um: fix parallel building with O= option

[ Upstream commit 13d3d01e26b942ada7cfced68ccb6db49597874a ]

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,

  $(Q)$(MAKE) KBUILD_SRC= ARCH=$(HEADER_ARCH) archheaders

... 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.

Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Richard Weinberger <richard@nod.at>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/um/Makefile b/arch/um/Makefile
index e54dda8a036388438bcac0302d6dcf0259ef8a96..de340e41f3b2875224c63618aaa580fe96d10e32 100644 (file)
--- 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 \
                    obj=$(HOST_DIR)/include/generated/uapi/asm
-       $(Q)$(MAKE) KBUILD_SRC= ARCH=$(HEADER_ARCH) archheaders
-
+       $(Q)$(MAKE) -f $(srctree)/Makefile ARCH=$(HEADER_ARCH) archheaders
 
 archprepare: include/generated/user_constants.h