On Fri, 16 Mar 2018, Masahiro Yamada wrote:
> If CONFIG_TRIM_UNUSED_KSYMS is enabled and the kernel is built from
> a pristine state, the vmlinux is linked twice.
>
> [1] A user runs "make"
>
> [2] First build with empty autoksyms.h
>
> [3] adjust_autoksyms.sh updates autoksyms.h and recurses "make vmlinux"
>
> --------(begin sub-make)--------
> [4] Second build with new autoksyms.h
>
> [5] link-vmlinux.sh is invoked because "vmlinux" is missing
> ---------(end sub-make)---------
>
> [6] link-vmlinux.sh is invoked again despite "vmlinux" is up-to-date.
>
> The reason of [6] is probably because Make already decided to update
> "vmlinux" at the time of [2] because "vmlinux" was missing when Make
> built up the dependency graph.
>
> Because 'if_changed' is implemented based on '$?', this issue can be
> narrowed down to how Make handles '$?'.
>
> You can test it with the following simple code:
>
> [Test Makefile]
> A: B
> @echo newer prerequisite: $?
> cp B A
>
> B: C
> cp C B
> touch A
>
> [Result]
> $ rm -f A B
> $ touch C
> $ make
> cp C B
> touch A
> newer prerequisite: B
> cp B A
>
> Here, 'A' has been touched in the recipe of 'B'. So, the dependency
> 'A: B' has already been met before the recipe of 'A' is executed.
> However, Make does not notice the fact that the recipe of 'B' also
> updates 'A' as a side-effect.
>
> The situation is similar in this case; 'vmlinux' has actually been
> updated in the 'vmlinux_prereq' target. Make cannot predict this, so
> judges 'vmlinux' is old.
>
> link-vmlinus.sh is costly, so it is better to not run it when unneeded.
> Split CONFIG_TRIM_UNUSED_KSYMS recursion to a dedicated target.
>
> The reason of commit 2441e78b1919 ("kbuild: better abstract vmlinux
> sequential prerequisites") was to cater to CONFIG_BUILD_DOCSRC, but
> it was later removed by commit 184892925118 ("samples: move blackfin
> gptimers-example from Documentation").
>
> Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>
Acked-by: Nicolas Pitre <nico@xxxxxxxxxx>
> ---
>
> Changes in v3:
> - autoksyms_recursive should be surrounded by
> ifdef CONFIG_TRIM_UNUSED_KSYMS
>
> Changes in v2:
> - Discard my wrong change to adjust_autoksyms.sh
> - Add more commit log to explain how Make is working
>
> Makefile | 22 ++++++++++------------
> 1 file changed, 10 insertions(+), 12 deletions(-)
>
> diff --git a/Makefile b/Makefile
> index 5fee703..ff62b8e 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -998,17 +998,9 @@ export KBUILD_ALLDIRS := $(sort $(filter-out arch/%,$(vmlinux-alldirs)) arch Doc
>
> vmlinux-deps := $(KBUILD_LDS) $(KBUILD_VMLINUX_INIT) $(KBUILD_VMLINUX_MAIN) $(KBUILD_VMLINUX_LIBS)
>
> -# Include targets which we want to execute sequentially if the rest of the
> -# kernel build went well. If CONFIG_TRIM_UNUSED_KSYMS is set, this might be
> -# evaluated more than once.
> -PHONY += vmlinux_prereq
> -vmlinux_prereq: $(vmlinux-deps) FORCE
> -ifdef CONFIG_HEADERS_CHECK
> - $(Q)$(MAKE) -f $(srctree)/Makefile headers_check
> -endif
> -ifdef CONFIG_GDB_SCRIPTS
> - $(Q)ln -fsn $(abspath $(srctree)/scripts/gdb/vmlinux-gdb.py)
> -endif
> +# Recurse until adjust_autoksyms.sh is satisfied
> +PHONY += autoksyms_recursive
> +autoksyms_recursive: $(vmlinux-deps)
> ifdef CONFIG_TRIM_UNUSED_KSYMS
> $(Q)$(CONFIG_SHELL) $(srctree)/scripts/adjust_autoksyms.sh \
> "$(MAKE) -f $(srctree)/Makefile vmlinux"
> @@ -1034,7 +1026,13 @@ cmd_link-vmlinux = \
> $(CONFIG_SHELL) $< $(LD) $(LDFLAGS) $(LDFLAGS_vmlinux) ; \
> $(if $(ARCH_POSTLINK), $(MAKE) -f $(ARCH_POSTLINK) $@, true)
>
> -vmlinux: scripts/link-vmlinux.sh vmlinux_prereq $(vmlinux-deps) FORCE
> +vmlinux: scripts/link-vmlinux.sh autoksyms_recursive $(vmlinux-deps) FORCE
> +ifdef CONFIG_HEADERS_CHECK
> + $(Q)$(MAKE) -f $(srctree)/Makefile headers_check
> +endif
> +ifdef CONFIG_GDB_SCRIPTS
> + $(Q)ln -fsn $(abspath $(srctree)/scripts/gdb/vmlinux-gdb.py)
> +endif
> +$(call if_changed,link-vmlinux)
>
> # Build samples along the rest of the kernel
> --
> 2.7.4
>
>
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