Re: [PATCH RESEND] arm64: fix vdso-offsets.h dependency

[Catalin Marinas <catalin.marinas@arm.com>]

On Thu, Jul 07, 2016 at 12:23:25PM +0100, Catalin Marinas wrote:
> On Thu, Jul 07, 2016 at 11:26:01AM +0100, Catalin Marinas wrote:
> > On Wed, Jul 06, 2016 at 06:57:39PM +0100, Catalin Marinas wrote:
> > > On Thu, May 12, 2016 at 05:39:15PM +0100, Kevin Brodsky wrote:
> > > > +# We need to generate vdso-offsets.h before compiling certain files in kernel/.
> > > > +# In order to do that, we should use the archprepare target, but we can't since
> > > > +# asm-offsets.h is included in some files used to generate vdso-offsets.h, and
> > > > +# asm-offsets.h is built in prepare0, for which archprepare is a dependency.
> > > > +# Therefore we need to generate the header after prepare0 has been made, hence
> > > > +# this hack.
> > > > +prepare: vdso_prepare
> > > > +vdso_prepare: prepare0
> > > > +	$(Q)$(MAKE) $(build)=arch/arm64/kernel/vdso include/generated/vdso-offsets.h
> > > 
> > > This indeed looks dodgy. I'm not sure about the makefile rules but would
> > > the above override the "prepare" target in the top Makefile?
> > > 
> > > I think a dependency problem we have is that arch/arm64/kernel/signal.o
> > > depends on include/generated/vdso-offsets.h. However, we don't have any
> > > target for the latter, only for
> > > $(objtree)arch/arm64/kernel/vdso/vdso-offsets.h which no-one is
> > > including. Because of this, we have a fake dependency in
> > > arch/arm64/kernel/Makefile.
> > > 
> > > -----------8<-------------------------
> > > diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
> > > index 7700c0c23962..9bdacbf59091 100644
> > > --- a/arch/arm64/kernel/Makefile
> > > +++ b/arch/arm64/kernel/Makefile
> > > @@ -55,5 +55,4 @@ head-y					:= head.o
> > >  extra-y					+= $(head-y) vmlinux.lds
> > >  
> > >  # vDSO - this must be built first to generate the symbol offsets
> > > -$(call objectify,$(arm64-obj-y)): $(obj)/vdso/vdso-offsets.h
> > > -$(obj)/vdso/vdso-offsets.h: $(obj)/vdso
> > > +$(objtree)/include/generated/vdso-offsets.h: $(obj)/vdso
> > 
> > This didn't work either. Basically, when building
> > arch/arm64/kernel/signal.o for example, it doesn't figure out that
> > vdso-offsets.h has additional dependencies like the vdso.so.dbg so that
> > it builds the vdso object first. I'll look for a little longer, maybe I
> > find a better workaround.
> 
> If I also add the patch below, it works fine but I'm not sure we can
> guarantee that kbuild first dives into subdirectories with a parallel
> build:
> 
> --- a/arch/arm64/kernel/Makefile
> +++ b/arch/arm64/kernel/Makefile
> @@ -49,11 +49,7 @@ arm64-obj-$(CONFIG_HIBERNATION)		+= hibernate.o hibernate-asm.o
>  arm64-obj-$(CONFIG_KEXEC)		+= machine_kexec.o relocate_kernel.o	\
>  					   cpu-reset.o
>  
> -obj-y					+= $(arm64-obj-y) vdso/
> +obj-y					+= vdso/ $(arm64-obj-y)

This one works "most of the time" but not always. What I found to work
reliably (until further notice ;)) is to force the vdso build like
below:

----------------------8<----------------------------
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 7700c0c23962..366e4f26071f 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -55,5 +55,5 @@ head-y					:= head.o
 extra-y					+= $(head-y) vmlinux.lds
 
 # vDSO - this must be built first to generate the symbol offsets
-$(call objectify,$(arm64-obj-y)): $(obj)/vdso/vdso-offsets.h
-$(obj)/vdso/vdso-offsets.h: $(obj)/vdso
+$(objtree)/include/generated/vdso-offsets.h: $(obj)/vdso
+	$(Q)$(MAKE) $(build)=$< $@
diff --git a/arch/arm64/kernel/vdso/Makefile b/arch/arm64/kernel/vdso/Makefile
index b467fd0a384b..238bb2db76be 100644
--- a/arch/arm64/kernel/vdso/Makefile
+++ b/arch/arm64/kernel/vdso/Makefile
@@ -23,7 +23,7 @@ GCOV_PROFILE := n
 ccflags-y += -Wl,-shared
 
 obj-y += vdso.o
-extra-y += vdso.lds vdso-offsets.h
+extra-y += vdso.lds ../../../../include/generated/vdso-offsets.h
 CPPFLAGS_vdso.lds += -P -C -U$(ARCH)
 
 # Force dependency (incbin is bad)
@@ -42,11 +42,10 @@ $(obj)/%.so: $(obj)/%.so.dbg FORCE
 gen-vdsosym := $(srctree)/$(src)/gen_vdso_offsets.sh
 quiet_cmd_vdsosym = VDSOSYM $@
 define cmd_vdsosym
-	$(NM) $< | $(gen-vdsosym) | LC_ALL=C sort > $@ && \
-	cp $@ include/generated/
+	$(NM) $< | $(gen-vdsosym) | LC_ALL=C sort > $@
 endef
 
-$(obj)/vdso-offsets.h: $(obj)/vdso.so.dbg FORCE
+$(objtree)/include/generated/vdso-offsets.h: $(obj)/vdso.so.dbg FORCE
 	$(call if_changed,vdsosym)
 
 # Assembly rules for the .S files
----------------------8<----------------------------

This was my last attempt at solving this. Unless someone has a better
idea or thinkg the above doesn't work (or it's uglier than Kevin's
original patch) I plan to merge it.

Other advantages are that it only generates a single vdso-offets.h file.
It also only rebuilds those objects that depend on vdso-offests.h rather
than everything under arch/arm64/kernel/ (Kevin's patch was also solving
this).

-- 
Catalin