Re: [PATCH] kbuild: prevent building single targets twice

[Nicolas Schier <nsc@kernel.org>]

On Wed, Oct 21, 2026 at 08:53:56AM +0200, Mathias Krause wrote:
> On 23.09.26 14:39, Mathias Krause wrote:
> > When building individual targets via 'make path/to/target.o', target.o
> > may be build multiple times concurrently, leading to build errors like
> > below:
> > 
> > $ make O=~/O/vanilla-7.1/ -j$(nproc) arch/x86/pci/intel_mid.o
> > make[1]: Entering directory '/scratch/minipli/obj/vanilla-7.1'
> >   DESCEND objtool
> >   DESCEND bpf/resolve_btfids
> >   CC      arch/x86/pci/intel_mid.o
> >   CC      arch/x86/pci/intel_mid.o
> > fixdep: error opening file: arch/x86/pci/.intel_mid.o.d: No such file or directory
> > make[4]: *** [/scratch/minipli/src/linux/scripts/Makefile.build:290: arch/x86/pci/intel_mid.o] Error 2
> > make[4]: *** Deleting file 'arch/x86/pci/intel_mid.o'
> > make[3]: *** [/scratch/minipli/src/linux/scripts/Makefile.build:551: arch/x86] Error 2
> > make[3]: *** Waiting for unfinished jobs....
> > make[2]: *** [/scratch/minipli/src/linux/Makefile:2229: .] Error 2
> > make[1]: *** [/scratch/minipli/src/linux/Makefile:248: __sub-make] Error 2
> > make[1]: Leaving directory '/scratch/minipli/obj/vanilla-7.1'
> > make: *** [Makefile:248: __sub-make] Error 2
> > 
> > Please note that arch/x86/pci/intel_mid.o gets output twice and, in
> > fact, tried to get built twice, causing the two parallel builds to
> > interfere with each other.
> > 
> > The reason for the double build attempt is that the target can be
> > reached twice, via arch/x86/'s Makefile and via arch/x86/pci/'s which
> > both end up in '$(single-subdirs)' as both fit the prefix match.
> > 
> > Only the latter should be used as it contains a cflags override for
> > CONFIG_PCI_DEBUG=y which would be skipped when using arch/x86/Makefile
> > to build arch/x86/pci/intel_mid.o.
> > 
> > Fix the multi-match issue by using only the deepest match instead of all
> > possible ones, which requires two changes:
> > 1. Limiting 'single-subdirs' to a single match per target and
> > 2. filtering out deeper targets in the recursive make invocation.
> > 
> > Also de-duplicate the list of target subdirectories to simplify the
> > use of '$(single-subdirs)' in filter expressions for multiple targets in
> > the same directory.
> > 
> > Fixes: cc306abd19e8 ("kbuild: fix and refactor single target build")
> > Assisted-by: LLM
> > Signed-off-by: Mathias Krause <minipli@grsecurity.net>
> > ---
> > I ran into this quite a few times in the past and only now gave the LLM
> > a try to dig into it. The bug only affects certain targets and only when
> > mentioned on the command line, so regular 'make -j$(nproc)' isn't
> > affected.
> > 
> >  scripts/Makefile.build | 8 ++++++--
> >  1 file changed, 6 insertions(+), 2 deletions(-)
> > 
> > diff --git a/scripts/Makefile.build b/scripts/Makefile.build
> > index 0f4fdc119e58..d5f58513d02a 100644
> > --- a/scripts/Makefile.build
> > +++ b/scripts/Makefile.build
> > @@ -534,7 +534,10 @@ endif
> >  # Single targets
> >  # ---------------------------------------------------------------------------
> >  
> > -single-subdirs := $(foreach d, $(subdir-ym), $(if $(filter $d/%, $(MAKECMDGOALS)), $d))
> > +# A single target may match nested entries in subdir-ym.  Descend through only
> > +# the deepest match so that two recursive makes do not build the same target.
> > +single-subdir = $(lastword $(sort $(foreach d, $(subdir-ym), $(if $(filter $d/%, $(1)), $d))))
> > +single-subdirs := $(sort $(foreach g, $(MAKECMDGOALS), $(call single-subdir, $g)))
> >  single-subdir-goals := $(filter $(addsuffix /%, $(single-subdirs)), $(MAKECMDGOALS))
> >  
> >  $(single-subdir-goals): $(single-subdirs)
> > @@ -548,7 +551,8 @@ $(subdir-ym):
> >  	$(Q)$(MAKE) $(build)=$@ \
> >  	need-builtin=$(if $(filter $@/built-in.a, $(subdir-builtin)),1) \
> >  	need-modorder=$(if $(filter $@/modules.order, $(subdir-modorder)),1) \
> > -	$(filter $@/%, $(single-subdir-goals))
> > +	$(filter-out $(addsuffix /%, $(filter $@/%, $(single-subdirs))), \
> > +		$(filter $@/%, $(single-subdir-goals)))
> >  
> >  # Add FORCE to the prerequisites of a target to force it to be always rebuilt.
> >  # ---------------------------------------------------------------------------
> 
> Ping! I know, it's the merge window but this patch may have fallen
> through the cracks as there was no response so far. Just trying to take
> care it doesn't get lost.

thanks for the ping.  The patch is in my review queue for the upcoming
cycle.

-- 
Nicolas