RE: [PATCH 1/3] iio: buffer: remove usage of list iterator variable for list_for_each_entry_continue_reverse()

[Sa, Nuno]

> From: Jakob Koschel <jakobkoschel@xxxxxxxxx>
> Sent: Friday, April 1, 2022 3:55 PM
> To: Sa, Nuno <Nuno.Sa@xxxxxxxxxx>
> Cc: Jonathan Cameron <jic23@xxxxxxxxxx>; Lars-Peter Clausen
> <lars@xxxxxxxxxx>; Dan Carpenter <dan.carpenter@xxxxxxxxxx>;
> linux-iio@xxxxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Mike
> Rapoport <rppt@xxxxxxxxxx>; Brian Johannesmeyer
> <bjohannesmeyer@xxxxxxxxx>; Cristiano Giuffrida
> <c.giuffrida@xxxxx>; Bos, H.J. <h.j.bos@xxxxx>
> Subject: Re: [PATCH 1/3] iio: buffer: remove usage of list iterator
> variable for list_for_each_entry_continue_reverse()
> 
> [External]
> 
> 
> 
> > On 1. Apr 2022, at 14:40, Sa, Nuno <Nuno.Sa@xxxxxxxxxx> wrote:
> >
> > Hi Jakob,
> >
> >> -----Original Message-----
> >> From: Jakob Koschel <jakobkoschel@xxxxxxxxx>
> >> Sent: Friday, April 1, 2022 1:07 AM
> >> To: Jonathan Cameron <jic23@xxxxxxxxxx>
> >> Cc: Lars-Peter Clausen <lars@xxxxxxxxxx>; Dan Carpenter
> >> <dan.carpenter@xxxxxxxxxx>; Jakob Koschel
> >> <jakobkoschel@xxxxxxxxx>; linux-iio@xxxxxxxxxxxxxxx; linux-
> >> kernel@xxxxxxxxxxxxxxx; Mike Rapoport <rppt@xxxxxxxxxx>; Brian
> >> Johannesmeyer <bjohannesmeyer@xxxxxxxxx>; Cristiano
> Giuffrida
> >> <c.giuffrida@xxxxx>; Bos, H.J. <h.j.bos@xxxxx>
> >> Subject: [PATCH 1/3] iio: buffer: remove usage of list iterator
> variable
> >> for list_for_each_entry_continue_reverse()
> >>
> >> [External]
> >>
> >> In preparation to limit the scope of the list iterator variable to the
> >> list traversal loop, use a dedicated pointer to iterate through the
> >> list [1].
> >>
> >> Since that variable should not be used past the loop iteration, a
> >> separate variable is used to 'remember the current location within
> the
> >> loop'.
> >>
> >> To either continue iterating from that position or start a new
> >> iteration (if the previous iteration was complete)
> list_prepare_entry()
> >> is used.
> >>
> >> Link:
> https://urldefense.com/v3/__https://lore.kernel.org/all/CAHk-
> >> =wgRr_D8CB-D9Kg-
> >>
> c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@xxxxxxxxxxxxxx/__;!!A3Ni8CS0y
> >>
> 2Y!q8llw5UCaMIsAU7tPtPDhwVor0wy032I7FJHv0VxBZksNuRJF04HjWe
> >> 0XYG7OQ$  [1]
> >> Signed-off-by: Jakob Koschel <jakobkoschel@xxxxxxxxx>
> >> ---
> >> drivers/iio/industrialio-buffer.c | 7 +++++--
> >> 1 file changed, 5 insertions(+), 2 deletions(-)
> >>
> >> diff --git a/drivers/iio/industrialio-buffer.c b/drivers/iio/industrialio-
> >> buffer.c
> >> index 208b5193c621..151a77c2affd 100644
> >> --- a/drivers/iio/industrialio-buffer.c
> >> +++ b/drivers/iio/industrialio-buffer.c
> >> @@ -1059,7 +1059,7 @@ static int iio_enable_buffers(struct
> iio_dev
> >> *indio_dev,
> >> 	struct iio_device_config *config)
> >> {
> >> 	struct iio_dev_opaque *iio_dev_opaque =
> >> to_iio_dev_opaque(indio_dev);
> >> -	struct iio_buffer *buffer;
> >> +	struct iio_buffer *buffer, *tmp = NULL;
> >> 	int ret;
> >>
> >> 	indio_dev->active_scan_mask = config->scan_mask;
> >> @@ -1097,8 +1097,10 @@ static int iio_enable_buffers(struct
> iio_dev
> >> *indio_dev,
> >>
> >> 	list_for_each_entry(buffer, &iio_dev_opaque->buffer_list,
> >> buffer_list) {
> >> 		ret = iio_buffer_enable(buffer, indio_dev);
> >> -		if (ret)
> >> +		if (ret) {
> >> +			tmp = buffer;
> >> 			goto err_disable_buffers;
> >> +		}
> >> 	}
> >>
> >> 	if (indio_dev->currentmode == INDIO_BUFFER_TRIGGERED) {
> >> @@ -1125,6 +1127,7 @@ static int iio_enable_buffers(struct
> iio_dev
> >> *indio_dev,
> >> 					     indio_dev->pollfunc);
> >> 	}
> >> err_disable_buffers:
> >> +	buffer = list_prepare_entry(tmp, &iio_dev_opaque-
> >>> buffer_list, buffer_list);
> >
> > Ok, it's Friday so I might be seeing ghosts... But looking at
> 'list_prepare_entry()'...
> > If tmp != NULL, then all goes well but if tmp == NULL, then we get
> >
> > list_entry(&iio_dev_opaque->buffer_list, struct iio_buffer,
> buffer_list) which
> > would require 'struct iio_dev_opaque'. It looks like like
> 'list_prepare_entry()'
> > assumes that pos and head are of the same type...
> >
> > Am I missing something?
> 
> The list iterators are weird in this perspective...
> 
> If you look at the original code,
> list_for_each_entry_continue_reverse() is called on 'buffer'.
> 
> 'buffer' would be a valid struct element of &iio_dev_opaque-
> >buffer_list if the break is hit,
> but if no break is hit in the earlier list_for_each_entry() buffer is not a
> valid entry.
> 
> Before the terminating condition of list_for_each_entry() is met, it
> essentially does:
> 
> 	buffer = list_entry(&iio_dev_opaque->buffer_list,
> typeof(*buffer), buffer_list);
> 
> the buffer returned here is not a valid pointer to struct however.
> But since list_for_each_entry_continue_reverse() immediately calls
> list_prev_entry(buffer, buffer_list)
> on it you end up with the last entry of the list again and start iterating
> with that one.

Ahh I see it now... I really never had noticed this dance but in fact the
same dance is also used to detect the end of list_for_each_entry()...

Ok, then:

Reviewed-by: Nuno Sá <nuno.sa@xxxxxxxxxx>