linux-kernel - Re: [PATCH 2/2] kbuild: warn if FORCE is missing for if_changed(_dep,

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Date:   Tue, 17 Aug 2021 10:25:23 +0200
From:   Nicolas Schier <n.schier@....de>
To:     Masahiro Yamada <masahiroy@...nel.org>
Cc:     linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michal Marek <michal.lkml@...kovi.net>
Subject: Re: [PATCH 2/2] kbuild: warn if FORCE is missing for
 if_changed(_dep,_rule) and filechk

On Friday, 2021-08-13 Masahiro Yamada wrote:
> if_changed, if_changed_dep, and if_changed_rule must have FORCE as a
> prerequisite so the command line change is detected.
> 
> Documentation/kbuild/makefiles.rst clearly explains it:
> 
>   Note: It is a typical mistake to forget the FORCE prerequisite.
> 
> However, not all people read the document, or understand what is written
> in it.
> 
> People repeated this mistake over again, and I determined a compelling
> force is needed.
> 
> Show a warning if FORCE is missing in the prerequisite of if_changed
> and friends. Same for filechk.
> 
> Signed-off-by: Masahiro Yamada <masahiroy@...nel.org>
> ---
> 
>  scripts/Kbuild.include | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)
> 
> diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
> index c3c975a92318..dd48e68965f8 100644
> --- a/scripts/Kbuild.include
> +++ b/scripts/Kbuild.include
> @@ -57,6 +57,7 @@ kecho := $($(quiet)kecho)
>  # - If the content differ the new file is used
>  # - If they are equal no change, and no timestamp update
>  define filechk
> +	$(check-FORCE)
>  	$(Q)set -e;						\
>  	mkdir -p $(dir $@);					\
>  	trap "rm -f $(dot-target).tmp" EXIT;			\
> @@ -130,7 +131,11 @@ make-cmd = $(call escsq,$(subst $(pound),$$(pound),$(subst $$,$$$$,$(cmd_$(1))))
>  # PHONY targets skipped in both cases.
>  newer-prereqs = $(filter-out $(PHONY),$?)
>  
> -if-changed-cond = $(newer-prereqs)$(cmd-check)
> +# It is a typical mistake to forget the FORCE prerequisite. Check it here so

prerequsite -> prerequisite?

Successfully found the missing FORCE in arch/x86/entry/vdso/Makefile:135.

Tested-by: Nicolas Schier <n.schier@....de>

> +# no more breakage will slip in.
> +check-FORCE = $(if $(filter FORCE, $^),,$(warning FORCE prerequsite is missing))
> +
> +if-changed-cond = $(newer-prereqs)$(cmd-check)$(check-FORCE)
>  
>  # Execute command if command has changed or prerequisite(s) are updated.
>  if_changed = $(if $(if-changed-cond),                                        \
> -- 
> 2.30.2
>