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

From: Julia Lawall
Date: Sun Jan 28 2024 - 13:07:44 EST




On Sun, 28 Jan 2024, Jonathan Cameron wrote:

> From: Jonathan Cameron <Jonathan.Cameron@xxxxxxxxxx>
>
> +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@xxxxxxxxxx/
> [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@xxxxxxxxxx/
>
> 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
>
>