Re: [PATCH 2/4] kbuild: avoid too many execution of scripts/pahole-flags.sh

[Martin Rodriguez Reboredo <yakoyoku@gmail.com>]

On 10/17/23 07:37, Masahiro Yamada wrote:
> scripts/pahole-flags.sh is executed so many times.
> 
> You can check how many times it is invoked during the build, as follows:
> 
>    $ cat <<EOF >> scripts/pahole-flags.sh
>    > echo "scripts/pahole-flags.sh was executed" >&2
>    > EOF
> 
>    $ make -s
>    scripts/pahole-flags.sh was executed
>    scripts/pahole-flags.sh was executed
>    scripts/pahole-flags.sh was executed
>    scripts/pahole-flags.sh was executed
>    scripts/pahole-flags.sh was executed
>      [ lots of repeated lines suppressed... ]
> 
> This scripts is exectuted more than 20 times during the kernel build
> because PAHOLE_FLAGS is a recursively expanded variable and exported
> to sub-processes.
> 
> With the GNU Make >= 4.4, it is executed more than 60 times because
> exported variables are also passed to other $(shell ) invocations.
> Without careful coding, it is known to cause an exponential fork
> explosion. [1]
> 
> The use of $(shell ) in an exported recursive variable is likely wrong
> because $(shell ) is always evaluated due to the 'export' keyword, and
> the evaluation can occur multiple times by the nature of recursive
> variables.
> 
> Convert the shell script to a Makefile, which is included only when
> CONFIG_DEBUG_INFO_BTF=y.
> 
> [1]: https://savannah.gnu.org/bugs/index.php?64746
> 
> Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
> ---
> [...]
> +include-$(CONFIG_DEBUG_INFO_BTF)+= scripts/Makefile.btf

Would have used a tab.

> [...]
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>