Re: [PATCH v4 2/2] kbuild: Support directory targets for building DTBs

[Rob Herring <robh@kernel.org>]

On Sat, Oct 17, 2026 at 07:49:53PM +0000, sashiko-bot@kernel.org wrote:
> Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
> - [High] kbuild: parallel build race in dtbs target and extra boot/dts traversal
> --
> 
> commit 3f00c18c0d96a2ac1d59c7335959fec4fa041ff7
> Author: Rob Herring (Arm) <robh@kernel.org>
> 
> kbuild: Support directory targets for building DTBs
> 
> This commit enables building DTBs using directory targets so that all DTBs
> for a vendor directory can be built, including base+overlay DTB targets,
> and updates the dtbs target to use the directory target rule.
> 
> > diff --git a/Makefile b/Makefile
> > index c307ee9293bcd..6d1d1c803ee86 100644
> > --- a/Makefile
> > +++ b/Makefile
> > @@ -1664,8 +1664,8 @@ dtbindingtree := Documentation/devicetree/bindings
> >  ifneq ($(dtstree),)
> >  
> >  PHONY += dtbs dtbs_prepare dtbs_install dtbs_check
> > -dtbs: dtbs_prepare
> > -	$(Q)$(MAKE) $(build)=$(dtstree) need-dtbslist=1
> > +dtbs:
> > +	$(Q)$(MAKE) $(dtstree)/
> 
> [Severity: High]
> Does dropping dtbs_prepare from the dtbs target prerequisites and spawning a
> separate top-level make for $(dtstree)/ introduce a parallel build race when
> vmlinux and dtbs are built together (for example, during make -j all when
> CONFIG_OF_EARLY_FLATTREE=y, make -j vmlinux when CONFIG_GENERIC_BUILTIN_DTB=y,
> or make -j image.fit on arm64)?

I think I just need to add back dtbs_prepare here.

> Previously, vmlinux (via . -> prepare -> prepare0 -> archprepare) and dtbs
> (via dtbs_prepare) shared the same top-level make dependency graph, which
> serialized include/config/kernel.release, scripts_basic, and scripts_dtc:
> 
> Makefile:
>     scripts: scripts_basic scripts_dtc
>     	$(Q)$(MAKE) $(build)=$(@)
>     ...
>     archprepare: outputmakefile archheaders archscripts scripts include/config/kernel.release \
>     ...
>     dtbs_prepare: include/config/kernel.release scripts_dtc
> 
> With dtbs running $(MAKE) $(dtstree)/, the child make enables single-build
> and evaluates $(build-dir): $(single-build-prep) where single-build-prep is
> dtbs_prepare. In a parallel build, the parent make (building archprepare) and
> the child make (building dtbs_prepare) have independent dependency graphs and
> can run concurrently:
> 
> 1. Both make instances can run filechk for include/config/kernel.release at
>    the same time, racing on the shared include/config/.tmp_kernel.release
>    temporary file in scripts/Kbuild.include:
> 
>    define filechk
>    	$(check-FORCE)
>    	$(Q)set -e;						\
>    	mkdir -p $(dir $@);					\
>    	trap "rm -f $(tmp-target)" EXIT;			\
>    	{ $(filechk_$(1)); } > $(tmp-target);			\
>    	if [ ! -r $@ ] || ! cmp -s $@ $(tmp-target); then	\
>    		$(kecho) '  UPD     $@';			\
>    		mv -f $(tmp-target) $@;				\
>    	fi
>    endef
> 
> 2. Both make instances can concurrently invoke $(MAKE) $(build)=scripts/basic
>    and $(MAKE) $(build)=scripts/dtc in the same output directory, racing on
>    building the host binaries (fixdep, dtc, fdtoverlay) and their .*.cmd
>    files.
> 
> [ ... ]
> 
> > diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> > index 4349108e75e1f..d22b3b2d43b32 100644
> > --- a/scripts/Makefile.build
> > +++ b/scripts/Makefile.build
> > @@ -38,6 +38,13 @@ include $(srctree)/scripts/Makefile.compiler
> >  include $(kbuild-file)
> >  include $(srctree)/scripts/Makefile.lib
> >  
> > +# Make DT directory targets reachable for architectures with a DTS tree.
> > +ifeq ($(obj),arch/$(SRCARCH))
> > +ifneq ($(wildcard $(src)/boot/dts/),)
> > +subdir-y += boot/dts
> > +endif
> > +endif
> > +
> 
> [Severity: High]
> Because subdir-y += boot/dts is added unconditionally whenever $(obj) is
> arch/$(SRCARCH), won't the main "." build pass (as well as non-DT builds like
> make vmlinux or make modules) now recurse into arch/$(SRCARCH)/boot/dts and
> all vendor subdirectories even when need-dtbslist is not set?
> 
> During make -j, the parent make's "." traversal will descend into
> arch/$(SRCARCH)/boot/dts in parallel with the child $(MAKE) $(dtstree)/
> spawned by the dtbs target. In every vendor directory where dtb-y or dtb- is
> non-empty, scripts/Makefile.build includes scripts/Makefile.dtbs (which adds
> $(real-dtb-y) to targets) and then includes the .cmd files for existing
> targets:
> 
> scripts/Makefile.build:
>     ifneq ($(need-dtbslist)$(dtb-y)$(dtb-)$(filter %.dtb %.dtb.o %.dtbo.o,$(targets)),)
>     include $(srctree)/scripts/Makefile.dtbs
>     endif
>     ...
>     existing-targets := $(wildcard $(sort $(targets)))
> 
>     -include $(foreach f,$(existing-targets),$(dir $(f)).$(notdir $(f)).cmd)
> 
> If the parallel dtbs sub-make has just created a .dtb file and cmd_and_fixdep
> is still writing the corresponding .*.dtb.cmd file while the "." build walk
> is evaluating that directory, can the "." sub-make read a partially written
> .*.dtb.cmd file and fail with a syntax error?

This should fix the issue:

diff --git a/scripts/Makefile.build b/scripts/Makefile.build
index d22b3b2d43b3..22fbd5199091 100644
--- a/scripts/Makefile.build
+++ b/scripts/Makefile.build
@@ -40,7 +40,7 @@ include $(srctree)/scripts/Makefile.lib
 
 # Make DT directory targets reachable for architectures with a DTS tree.
 ifeq ($(obj),arch/$(SRCARCH))
-ifneq ($(wildcard $(src)/boot/dts/),)
+ifdef need-dtbslist
 subdir-y += boot/dts
 endif
 endif