kbuild: link vmlinux only once for CONFIG_TRIM_UNUSED_KSYMS

masahir0y · masahir0y · commit 3fdc7d3fe4c0 · 2018-03-26T02:01:24.000+09:00
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; 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 2441e78 ("kbuild: better abstract vmlinux sequential prerequisites") was to cater to CONFIG_BUILD_DOCSRC, but it was later removed by commit 1848929 ("samples: move blackfin gptimers-example from Documentation"). Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com> Acked-by: Nicolas Pitre <nico@linaro.org>
diff --git a/Makefile b/Makefile
@@ -996,17 +996,9 @@ 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"
@@ -1032,7 +1024,13 @@ 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
