linux-kernel - RE: [PATCH 1/3] iio: buffer: remove usage of list iterator variable for list_for_each_entry_continue

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Message-ID: <PH0PR03MB6786326401D38B60141CAC9799E09@PH0PR03MB6786.namprd03.prod.outlook.com>
Date:   Fri, 1 Apr 2022 12:40:59 +0000
From:   "Sa, Nuno" <Nuno.Sa@...log.com>
To:     Jakob Koschel <jakobkoschel@...il.com>,
        Jonathan Cameron <jic23@...nel.org>
CC:     Lars-Peter Clausen <lars@...afoo.de>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        "linux-iio@...r.kernel.org" <linux-iio@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mike Rapoport <rppt@...nel.org>,
        Brian Johannesmeyer <bjohannesmeyer@...il.com>,
        Cristiano Giuffrida <c.giuffrida@...nl>,
        "Bos, H.J." <h.j.bos@...nl>
Subject: RE: [PATCH 1/3] iio: buffer: remove usage of list iterator variable
 for list_for_each_entry_continue_reverse()

Hi Jakob,

> -----Original Message-----
> From: Jakob Koschel <jakobkoschel@...il.com>
> Sent: Friday, April 1, 2022 1:07 AM
> To: Jonathan Cameron <jic23@...nel.org>
> Cc: Lars-Peter Clausen <lars@...afoo.de>; Dan Carpenter
> <dan.carpenter@...cle.com>; Jakob Koschel
> <jakobkoschel@...il.com>; linux-iio@...r.kernel.org; linux-
> kernel@...r.kernel.org; Mike Rapoport <rppt@...nel.org>; Brian
> Johannesmeyer <bjohannesmeyer@...il.com>; Cristiano Giuffrida
> <c.giuffrida@...nl>; Bos, H.J. <h.j.bos@...nl>
> 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@...l.gmail.com/__;!!A3Ni8CS0y
> 2Y!q8llw5UCaMIsAU7tPtPDhwVor0wy032I7FJHv0VxBZksNuRJF04HjWe
> 0XYG7OQ$  [1]
> Signed-off-by: Jakob Koschel <jakobkoschel@...il.com>
> ---
>  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?

- Nuno Sá