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

  $ 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.

link-vmlinux.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.masah...@socionext.com>
Acked-by: Nicolas Pitre <n...@linaro.org>

Changes in v4:
  - typo fix link-vmlinus.sh -> link-vmlinux.sh

Changes in v3:
  - autoksyms_recursive should be surrounded by

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
-# 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/vmlinux-gdb.py)
+# Recurse until adjust_autoksyms.sh is satisfied
+PHONY += autoksyms_recursive
+autoksyms_recursive: $(vmlinux-deps)
        $(Q)$(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
+       $(Q)$(MAKE) -f $(srctree)/Makefile headers_check
+       $(Q)ln -fsn $(abspath $(srctree)/scripts/gdb/vmlinux-gdb.py)
        +$(call if_changed,link-vmlinux)
 # Build samples along the rest of the kernel

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