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

[Nicolas Schier]

> 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 '$(single-subdirs)' in filter expressions for multiple targets in
> the same directory.
> 
> Fixes: cc306abd19e8 ("kbuild: fix and refactor single target build")
> Assisted-by: LLM
> Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> ---
> I ran into this quite a few times in the past and only now gave the LLM
> a try to dig into it. The bug only affects certain targets and only when
> mentioned on the command line, so regular 'make -j$(nproc)' isn't
> affected.
> 
>  scripts/Makefile.build | 8 ++++++--
>  1 file changed, 6 insertions(+), 2 deletions(-)
>

Thanks a lot for the patch!  I will happily take it for the v7.5 cycle,
and add a

Cc: stable@xxxxxxxxxxxxxxx



Reviewed-by: Nicolas Schier <nsc@xxxxxxxxxx>
Tested-by: Nicolas Schier <nsc@xxxxxxxxxx>

-- 
Nicolas