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>
---
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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