Re: [PATCH 1/2] kbuild: Require a 'make clean' if we detect gcc changed underneath us

[Guenter Roeck <linux@roeck-us.net>]

On Thu, Dec 21, 2017 at 05:53:02PM -0800, Douglas Anderson wrote:
> Several people reported that the commit 3298b690b21c ("kbuild: Add a
> cache for generated variables") caused them problems when they updated
> gcc versions.  Specifically the reports all looked something similar
> to this:
> 
> > In file included from ./include/uapi/linux/uuid.h:21:0,
> >                  from ./include/linux/uuid.h:19,
> >                  from ./include/linux/mod_devicetable.h:12,
> >                  from scripts/mod/devicetable-offsets.c:2:
> > ./include/linux/string.h:8:20: fatal error: stdarg.h: No such file or
> >                  directory
> >  #include <stdarg.h>
> 
> Masahiro Yamada determined that the problem was with:
> 
>   NOSTDINC_FLAGS += -nostdinc -isystem $(call shell-cached,$(CC)
>   -print-file-name=include)
> 
> Specifically that the stale result of -print-file-name is stored in
> the cache file.  It was determined that a "make clean" fixed the
> problems in all cases.
> 
> In this particular case we could certainly try to clean just the cache
> when we detect a gcc update, but it seems like overall it's a bad idea
> to do an incremental build when gcc changes.  We should warn the user
> and tell them that they need a 'make clean'.
> 
> Fixes: 3298b690b21c ("kbuild: Add a cache for generated variables")
> Reported-by: Yang Shi <yang.s@alibaba-inc.com>
> Reported-by: Dave Hansen <dave.hansen@intel.com>
> Reported-by: Mathieu Malaterre <malat@debian.org>
> Signed-off-by: Douglas Anderson <dianders@chromium.org>

Unfortunately, it doesn't work as-is. Trying to build for mips with two
different compilers, and trying to run "make clean" with the 2nd compiler
version, results in:

scripts/Kbuild.include:490: *** Detected new CC version (050400 vs 040803).
Please 'make clean'.  Stop.
Makefile:1297: recipe for target '_clean_.' failed
make: *** [_clean_.] Error 2
make: *** Waiting for unfinished jobs....
scripts/Kbuild.include:490: *** Detected new CC version (050400 vs 040803).
Please 'make clean'.  Stop.
Makefile:1297: recipe for target '_clean_arch/mips' failed
make: *** [_clean_arch/mips] Error 2
scripts/Kbuild.include:490: *** Detected new CC version (050400 vs 040803).
Please 'make clean'.  Stop.
Makefile:1297: recipe for target '_clean_arch/mips/boot/dts' failed
make: *** [_clean_arch/mips/boot/dts] Error 2
make: *** wait: No child processes.  Stop.

Guenter