Re: [RFC PATCH 0/5] of: automate of_node_put() - new approach to loops.

[Julia Lawall <julia.lawall@inria.fr>]

On Sun, 28 Jan 2024, Jonathan Cameron wrote:

> From: Jonathan Cameron <Jonathan.Cameron@huawei.com>
>
> +CC includes peopleinterested in property.h equivalents to minimize
> duplication of discussion.  Outcome of this discussion will affect:
> https://lore.kernel.org/all/20240114172009.179893-1-jic23@kernel.org/
> [PATCH 00/13] device property / IIO: Use cleanup.h magic for fwnode_handle_put() handling.
>
> In discussion of previous approach with Rob Herring we talked about various
> ways to avoid a disconnect between the declaration of the __free(device_node)
> and the first non NULL assignment. Making this connection clear is useful for 2
> reasons:
> 1) Avoids out of order cleanup with respect to other cleanup.h usage.
> 2) Avoids disconnect between how cleanup is to be done and how the reference
>    was acquired in the first place.
>
> https://lore.kernel.org/all/20240117194743.GA2888190-robh@kernel.org/
>
> The options we discussed are:
>
> 1) Ignore this issue and merge original set.
>
> 2) Always put the declaration just before the for loop and don't set it NULL.
>
> {
> 	int ret;
>
> 	ret = ... and other fun code.
>
> 	struct device_node *child __free(device_node);
> 	for_each_child_of_node(np, child) {
> 	}
> }
>
> This works but careful review is needed to ensure that this unusual pattern is
> followed.  We don't set it to NULL as the loop will do that anyway if there are
> no child nodes, or the loop finishes without an early break or return.
>
> 3) Introduced the pointer to auto put device_node only within the
>    for loop scope.
>
> +#define for_each_child_of_node_scoped(parent, child) \
> +	for (struct device_node *child __free(device_node) =		\
> +	     of_get_next_child(parent, NULL);				\
> +	     child != NULL;						\
> +	     child = of_get_next_available_child(parent, child))
> +
>
> This series is presenting option 3.  I only implemented this loop out of
> all the similar ones and it is only compile tested.
>
> Disadvantage Rob raised is that it isn't obvious this macro will instantiate
> a struct device_node *child.  I can't see a way around that other than option 2
> above, but all suggestions welcome.  Note that if a conversion leaves an
> 'external' struct device_node *child variable, in many cases the compiler
> will catch that as an unused variable. We don't currently run shaddow
> variable detection in normal kernel builds, but that could also be used
> to catch such bugs.
>
> All comments welcome.

It looks promising to get rid of a lot of clunky and error-prone
error-handling code.

I guess that

for_each_child_of_node_scoped(parent, struct device_node *, child)

would seem too verbose?

There are a lot of opportunities for device_node loops, but also for some
more obscure loops over other types.  And there are a lot of of_node_puts
that could be eliminated independent of loops.

julia

>
> Jonathan Cameron (5):
>   of: Add cleanup.h based auto release via __free(device_node) markings.
>   of: Introduce for_each_child_of_node_scoped() to automate
>     of_node_put() handling
>   of: unittest: Use __free(device_node)
>   iio: adc: fsl-imx25-gcq: Use for_each_child_node_scoped()
>   iio: adc: rcar-gyroadc: use for_each_child_node_scoped()
>
>  drivers/iio/adc/fsl-imx25-gcq.c | 13 +++----------
>  drivers/iio/adc/rcar-gyroadc.c  | 21 ++++++---------------
>  drivers/of/unittest.c           | 11 +++--------
>  include/linux/of.h              |  8 ++++++++
>  4 files changed, 20 insertions(+), 33 deletions(-)
>
> --
> 2.43.0
>
>