resource" looks a
common suffix for this kind of pattern,
iio_read_avail_channel_release_resource() seems in fact extremely long
and I would go for something like iio_read_avail_channel_release(). At
that point I would also shorten the iio_info release function name for
consistency, like read_avail_release_resource() => read_avail_release().
I hope such names would be clear enough though. Any feedback on this?

> - Nuno Sá

Thanks,
Matteo Martelli