[PATCH 1/3] um: fix parallel building with O= option

From: Masahiro Yamada
Date: Sat Aug 04 2018 - 00:49:21 EST

Randy Dunlap reports UML occasionally fails to build with -j<N> and
O=<builddir> options.

make[1]: Entering directory '/home/rdunlap/mmotm-2018-0802-1529/UM64'
UPD include/generated/uapi/linux/version.h
WRAP arch/x86/include/generated/asm/dma-contiguous.h
WRAP arch/x86/include/generated/asm/export.h
WRAP arch/x86/include/generated/asm/early_ioremap.h
WRAP arch/x86/include/generated/asm/mcs_spinlock.h
WRAP arch/x86/include/generated/asm/mm-arch-hooks.h
WRAP arch/x86/include/generated/uapi/asm/bpf_perf_event.h
WRAP arch/x86/include/generated/uapi/asm/poll.h
GEN ./Makefile
make[2]: *** No rule to make target 'archheaders'. Stop.
arch/um/Makefile:119: recipe for target 'archheaders' failed
make[1]: *** [archheaders] Error 2
make[1]: *** Waiting for unfinished jobs....
UPD include/config/kernel.release
make[1]: *** wait: No child processes. Stop.
Makefile:146: recipe for target 'sub-make' failed
make: *** [sub-make] Error 2

The cause of the problem is the use of '$(MAKE) KBUILD_SRC=',
which recurses to the top Makefile via the $(objtree)/Makefile
generated by scripts/mkmakefile.

When you run "make -j<N> O=<builddir> ARCH=um", Make can execute
'archheaders' and 'outputmakefile' targets simultaneously because
there is no dependency between them.

If it happens,


... tries to run $(objtree)/Makefile that is being updated.

The correct way for the recursion is

$(Q)$(MAKE) -f $(srctree)/Makefile ARCH=$(HEADER_ARCH) archheaders

..., which does not rely on the generated Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@xxxxxxxxxxxxx>

arch/um/Makefile | 3 +--
1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/arch/um/Makefile b/arch/um/Makefile
index e54dda8..de340e4 100644
--- a/arch/um/Makefile
+++ b/arch/um/Makefile
@@ -122,8 +122,7 @@ archheaders:
$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic \
kbuild-file=$(HOST_DIR)/include/uapi/asm/Kbuild \
- $(Q)$(MAKE) KBUILD_SRC= ARCH=$(HEADER_ARCH) archheaders
+ $(Q)$(MAKE) -f $(srctree)/Makefile ARCH=$(HEADER_ARCH) archheaders

archprepare: include/generated/user_constants.h