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Subject[PATCH v4 7/7] kbuild: link vmlinux only once for CONFIG_TRIM_UNUSED_KSYMS
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] updates autoksyms.h and recurses 'make vmlinux'

--------(begin sub-make)--------
[4] Second build with new autoksyms.h

[5] is invoked because vmlinux is missing
---------(end sub-make)---------

[6] 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

$ 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; the vmlinux has actually been
updated in the vmlinux_prereq target. Make cannot predict this, so
judges the vmlinux is old. 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 <>
Acked-by: Nicolas Pitre <>

Changes in v4:
- typo fix ->

Changes in v3:
- autoksyms_recursive should be surrounded by

Changes in v2:
- Discard my wrong change to
- 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


-# 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
- $(Q)$(MAKE) -f $(srctree)/Makefile headers_check
- $(Q)ln -fsn $(abspath $(srctree)/scripts/gdb/
+# Recurse until is satisfied
+PHONY += autoksyms_recursive
+autoksyms_recursive: $(vmlinux-deps)
$(Q)$(CONFIG_SHELL) $(srctree)/scripts/ \
"$(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/ vmlinux_prereq $(vmlinux-deps) FORCE
+vmlinux: scripts/ autoksyms_recursive $(vmlinux-deps) FORCE
+ $(Q)$(MAKE) -f $(srctree)/Makefile headers_check
+ $(Q)ln -fsn $(abspath $(srctree)/scripts/gdb/
+$(call if_changed,link-vmlinux)

# Build samples along the rest of the kernel
 \ /
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